Lab 19 – Poriferan Anatomy
Overview of Animal Form and Function
AP Biology
1st ed. 2008
Taken from Montgomery High Schools AP Biology Lab Manual
Compiled by Christopher Resch
Lab 19 – Poriferan Anatomy
Introduction:
PHYLUM PORIFERA - the sponges
Class Calcarea - sponges with spicules composed of calcium carbonate.
Class Hexactinellida - spicules composed of silica.
Class Demospongiae - skeleton composed of silica or spongin fibers or both
Sponges are the simplest of all metazoan animals. Neither true tissues nor organs are present, and the cells show considerable independence. All members of this phylum are sessile, are filter feeders, and exhibit little detectable movement. Primitive sponges appear radially symmetrical, but most sponges are asymmetrical in body shape, being shaped primarily by enviromental factors. Except for two families, the phylum is entirely marine.
In addition to their taxonomic classes, the sponges are organized into three structural grades of increasing complexity:
Asconoid - the simplest structure, comprised of a single chamber lined with flagellated choanocytes and single exit pore or osculum (ex. Leucosolenia).
Syconoid - sponges with many flagellated canals, but a single osculum (ex. Grantia or Scypha)
Leuconoid - complex sponges with flagellated chambers and numberous oscula (this is the most common structural grade). (Ex. Rhabdodermella, commercial sponges)
Anatomy of a sponge:
The overall body of a sponge is extremely simple. A sponge consists of a body wall(s) that surrounds an opening within the sponge called the spongoceol. Sponges are filter feeders that draw water into their spongoceol through pores along the side of the sponge called ostia (sing. ostium). From the ostia the water flows into the spongoceol and then out of the sponge through an opening called the osculum.
The body walls of asconoid sponges are thinnest while leuconoid sponges have the thickest walls. Syconoid sponges show intermediate and varying wall size between the two. Some sponges, like the glass-sponges (Class Hexactinellida), have a body wall that is composed of syncytium as opposed to cells. Syncytium is a large multinucleated cytoplasm enclosed by an external membrane but not divided into cells by internal membranes.
The two types of tissues that make up the Desmosponges and Calcareans (calcareous sponges), include the epithelioid tissue and the connective tissue. Epithelioid tissues look like epithelium but lacks does not have epiteliums intercellular junctions or a basement membrane. The epithelioid is divided into the pinacoderm (the layer that covers the outer potion of the sponge) and the choanoderm, which lines the inner portion of the sponge. The connective-tissue layer is located between the pinacoderm and the choanoderm. It is composed of mesohyl, a gelatinous like substance that aids in diffusion of nutrients and other molecules, and the spicules, the structural elements of the sponge.
The basic types of cells that are present in a sponge are the following:
|Pinacocytes |The epidermal cells that line the outer layer of the sponges. |
|Porocytes |Cells that create the ostium (pl. ostia), the pores that allow water to flow |
| |into the spongoceol, or the large opening within the sponge. |
|Ameobocytes |Cells that can differentiate into many different types of cells, including |
| |spermatocytes, oocytes, and lophocytes. These cells wander through the mesohyl|
| |and differentiate into a cell type depending on the needs of the organisms |
| |(i.e. if sperm production is needed, it will differentiate into a |
| |spermatocyte). They can then differentiate back into an ameobocytes. Are also|
| |responsible for digesting and distributing carbohydrates and any other digested|
| |molecules. |
|Choanocytes |Also called “collar cells”, these cells make up the inner choanoderm. The name|
| |“collar cell” comes from there collar of microvilli around a single flagella. |
| |Responsible for producing water flow through a sponge via the beating of their |
| |flagella. |
Reproduction:
There are two main ways that sponges can reproduce: either sexually or clonally. Sponges reproduce clonally via budding, fragmentation (when an organisms body becomes fragmented and each piece develops into a new organism), or via the formation of gemmules.
Many freshwater sponges will produce hundreds to thousands of sporelike structures called gemmules. This happens typically during the fall portion of the year. These autumn gemules are in a suspended state of animation called diapauses, which allows it to endure extreme changes in environmental conditions such as temperature, salinity, or and dessication.
EXERCISES
1. Grantia (Scvpha), a common syconoid sponge, will serve to illustrate the different cell types found in sponges. Look at the prepared slide of a cross section (x.s.), for the radial canals (additional canals as a result of an increase in size of certain groups) and their lining of flagellated choanocyte cells. The beating of the flagellae in these cells creates water currents, bringing food particles through the canals from the incurrent pore. Each canal empties into the spongocoel in the center, which empties out through the single osculum. Look at the longitudinal section (I.s) slide and see if you can figure out the overall body design of this sponge.
2. Spicules, skeleton - the skeleton of sponges may be made up of spicules (calcium carbonate or silica), spongin fibers (protein), or both. Examine the slide of spicules of Grantia to see the different shapes and how they hold together. On the slide of Spongilla gemmules, there can be seen characteristic amphidisc spicules. What is a gemmule? What role do gemmules play in the life cycle of sponges?
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Lab 20 – Cnidarian Anatomy
Introduction:
PHYLUM CNIDARIA
Class Hydrozoa - the hydrozoans, colonial or solitary coelenterates with the polyp as the predominant form.
Class Scyphozoa - the jelly fish, characterized by the mobile, floating medusoid form.
Class Anthozoa - the.anemones, corals, sea pens, etc., polypoid forms often with supporting skeletons.
Subclass A1cyonaria (Octocorallia) - "soft corals" with 8 tentacles.
Subclass Zoantharia - "hard corals" with more than 8 tentacles.
The Cnidaria are the most primitive of all the eumetazoa (true multicellular animals). Except for a handful of species, the phylum is marine. They are generally radially symmetrical, and have tentacles armed with exploding cells (cnidoblasts).
Cnidarians are diploblastic (two cell layers), and have a single body cavity with one opening (the coelenteron). There are two structural forms: the polyp and the medusa, which may alternate as the vegetative and reproductive generations in the reproductive cycle.
a. External Anatomy
1. Hydra is a solitary, freshwater polyp-type hydrozoan, and is commonly used as an example of cnidarian anatomy. Obtain a living specimen, place it in a watch glass, and study it with the dissecting microscope at low power. They have a basal disc or aboral end, which is attached, and a free end, the oral end, which bears the mouth on the hypostome and is surrounded by tentacles.
2. At higher magnification, the clusters of stinging cells, the cnidocytes, can be seen. These cells contain explosive cell organelles called nematocysts, unique to this phylum To study nematocysts, place the Hydra on a microscope slide, preferably one with a concavity, then add a drop of water and a cover glass. Examine the cells with the low power of your compound microscope. Then introduce a drop of 1% acetic acid to the edge of the cover slip and draw the water through from the other side with a piece of paper towel. This should cause some of the nematocysts to fire, enabling you to compare exploded and unexploded cells. Using a higher magnification should enable you to see the structure of nematocysts, including the trigger of cnidocil and the filament.
3. Study the prepared microscope slides under the dissecting scope and the compound scope to examine the internal anatomy of Hydra. The central, inner body cavity is the coelenteron or gastrovascular cavity. This cavity is lined with a layer of cells called the gastrodermis. The outer side of the body is covered with a layer of cells called the epidermis. Between them is the transparent, non-cellular layer of mesoglea. The epidermis contains cells that serve both covering and contraction functions, the epitheliomuscular cells. The gastrodermis contains cells with several roles called nutritive muscular cells. Some nematocysts can be found in the body wall, but for the most part they are in the tentacles. Also between the cell layers is the nerve net of Hydra, but it cannot be seen well without staining.
4. Reproduction - Hydra reproduces sexually or asexually. Obtain a slide of the hydra budding for a view of the asexual form of reproduction. Obtain a slide of Obelia and identify the ephyra and other relative parts. Ova are produced in conical enlargements, the ovaries, at the aboral portion of the trunk. Sperm is produced in a testis in the distal third of the body. Can you find ovaries and testes in the same animal?
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Lab 21-Platyhelminthes Anatomy
Introduction:
PHYLUM PLATYHELMINTES
Class Turbellaria
Order Seriata
Family Planariidae
Family Planariidae
Planaria are common freshwater, non-parasitic flatworms of the phylum Platyhelminthes, class Turbellaria. It should be noted that the term "planaria" is most often used as a common name. It is also the name of a genus within the family Planariidae.
Planaria are common to many parts of the world and reside in fresh water ponds and rivers. They are also found commonly residing on plants.
The planarian has very simple organ systems. The digestive system consists of a mouth, pharynx, and a gastrovascular cavity. The mouth is located in the center of the underside of the body. Digestive enzymes secrete from mouth to begin external digestion. The pharynx connects the mouth to the gastrovascular cavity. This structure branches throughout the body allowing nutrients from food to reach all extremities. They eat living or dead small animals that they suck with their muscular mouth. From there, the food passes through the pharynx into the intestines and digesting of the food takes place in the cells lining the intestine, which then diffuses to the rest of the body.
Planare receive oxygen and release carbon dioxide by diffusion. The excretory system is made of many tubes with many flame cells and excretory pores on them. Flame cells remove unwanted liquids from the body by passing them through ducts that lead to excretory pores where the waste is released on the dorsal surface of the planarian.
At the head of the planarian there is a ganglion under the eyespots. From the ganglion there are two nerve cords which connect at the tail. There are many transverse nerves connected to the nerve cords which make it look like a ladder. With a ladder-like nerve system, it is able to respond in a coordinated manner.
Reproduction
A planaria can reproduce either asexually or sexually. In asexual reproduction the planarian detaches its tail end and each half regrows the lost parts. However several problems can occur so this is not done as often. Instead, in sexual reproduction, the planaria transports its excretion to the next planaria. Each Planaria gives and receives sperm. Planaria have both testes and ovaries and, therefore, are hermaphrodites. They can reproduce with their own gametes, or they can mate with another worm. Mating is desirable in order to enhance the survival of the species by altering DNA. Eggs develop inside the body and are shed in capsules. Weeks later the eggs hatch and grow into adults. Planarians can also reproduce by regeneration. Regeneration may occur when a Planarian is cut into two halves. Each half may become a new Planarian.
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Using prepared slides identify the major anatomical features.
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Lab 22 – Nematoda Anatomy
Introduction:
PHYLUM NEMATODA- the round worms
The nematodes or roundworms are one of the most common phyla of animals, with over 80,000 different described species (over 15,000 are parasitic). They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species counts, and are found in locations as diverse as Antarctica and oceanic trenches. Further, there are a great many parasitic forms, including pathogens in most plants, animals, and also in humans.
Nematodes are unsegmented, bilaterally symmetric and triploblastic protostomes with a complete digestive system. Roundworms have no circulatory or respiratory systems so they use diffusion to breathe. Although they lack a circulatory system, nutrients are transported throughout the body via fluid in the pseudocoelom. Nematodes are one of the simplest animal groups to have a complete digestive system, with a separate orifice for food intake and waste excretion, a pattern followed by all subsequent, more complex animals. The body cavity is a pseudocoelom. The mouth is often surrounded by various flaps or projections used in feeding and sensation. The portion of the body past the anus or cloaca is called the "tail." As they grow, their cells get larger, but the total number is constant, called eutely.
The epidermis secretes a layered cuticle made of three layers of collagen that protects the body from drying out, from digestive juices, or from other harsh environments. Although this cuticle allows movement and shape changes via a hydrostatic skeletal system, it is very inelastic so it does not allow the volume of the worm to increase. Therefore, as the worm grows, it has to molt and form new cuticles.
Nematodes have a simple nervous system, with a main ventral nerve cord and a smaller dorsal nerve cord. Sensory structures at the anterior end are called amphids, while sensory structures at the posterior end are called phasmids.
Most free-living nematodes are microscopic, though a few parasitic forms can grow to over a meter in length (typically as parasites of very large animals such as whales).
Nematodes generally eat bacteria, fungi and protozoans, although some are filter feeders. Excretion is through a separate excretory pore. Nematodes also contract bacterial infections within excretion pores.
Reproduction
Reproduction is usually sexual. Males are usually smaller than females (often much smaller) and often have a characteristically bent tail for holding the female for copulation. During copulation, one or more chitinized spicules move out of the cloaca and are inserted into genital pore of the female. Amoeboid sperm crawl along the spicule into the female worm. Nematode sperm is thought to be the only eukaryotic cell without the globular protein G-actin.
Eggs may be embryonated or unembryonated when passed by the female, meaning that their fertilized eggs may not yet be developed. In free-living roundworms, the eggs hatch into larva, which eventually grow into adults; in parasitic roundworms, the life cycle is often much more complicated.
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Some nematodes, specifically Heterorhabditis spp., undergo a process called endotokia matricida; intrauterine birth causing maternal death. The hermaphroditic nematode keeps its self-fertilized eggs inside its uterus until they hatch. The juvenile nematodes will then ingest the parent nematode. This process is significantly promoted in environments with a low or reducing food supply.
Free-living species
In free-living species, development usually consists of four molts of the cuticle during growth. Different species feed on materials as varied as algae, fungi, small animals, fecal matter, dead organisms and living tissues. Free-living marine nematodes are important and abundant members of the meiobenthos. They play an important role in the decomposition process, aid in recycling of nutrients in marine environments and are sensitive to changes in the environment caused by pollution. One roundworm of note is Caenorhabditis elegans, which lives in the soil and has found much use as a model organism. C. elegans has had its entire genome sequenced, as well as the developmental fate of every cell determined, and every neuron mapped.
Some Nematodes can undergo cryptobiosis.
Parasitic species
Nematodes commonly parasitic on humans include whipworms, hookworms, pinworms, ascarids, and filarids. The species Trichinella spiralis, commonly known as the trichina worm, occurs in rats, pigs, and humans, and is responsible for the disease trichinosis. Baylisascaris usually infests wild animals but can be deadly to humans as well. Haemonchus contortus is one of the most abundant infectious agents in sheep around the world, causing great economic damage to sheep farms. In contrast, entomopathogenic nematodes parasitize insects and are considered by humans to be beneficial.
One form of nematode is entirely dependent upon the wasps which are the sole source of fig fertilization. They prey upon the wasps, riding them from the ripe fig of the wasp's birth to the fig flower of its death, where they kill the wasp, and their offspring await the birth of the next generation of wasps as the fig ripens.
Plant parasitic nematodes include several groups causing severe crop losses. The most common genera are: Aphelenchoides (foliar nematodes), Meloidogyne (root-knot nematodes), Heterodera, Globodera (cyst nematodes) such as the potato cyst nematode, Nacobbus, Pratylenchus (lesion nematodes), Ditylenchus, Xiphinema, Longidorus, Trichodorus. Several phytoparasitic nematode species cause histological damages to roots, including the formation of visible galls (Meloidogyne) which are useful characters for their diagnostic in the field. Some nematode species transmit plant viruses through their feeding activity on roots. One of them is Xiphinema index, vector of GFLV (Grapevine Fanleaf Virus), an important disease of grapes.
Other nematodes break down bark and forest trees. The most important representative of this group is Bursaphelenchus xylophilus, the pine wood nematode, present in Asia and America and recently discovered in Europe.
The largest nematode ever recorded, Placentonema gigantissima, was discovered parasitizing the placenta of a sperm whale, measuring 8.5 m in length with a diameter of 0.3 mm, and containing 32 ovaries. Other large nematodes include: Dioctophyma renale, the giant kidney worm, a parasite most commonly found in mink but also in dogs and humans, that can reach up to 103 cm in length.
Male with Ascaris infection
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We will be looking at the prepared slides of Ascaris a common pathogenic nematode. Can you identify the body structures on the slide? Where is the pseudoceoel on your slide? Identify the organs in the cross sectional prepared slide. Also examine the preserved round worms. Is it male or female? How can you tell?
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Lab 23 – Annelid Anatomy
Introduction:
PHYLUM ANNELIDA
Class Clitellata
Subclass Oligochaeta
Subclass Hirudinea
Class Polychaeta
SUBCLASS OLIGOCHAETA - the earthworms, bristle-footed worms
I. Earthworm
a. If available*, let a live earthworm crawl in a pan, on a moist towel. What movements bring about forward motion? Which way does the wave pass? What muscle systems do they move with the body contraction? Watch a single somite in motion. Put an earthworm on some dry paper and listen to the scratching of the setae; on a glass plate to see if they can still move.
Hang an earthworm loosely over your finger (posterior end up). Stick a small piece of filter paper (2cm2) on the anterior. Why does the paper move up toward the tail?
Let the earthworm rest on the paper. Touch lightly or jar the paper. The animal will respond with a reflex, made possible by giant nerve fibers, of importance in rapidly withdrawing within the burrow.
b. If you have never dissected or studied an earthworm, start here; otherwise refresh your memory by examining your neighbor's dissected earthworm. Use a preserved specimen or an (anesthetized) live one for dissection. Each annulus (ring) marks one somite. The tip of the worm anterior to the mouth is the prostomium. The next segment is the first somite. The last somite bears the anus. Some somites at the anterior of the worm are swollen, forming a glandular area, the clitellum. It secretes the mucous ring which binds worms together when copulating, the albumin in which eggs are laid in the cocoon, and the outer wall of the cocoon itself. Study one somite under the dissecting microscope and see the distribution of the setae (the small hair like projections on the ventral side of the worm). How many setae per segment, on what surface, and which way do they point? What is their structure?
Earthworms are hermaphroditic. On the ventral side of the 15th somite are 2 small raised areas, which bear the male gonopores. The female gonopores are on the 14th somite toward the ventral median line and are small. Two pairs of tiny openings of the seminal receptacles, or spermathecae are located between the 9-10th and l0-llth somites. The spermathecae receive and store sperm from the male, and it is usually the site of fertilization when the oocytes are ready.
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c. Make an incision on the dorsal surface (not mid-dorsal). Cut slowly posteriorly from the 2nd somite to the posterior end of clitellum pulling the body wall apart. Notice the septum (divider) between each of two somites.
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Carefully cut the septa and pin the body wall back on the dissecting pan. (Keep the dissection completely submerged under water.) Why not cut mid-dorsally? (the dorsal blood vessel?)
The pharynx is the most anterior, muscular region of the gut. The pharynx connects to the esophagus. The seminal vesicles form three pairs of sacs, the most anterior and smallest pair sometimes covering the esophagus. Two pairs of small spherical seminal receptacles are located lateral and ventral to the vesicles. What is the function of these two organ systems?
After removing connective tissue holding the vesicles in place, you can find the hearts. Filled with blood, they appear as dark brown tubes connecting the dorsal vessel and the subintestinal vessel in the 11th and 12th segments there are 2 pairs of whitish calciferous glands on the sides of the esophagus. They are excretory organs, and regulate the Ca++ concentration and pH of the blood and coelomic fluid.
Posteriorly the esophagus connects to the large crop and this in turn adjoins the muscular gizzard. The gizzard continues into the intestine. If you lift the gut out you may be able to see the two very small ovaries in the 13th segment and the oviduct with funnel opening on the ventral side of the 14th somite. The two pairs of testes are close to the median line in the 10th and 11th somites ventral within the seminal vesicles. The sperm escapes through the sperm duct opening on somite 15.
After removing the intestine from a posterior somite notice the coiled nephridium. The nephrostome collects the excretory material and opens through the nephropore on the ventral side of segment. Do all segments have a
nephridium?
On the surface of the ventral body wall notice the thin white nerve cord. Trace it anteriorly to the 3rd somite and expose the subpharyngeal ganglion. The nerve cord divides anteriorly, uniting again above the pharynx to form the suprapharyngea1 ganglion (brain). Different species of earthworm differ from each other by having organs,
ducts, etc. in different somites.
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d. On a prepared slide of an earthworm cross-section find the outer acellular cuticle and the glandular epidermis underneath.
Under the epidermis is a layer of circular muscle and under this a layer of longitudinal muscle. The membrane towards the coelom is the peritoneum. The intestine is surrounded by chloragogen cells and is lined by glandular epithelium. The chloragogen cells are important in the metabolism of the worm and are functionally analogous to the vertebrate liver and kidney. They are the site of glycogen synthesis, urea and ammonia formation, and protein deamination. A dorsal fold, the typhlosole, increases the surface of intestine.
Notice the dorsal blood vessel, subintestinal blood vessel, ventral nerve cord, and subneural vessel.
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Lab 24 – Arthropod Anatomy
Introduction:
PHYLUM ARTHROPODA
Class Malacostraca-
Order Decapoda- These are the familiar shrimp, lobsters and crabs. The first three throacopods are modified as maxillipeds, leaving 5 pair of unmodified legs (thus the name deca, or 10). The gills are thoracic and are always enclosed by the carapace. The first walking legs are chelate, having a pair of claws (which are often enlarged).
Sub-Order Natantia -the shrimp- The body is laterally compressed. The pleopods are well adapted for swimming and the thoracopods are very slender.
Sub-Order Reptantia - the Steeping decapods. The abdomen is reduced, and they usually have an enlarged cheliped.
Section Macrura - crayfish have conspicuous uropods and large abdomen.
Section Anomura - hermit crabs and mole crabs have a reduced abdomen, ins also soft.
Section Brachyura - the crabs, in which the abdomen is greatly reduced and flexed under.
1. Cambarus the freshwater crayfish (or "crawdad") will serve as a representative decapod crustacean for dissection.
a. External Anatomy
The 5 head segments and 8 thoracic segments are fused to form the cephaothorax covered dorsally by the hard carapace. Dorsally on the carapace you can find the anterior projection between the compound eyes, the rostrum. In the middle of the carapace is the transverse cervical groove; behind it are 2 branchiocardiac grooves.
The abdomen is composed of 6 movable somites, each bearing a pair of jointed appendages, and the telson which lacks appendages but has the anus on its ventral side. Each somite has a tergum (a dorsal plate) with a pleuron (its lateral extension), a sternum (the ventral plate). Between the pleuron and the base of the appendages is the epimeron.
On each side of the rostrum are the stalked compound eyes. Anterior are the antennules (1st antennae) and the antennae (2nd antennae). There are 6 pairs of mouth appendages. The che1ipeds are the largest limbs, each with a large claw. Are the claws on each side exactly alike?
The cheliped on each side is followed by 4 pairs of walking legs, pereopods. The abdomen has 6 pairs of pleopods (swimmerets). In the male the first 2 pairs are modified. What is the sex of your specimen? The last abdominal somite bears the uropods.
Examine a cheliped and compare it to a walking leg and determine what parts form the chela or claw. Are the walking legs and chelipeds biramous appendages (limb, that branches into two, and each branch consists of a series of segments attached end-to-end) or uniramous (comprises a single series of segments attached end-to-end)?
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b. Mouthparts
Put the crayfish on the dissecting pan, ventral up, and find the mouth appendages. Use a probe to move the limbs about. Note how the appendages are associated:
Most anterior are the mandibles, almost completely covered by the posterior appendages. Are the teeth on the left and right mandible exactly alike?
The other mouthpart appendages are the first maxil1a (You may want to remove the posterior appendages first, but keep them in order), the second maxilla, and a small endopodite. It assists in water movement by the gil1s and 3 pairs of maxillipeds.
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c. Internal Anatomy
To remove the carapace, make a median cut from the middle of the posterior margin to the rostrum. Make lateral cuts inside the upper limits of the branchial chambers. Carefully cut off all structures that adhere to the carapace and remove it. Then remove the terga from the first two abdominal somites. Notice that the cheliped and some walking legs also have gills attached to the coxa. These gills hang into the branchial chamber and are protected by the carapace.
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Remove the epidermis and identify the sclerotized stomach near the anterior end of the cephalothorax. There are anterior stomach muscles (attached to the anterior of the stomach), posterior stomach muscles (attached to the posterior of the stomach), and mandibular muscles (lateral to the stomach). The digestive gland (hepatopancreas) is yellow-green and lateral to the mandibular muscles. The heart is shield-shaped and lies in the mid-thoracic region. Find the ostia through which the blood enters the heart from the surrounding pericardial sinus, and the antennary arteries running forward from each side of the heart.
Remove the longitudinal extensor muscles from the first two abdominal somites and find the posterior lobes of the digestive gland, and the dorsal abdominal artery. In males, the testes are on either side and below the dorsal abdominal. Locate the testes or ovaries below the heart and the oviduct or sperm duct running down to the appendages. Where are the gonopores in each sex?
Remove the stomach by cutting through the esophagus where it joins the stomach. Be careful not to injure the circumesophageal connectives of the nervous system which pass around the esophagus near this point. Cut the stomach open along the mid-ventral line and examine the gastric mill, the excretory organs (green glands).
Locate the nerve cord in the abdomen by carefully separating the two columns of muscles. Then cut away the muscles almost to the level of the nerve cord. Study the abdominal portion first and then work forward to the cephalothoracic region. The cord lies in the sternal sinus, the roof of which is formed by the sclerotized internal folding of the exoskeleton.
Remove this roof carefully to study the nerve cord and the nerves. Identify the two longitudinal nerve cords, the thickened segmental ganglia the commissures between the ganglia, the circumesophageal connectives, and the supraesophageal ganglion (brain) lying between the eye stalks. The optic nerves lead to the eyes, the antennary nerves to the 2nd antennae. Find the sub-esophageal ganglion and the 5 pairs of nerves that innervate the appendages.
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Grasshopper Anatomy
CLASS INSECTA - The insects
The insects are characterized as mandibulate arthropods with three tagma: head, thorax, and abdomen. The thorax bears three pairs of walking legs and wings when they are present. Not all insects have wings; some groups lack them altogether, while others have "lost" them.
As a representative insect, we will use the large lubber grasshopper, Romalea for dissection.
a. External Anatomy
The body of the grasshopper is divided into a head consisting of six fused segments (somites), a thorax of three somites to which are attached the legs and wings, and a long segmented abdomen that terminates with the reproductive organs. The outer exoskeleton consists largely of chitin which is secreted by the epidermis. In order to grow, the grasshopper periodically sheds this exoskeleton (molts) as do all arthropods; adults do not molt.
The head has one pair of slender, jointed antennae, two compound eyes, and three simple eyes or ocelli. The mouth parts are of the chewing type and include a broad upper lip or labrum; a tonguelike hypopharynx: two heavy blackish lateral jaws or mandibles, each with teeth along the inner lateral margin for chewing food; a pair of maxillae of several parts, including palps (sensory appendages) at the side; and a broad lower lip or labium with two short palps.
The thorax consists of three parts: a large anterior prothorax the mesothorax and the posterior metathorax. Each part bears a pair of jointed legs. Identify the leg segments indicated in the figure. The mesothorax and metathorax each bear a pair of wings. The anterior wings of the grasshopper are thick and shield the larger pair of flight wings. Both pair of wings are derived from the cuticle and have thick parts (veins) that strengthen them. Stretch out the wings and examine the anterior protective wings and the flight wings.
The slender abdomen consists of eleven somites, the posterior ones being modified for reproduction. The male has a blunt terminal segment, whereas the female has four sharp conical prongs, the ovipositors, which are used in egg laying. Along the lower sides of the thorax and abdomen are ten pairs of spiracles, the small openings of elastic air tubes, or tracheae that branch to all parts of the body and constitute the respiratory (tracheal) system of the grasshopper. This system of air tubes brings atmospheric oxygen directly to the cells of the body. The spiracles open and close to regulate the flow of air.
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b. Internal Anatomy
It is difficult to preserve the internal organs of the grasshopper because the preservative often fails to penetrate the exoskeleton. Careful dissection is necessary to study the internal anatomy.
After removing the wings, start at the posterior end and make two lateral cuts toward the head with a pair of scissors or fine scalpel as indicated in the figure. Remove the dorsal wall. Locate the muscles on the inside of the body wall and note their arrangement. What is their function?
A space between the body wall and digestive tract, the hemocoel, is filled with colorless blood. Study the digestive tract and identify its parts. Beginning at the anterior end, find the mouth, which is located between the mandibles and leads to a short esophagus followed by the crop.
Next is the midgut to which are attached six double finger-shaped digestive glands (gastric caeca); these glands produce enzymes that are secreted into the gut to aid digestion. The digestive tract continues as the hindgut which consists of a tapered anterior part, a slender middle part, and an enlarged rectum that opens to the outside at the anus. During feeding, food held by the forelegs, labium, and labrum is lubricated by secretions from the salivary glands and chewed by the mandibles and maxillae. Chewed food is stored in the crop. Because most of the digestive tract, except for the stomach and crop, is lined with chitin, which is impervious, digestion and absorption take place mainly in the midgut. Excess water is absorbed from any undigested food in the rectum.
The excretory system is made up of numerous tiny tubules – the Malpighian tubules - which empty their products into the anterior end of the intestine. The tubules remove urea and salts from the blood.
The sexes are separate, and their reproductive organs are in the terminal abdominal segments. In the male, each of the two testes is composed of a series of slender tubules, or follicles, and is located above the intestine; each testis is joined to a longitudinal vas deferens. The vas deferens are joined to a single ejaculatory duct to which accessory glands are attached. In the female, each ovary is composed of several tapering egg tubes (ovarioles), which produce the ova. Each ovary is joined to an oviduct leading to the vagina to which a pair of accessory glands and a single soermatheca is attached. The latter organ is used to store sperm received at copulation.
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Although the class Insecta comprises some 26 different orders (and nearly 1000 families!), the four orders described below represent the four most common and familiar.
ORDER COLEOPTERA
Beetles constitute the single largest order of insects (and animals) representing about 300,000 species or nearly one third of all known animal species. They are recognized most easily by their hard bodies and chewing mouthparts. Adults usual1y have two pair of wings, the front pair being modified as a hard protective covering (elytra). We have many of these preserved for you to observe.
ORDER LEPIDOPTERA
These insects include the familiar moths and butterflies. They have conspicuous scaled wings and mouthparts modified into a long coiled proboscis specialized for sucking flower nectar. We have many of these preserved for you to observe.
ORDER DIPTERA
Literally, diptera means "two wings". These are the true flies, Drosophila melanogaster being the most familiar. All have functional front wings, but the back wings are reduced and knoblike. These back wings (halteres) aid in turning in flight. The group also includes mosquitoes, gnats, midges, horseflies and other summer nuisances. We have many of these preserved for you to observe.
ORDER HYMENOPTERA
Bees, wasps, sawflies, and ants comprise this large order. All have chewing mouthparts. It is among the hymenopterans that social behavior in animals has reached its highest development. Many species are eusocial with distinct division of labor and reproduction within colonies. We have many of these preserved for you to observe.
Lab 25 – Mollusk Anatomy
Introduction:
PHYLUM MOLLUSCA
Class Polyplacophora (Amphineura)
Class Scaphopoda
Class Gastropoda
Subclass Prosobranchia
Subclass Opisthobranchia
Subclass Pulmonata
Class Pelecypoda (Bivalvia)
Class Cephalopoda
Subclass Nautiloidea
Subclass Ammonoidea
Subclass Coleoidea
The' phylum Mollusca is among the most conspicuous of the invertebrate phyla, they have been collected, eaten, and coveted since humans emerged. A collector of shells is considered a conchologist, whereas those who study the mollusca are malacologists. There is no apparent metamerism in these soft bodied animals, and the body form is frequently secondarily modified to an asymmetrical condition. The shell, when present, is secreted by the mantle, a flap of tissue covering the dorsal surface.
CLASS CEPHALOPODA
This is the class that we will be focusing on for this dissection. These are the largest, fastest and "smartest" invertebrates. Despite these achievements and their bizarre form, they nevertheless exhibit aI1 the basic molluscan features. The shell is usual1y reduced, the foot is variously modified as a siphon and tentacles, the circulatory system is closed and there is extensive cephalization. The nautiloids, cuttlefish, octopods, and squid make up the extant members of this class. On demonstration are a few extinct members that belong within the Subclass Nautiloidea- the Ortheceras. These, and another extinct Subclass, Ammonoidea (Ammonites), are found in the Cincinnati area.
Subclass Coleoidea
Members of this subclass are those you are probably most fami1ar. They include the squid, octopus, and cuttlefish. These molluscs are most easily recognized by their internal shells or greatly reduced external shells. The eight or ten appendages bear suckers and only possess one pair of gills.
Subclass Nautiloidea
The only extant representative of this subclass is the chambered Nautilus, which we have the shell on demonstration. These possess tentacles like those of the Coleoidea, but lack suckers. They also possess two pairs of gills.
Subclass Ammonoidea
This is an extinct subclass that in known mostly from their coiled shells. Superficial1y it resembles Nautiloids but have complex sutures and septa within the shell (contrast this with the smooth shell of the Nautilus).
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We will have squid (Loligo) for your dissection, as well as a large specimen on demonstration.
a. External Anatomy
Orient the squid in the dissecting pan so that the tentacles and arms are directing away from you. The dorsal part of the animal is now nearest you and the ventral part, the "foot" region, is farthest away. Figure out what is responsible for this bizarre conformation of bilateral symmetry.
Arrange the squid so that the surface with the fin is down and the end with the arms nearest you. Exclusive of the tentacles and fins, the body form is elongate and spindle-like. The head region appears as though it is inserted in a central tube. This tough, outer tubular structure is the muscular mantle.
Arrange the squid so that the tentacled end of the animal is facing you. Take the head in your hands and spread the arms and tentacles. Compare the arms as to size and shape. By spreading the appendages slightly, a ring of small finger projections will be found centrally located surrounding the mouth. Observe their arrangement and the suckers they bear. The appendages at the extreme oral region of the squid are made up of two kinds. The eight shorter appendages are the arms. The two long, laterally located club-shaped appendages are the tentacles. Note the position of the sucker cups on all the arms and the tentacles.
The cone-shaped, compressed, medial structure extending beyond the collar on the posterior surface is the siphon, sometimes called the funnel. The siphon is a part of the "foot" region. The animal moves about rapidly, by use of the mantle and the siphon. Water is drawn into the mantle cavity by mantle expansion. Constriction of the collar region by a special valve and contraction of the mantle force water from the mantle cavity through the siphon. The force of the expelled water “propels" the animal. Whichever position the open end of the siphon takes will determine the direction of propulsion. Lateral to the siphon are the eyes, which are very well developed and similar in some aspects of structure and function to vertebrate eyes.
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b. Internal Anatomy
Orient the squid with the siphon facing upward. Use a sharp heavy duty scalpel and make an incision from the collar to the far end of the body. It would be best to make a shallow cut first and follow this by complete separation of the mantle. Make sure the cut is mantle thickness deep so the internal organs are not disturbed and the medial mantle artery is severed as described above. Gently fold the mantle laterally, and expose the internal organs.
Locate the ink sac. The ink sac is filled with a melanin pigment, which if allowed to escape, will create messy conditions for dissection. The ink sac may carefully be removed without disturbing any other structures at this time so dissection can proceed uninterrupted. Observe the membranous tissue which binds the ink sac to the rectum. Using sharp fine pointed scissors and scalpel, separate the ink sac from the rectum, finally removing it. Place the sac in a pan of water and puncture. The clouded water indicates how the squid may use the pigment under normal circumstances for means or escape or hiding when being disturbed. The ink is also said to paralyze the olfactory cells of predatory fish for a brief period. If you have opened the animal properly, all the organs illustrated in the diagrams will be easily visible. Squids are dioecious, that is, the sexes are separate and morphogically distinct. Females have ovaries and nidamental glands (which produce egg coverings). Males lack these, but have testes, running along the length of the mantle alongside the gastric caecum. Note the ctenidiurn, or gill in the mantle cavity. Find the heart (right and left chambers) and the kidney.
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1. Funnel (siphon)
2. Mantle
3. Lateral fins
4. Tentacles
5. Arms
6. Gills
7. Intestinal cecum
8. Pen
Lab 26 – Echinoderm Anatomy
Introduction:
PHYLUM ECHINODERMATA
Class Asteroidea
Class Opbiuroidea
Class Ecbinoidea
Class Holotburoidea
Class Crinoidea
Class Blastoidea
The name of this phylum is derived from one of its distinguishing characteristics "spiny skin". The phylum is named because of the spiny plates (composed of calcareous ossicles) that form an internal skeleton just below the surface of the skin. These are familiar marine animals known as starfish, sea urchins, sand dollars, sea cucumbers, etc. Echinoderms are known for their pentaradial symmetry, which means that the body symmetry is based on multiples of 5. Count the arms on a sea star or a crinoid, and you'll see. They are coelomate, but in contrast to the protostomate phyla (whose coelom appears as a split in the mesoderm), the echinoderms develop a coelom as outpocketings of the embryonic mesoderm of the gut. The echinoderms are referred to as deuterostomate because during development, the mouth forms as the second opening in the embryo (the anus is the first). Because of their "ass-backwards" embryogeny, echinoderms are considered more closely related to chordates (including vertebrates) than they are to other invertebrate phyla. Another unique characteristic of the echinoderms is the water vascular system derived from the coelom, and used primarily in locomotion and food getting.
CLASS ASTEROIDEA - (sea stars, starfish, etc.)
Starrfishes have arms, but they differ from other echinoderm groups in having much more massive arms, into which the body cavity, and large caeca of the gut extend. The orientation also differs, since the mouth is turned downwards. The most primitive starfishes have a blind gut, with no anus, but in the majority of present day starfishes an anus has developed on the upper surface. The gonads lie inside the arms, and open to the exterior by way of definite gonoducts and gonopores. The larval stage resembles that of a sea cucumber, and is called by the same name (auricularia), but many starfishes have acquired a second or third larval stage which develops after the auricularia stage. StarDshes are predators, and are found all over the ocean, and descend into the greatest depths. Modern starfishes include many forms which so closely resemble Paleozoic forms that it is now thought very likely that most of the existing groups had already differentiated before the onset of Mesozoic times.
Dissection: we will use Asterias, a common starfsh, as a representative of this group.
a. External Anatomy
A starfish moves in any direction, chiefly through the action of the tubefeet, so we cannot distinguish anterior or posterior, left or right. The terms dorsal and ventral are commonly used, although oral and aboral are more correct. Since a starfish invariable orients itself so that the aboral side is uppermost, it becomes the "dorsal" side.
Identify the tough, flexible integument which invests the body, and which carries imbedded in it numerous discrete ossicles, bearing spines; note the central disk, the five radially arranged arms. On the upper, or aboral, surface identify the centrally placed anus, guarded by small calcareious plates. On the lower, oral surface note the central mouth, from which radiate five ambulacral grooves. The walls of the grooves are formed by serially arranged transverse ambulacral ossicles, which rest upon adambulacral ossicles, the latter bearing spines which protect the groove.
At the midline of the groove can be seen the radial water vessel, and the radial nerve is also imbedded in the ectoderm ofthe midline of the furrow; note the tube-feet which emerge between adjacent ambulacral plates.
Note the suckers (not all asteroids have suctorial tube-feet). At the extreme tip of the furrow look for a small pigmented light-sensitive eyespot (it is not capable of forming an image).
Under the dissecting microscope examine the bases of the spines on the upper surface, and identify the wreathes of stalked two-jawed pedicellariae which surround them. Note the retractile papulae which are found in the interstices between the platelets of the integument. A madreporite occupies one of the interradial areas of the disk.
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b. Internal Anatomy
With a strong, sharp scalpel cut a vertical cross section through one of the arms, about midway along the arm; note the ossicles imbedded in the dorsal surface, the papulae, the peritoneum lining the coelom, the arch formed by the ambulacral ossicles, and their manner of support upon the adambulacral ossicles, the ambulacral radial water vessel, linked to the ampulae which lie inside the coelom. The ampulae protrude through the ossicles and can be seen on the inner lower surface of the starfish's arms. Within the coelom of the arm, note the branched pyloric ceca of the alimentary system. If the reproductive organs are in season, the ovary (or testis) may also be seen in the section.
Make an incision with strong scissors along both sides of one or more of the remaining arms continuing the incision from the tips of the arms to the lateral margin of the disk. Carefully raise the dorsal integument which has been thus disconnected. Hold the dorsal body wall to a light, and note the thin regions between the skeletal mesh where the papulae are. Identify the rectum which runs upwards to the anus, and be careful not to break it, nor to damage the stone canal which runs upwards to the madreporite. Having now determined the positions of these delicate structures, more of the dorsal integument may be removed from the disk.
Probing carefully with a blunt instrument, follow the alimentary canal from the mouth which leads via a short esophagus into the cardiac stomach, a five-lot sac with folded walls, capable of being everted through the mouth and wrapped over some object selected as food. The retractor muscles of the stomach are attached to the ambulacral plates near the base of the arm, and enable the stomach's retraction within the body. Above the cardiac stomach is the pyloric stomach, a pentagonal chamber of which each angle is drawn out into a bifurcating pyloric caecum, which enters the adjacent arm. What is the function of the four stomachs?
Note the glandular diverticula of the caeca. From the pyloric stomach the intestine continues upwards, and from it there branch several short rectal caeca; above this region the canal becomes the rectum, as earlier noted.
The gut may now be removed in part, to facilitate examination of the water vascular system. Follow the stone canal downwards from the madreporite to where it joins the ring vessel, a pentagonal structure surrounding the mouth. In addition to the radial water vessels (already seen), small glandular Tiedemann bodies (Believed to produce coelomocytes) may sometimes be found. The wall of the stone canal is strongly calcified. The nerve ring and radial nerves follow the same courses as the ambulacral ring vessel and radial water vessels; they lie in the superficial integument, and are therefore hard to distinguish from the ectoderm covering the ambulacral system on the ventral surface.
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Lab 27 – Fetal Pig Anatomy
Introduction
In the following laboratory exercise, you will examine in some detail the external and internal anatomy of a fetal pig (Sus scrofa). As the pig is a mammal, many aspects of its structural and functional organization are identical with those of other mammals, including humans. Thus, a study of the fetal pig is in a very real sense, a study of humans.
The fetuses you will use in the following weeks were salvaged from pregnant sows being slaughtered for food. They are not raised specifically for dissection purposes. The fetuses are removed from the sow and embalmed with a preservative, which is injected through the umbilicus. Following this, the arterial and venous systems are injected under pressure with latex, a rubber-like compound. Arteries (red) are injected through the umbilicus; veins (blue) are injected through one of the jugular veins at the base of the throat.
With the possible exception of the abdominal cavity, organs rarely appear as they are presented in a diagram. If the purpose of this exercise were simply to have you memorize diagrams (or computer screens), we would do only that and bypass the expense, time, and controversy of dissecting! Dissection is a powerful teaching method, especially for concrete thinkers and visual learners. Only by dissecting can you really appreciate the structural and functional role of the many membranes, mesenteries, and connective tissues that will impede your progress every step of the way. Only by dissecting can you really appreciate the relationship between an organ's texture, location, and function. I do not take the life (or death) of your pig specimen lightly – this is why I demand that you take your dissection seriously and utilize your pig to the fullest extent possible.
During these exercises, keep several points in mind. First, be aware that "to dissect" does not mean "to cut up," but rather primarily "to expose to view." Actual cutting should be kept to a minimum. Tissues are picked and teased apart with needle probes, forceps, and blunt probes in order to trace the pathways of blood vessels, nerves, muscles, and other structures. Never cut or move more than is necessary to expose a given part. Second, pay particular attention to the spatial relationships of organs, glands, and other structures as you expose them. Realize that their positions are not random. Third, we encourage you to engage in collaborative discussions with your classmates and compare dissections.
Although the structures described below are identified on the accompanying figures, in some cases the figures contain more information than you need to know. Don't panic – this extra information is provided to help you identify what you do need to know. If you wish to explore your pig more thoroughly and identify additional structures (e.g., blood vessels), please do! Each lab group will be provided with a color photograph dissection manual to supplement this handout. By the end of this exercise you should have a very good grasp of the connections between physiological processes and organ structure/function.
At the end of each major section are a set of questions (Think about it). Additionally, there are boldface questions scattered through the text. Make sure you figure out the answers to these questions before moving on. All are fair game for the practical.
SAFETY AND HYGIENE
1. Practice safe hygiene when dissecting. Do not place your hands near your mouth or eyes while handling preserved specimens. Although most of the preservatives in use today are non-toxic to the skin, they may cause minor skin irritations. If the preservative gets on your skin, wash with soap and warm water.
2. If the preservative gets in your eyes, rinse them thoroughly with the safety eyewash.
3. Never splash the preservative in the pig buckets.
4. Wear lab gloves. Small, medium, and large sizes are available. These gloves are expensive--please don't waste them.
5. Lab gloves and paper towels go in the regular trash. Skin and pieces of pig go into the red plastic bag at the front of the room (not down the disposal).
6. After bagging your pig and placing it in the mortuary cabinet, rinse your tray and stack it neatly by the sink. Wipe up your station.
II. PreLab:
Outside of Class: Prepare for each lab by using the website:
This is the site for the Virtual Pig Dissection. You will need to install Shockwave on your computer in order to see the diagrams.
Anatomical References
Refer to the Virtual Pig Dissection website and study the anatomical references.
1. Draw a sketch of a pig on white, unlined paper. Draw each of the three planes on the pig. Color each plane lightly with a pastel map color and make a key listing the planes. Staple the drawing to this page.
2. After studying the review, match the following terms correctly:
____1) Proximal
____2) Anterior (cranial)
____3) Lateral
____4) Distal
____5) Posterior (caudal)
____6) Ventral
____7) Dorsal
____8) Medial
a. Toward the head
b. Toward the tail
c. Toward the backbone
d. Toward the belly
e. Toward the side; away from the midline
f. Toward the midline
g. Lying near the point of reference (usually the midline)
h. Lying further from the point of reference
Similarity to human structure -- Pigs are mammals. Consequently, all of the major structures found in humans are present in the fetal pig. With proper directions, they can all be readily found, especially with large, full term fetal pig specimens. There are a some differences in structural details, mostly relatively minor in nature. Some examples are given below.
Muscles
In almost every case, fetal pigs have the same muscles as humans, with some small variations in the size and location of some muscles related to the fact that pigs are quadrupedal and humans are bipedal. For example, the major chest and abdominal muscles found in humans are present in the pig. There are some differences in the location of chest muscles that attach to the shoulder girdle. In the hind limb, the pig has the same muscles as humans in the major thigh muscle groups.
Internal Organs
Pigs have all of the same thoracic and abdominal organs as humans. There are small differences in a few organs.
Liver - the human liver has four lobes: right, left, caudate and quadrate. The fetal pig liver has five lobes: right lateral, right central, left central, left lateral, and caudate.
Intestines: there is a significant difference in the structure of the fetal pig colon compared to the human colon. The pig colon is spiral.
Stomach, spleen, bile duct system, small intestines, kidneys, bladder, etc. - the remainder of the abdominal organs found in the fetal pig are basically the same as found in humans.
Thymus - the thymus is found in the same areas in pigs as in humans. However, it is much larger than most students1 expect. This is not a difference of pigs from other mammals. All mammals have a large (enormous) thymus gland during the fetal stage. It gradually shrinks, relative to the rest of the body, throughout life.
Lungs - Like humans, pigs have multi-lobed lungs.
Quiz: Humans have three lobes in the right lung, two lobes in the left lung. How many lobes are there in the lungs of the fetal pig?
Pericardium, vena cava, esophagus, phrenic nerve, etc. - these other thoracic organs are basically the same in pigs and humans.
Uterus - The fetal pig uterus is of a type called bicornate, compared to the simplex human uterus. This means that the pig uterus has two large horns in addition to the body. These horns are sometimes confused with the much smaller Fallopian tubes. It is the presence of these horns which allows pigs to have a litter of 8 or 10 pigs.
Urethra, ovaries, uterine tubes, labia, mesenteries, testes, epididymis, vas deferens, inguinal canal, prostate gland, etc. - these structures are basically the same in the fetal pig and human.
**Embryological note**
Hemiazygous vein/coronary sinus structure -- One of the most notable differences of pigs from humans is in the veins that drain the posterior chest wall. If you look at the back wall of a fetal pig heart, at the location where the coronary sinus is found in a human heart or a sheep heart, you will see a vessel that is enormous compared to the relatively small size of the fetal pig heart. In humans, there is an azygous vein which develops a connection to the heart through the right common cardinal vein (which becomes the superior vena cava in the adult). In humans, the proximal left common cardinal vein becomes very small, draining only the heart wall, as the coronary sinus. The hemiazygous vein develops a connection to the azygous vein through an anastamosis.
In pig development, there is no azygous vein, and the hemiazygous vein drains both sides of the posterior chest wall. The hemiazygous vein loses its connection to the right common cardinal vein, and maintains its connection through the left cardinal vein. The left cardinal vein, consequently, drains not only the heart wall, but also the posterior chest wall through the hemiazygous vein. Consequently, the left cardinal vein becomes very large in pigs. In the pig, one could say that the animal has a very large coronary sinus because it drains the hemiazygous vein as well as the chest wall. Alternatively, one could say that in pigs, the proximal hemiazygous vein drains the heart wall.
Pig Taxonomy
Domestic hogs belong to the same species as the European wild hog, Sus scrofa. Their Order is Artiodactyla (even-hoofed mammals). Other animals in this Order include cattle, sheep, goats, deer, camels giraffes and hippos.
What swimming mammal is somewhat related? _______________
Complete the classification of the domestic Pig using the internet as your reference:
Kingdom: _________________ Order: _________________
Phylum: _________________ Family: _________________
Class: _________________ Genus: _________________
Species: _________________
Procedure:
OBJECTIVES
1. Perform a whole-body dissection of a vertebrate.
2. Identify the major anatomical features of the vertebrate body in a dissected specimen.
3. Understand the relationship between structure and function in the vertebrate body and relate concepts covered in lecture to structures found in your pig.
4. Understand mammalian fetal circulation from a mechanical, physiological, and evolutionary perspective.
5. Apply knowledge and understanding acquired to problems in human physiology.
6. Apply knowledge and understanding acquired to explain organismal adaptive strategies.
EXTERNAL FEATURES
1. Determine the anatomical orientation of your specimen.
• *dorsal: toward the back of the body
• *ventral: toward the underside of the body
• *anterior (cranial): toward the head end of the body
• *posterior (caudal): toward the tail end of the body
• lateral: to the side of the body
• median: toward the center of the body
• right and left: the pig's right and left, not yours!
• proximal or basal: closer to the trunk
• distal: farther from the trunk
• superficial: lying closer to the body surface
• deep: lying under or below
*The terms anterior and posterior are sometimes used synonymously with ventral and dorsal, respectively, for humans.
2. Note the thin peeling layer of tissue covering the body of your pig. This layer is the epitrichium, a layer of embryonic skin that peels off as hair develops beneath it.
3. Identify the regions of the body (Fig. 1):
• head (cranial) region
• neck (cervical) region
• trunk region (thoracic region)
• tail (caudal) region (abdomoninal region)
4. Head: Find the following:
• pinna (auricle): external ear
• external nares (nostrils)
• upper and lower eyelids
• nictitating membrane (third eyelid)
5. Trunk: The terms sometimes used to describe the trunk vary whether one is discussing the dorsal or ventral surface. The trunk can be described using the terms associated with the vertebral column: thoracic (rib), lumbar (lower back), and sacral (pelvic) vertebrae. Ventrally, the abdominal region dominates the area posterior to the thorax. Note the umbilical cord; it connects the fetus to the placenta of the mother and later becomes the navel. Cut off the very tip (0.5 cm) of the umbilicus to more clearly see the following:
• umbilical arteries: two arteries, carry deoxygenated blood from fetus to placenta
• umbilical vein: a single large vein, carries oxygenated blood from placenta to fetus
• allantoic duct: channels urine to the allantois, an extra-embyronic sac
6. Appendages: Examine the legs of your pig. Find the following:
• On the forelimb find the shoulder, elbow, wrist, and digits.
• On the hindlimb find the hip, knee, ankle, heel, and digits.
7. Determining the sex of your pig:
1. Female: Look for a single urogenital opening just ventral to the anus. A prominent genital papilla projects from the urogenital opening.
2. Male: Look for the scrotum, a sac-like swelling containing the testes and located ventral to the anus. The male urogenital opening is faintly visible just posterior to the umbilicus. Note that males as well as females have multiple nipples = teats = mammary papillae.
Think about it
1. Notice how the number of toes is reduced in your pig. The middle two digits form hooves. Ungulates (hooved animals) like the pig walk with the weight of the body borne on the tips of the digits (unguligrade locomotion). Cats and dogs use digitigrade locomotion (walking on the balls of their feet). Humans typically use the entire foot for walking (plantigrade locomotion). What form of locomotion do you use when you sprint?
2. Although male mammals have nipples, as a general rule they do not lactate. From an ultimate (why?) rather than a proximate (how?) standpoint, why is male lactation the exception rather than the rule (HINT: there are very few monogamous mammals)?
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Figure 1. External anatomy of the fetal pig. A. Ventral view. B. Lateral view. C. Posterior view of female. D. Posterior view of male.
MUSCULAR SYSTEM
OBJECTIVES
1. Identify the major muscles groups on a fetal pig
2. Identify Origin and Insertion points of these muscle groups
Dissecting the fetal pig to identify musculature is approached differently than most fetal pig dissection books explain. This is due to development of the internal structures being the common focus of fetal pig dissection. Be careful to not cut too deeply when making incisions as the scalpel will easily damage subcutaneous tissue and musculature. The bilateral symmetry of the fetal pig allows for errors or practice on one side and clean dissection on the other.
Use Figure 4 as a guide for making the various incisions.
1. Begin your incision at the small tuft of hair on the upper portion of the throat (1) and continue the incision posteriorly to approximately 1.5 cm anterior to the umbilicus. You should cut through the muscle layer, but not too deeply or you will damage internal organs.
2. Whether your pig is male or female, make the second incision (2M) as a half circle anterior to the umbilicus and then proceed with two incisions posteriorly to the region between the hindlimbs. Do not make the 2F incision. If you have a male, be careful not to cut deeply into the scrotum.
3. Deepen incisions 1 and 2 until the body cavity is exposed. Make incisions 3 and 4 to produce lateral flaps that can be folded back. Pour excess fluid into the waste container and rinse out the body cavity.
4. Just below the lower margin of the rib cage, make a fifth (5) incision laterally in both directions. This should expose the diaphragm, which separates the thoracic and abdominal cavities. Using your scalpel, free the diaphragm, but do not remove it.
5. Carefully peel back flaps A, B, C, and D and pin them beneath your pig. It may be necessary to cut through the ventral part of the rib cage (very carefully) with a pair of scissors to separate flaps A and B.
6. Carefully remove any excess latex. To free the umbilicus, cut through the umbilical vein approximately 1 cm from where it enters the liver. Flap E can now be laid back and pinned. Do not cut off this flap--it contains important organs that we will examine later!!
For identification of muscles use a finger to smooth out the tissue and find the direction of the muscle striations. Where these change direction is a good indication of new musculature or a different section of the same musculature. Once the borders of the muscle are found, use a blunt probe and scissors to separate it from surrounding tissue.
Think about it
1) Where were many of the thinner muscles located? The larger muscles? Why?
Head ad Neck Muscles
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Neck and Shoulder Muscles
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Lateral view of Shoulder and Leg
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Lateral Side Dissection of Leg
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Medial Dissection of Leg
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Key to the Number Labels
Neck and Shoulder
1. Masseter
2. Submaxillary gland (Mandibular gland)
3. Parotid gland
4. Sublingual gland
5. Cleidomastoid muscle
6. Brachiocephalous muscle
7. Sternomastoid muscle
8. Sternohyoid muscle
9. Omohyoid muscle
10. Sternothyroid muscle
11. Trapezius muscle
12. Triceps brachii muscle
13. Deltoid
14. Lattissmus dorsi muscle
15. Digastric muscle
16. Mylohoid
17. Pectoralis
Lower limb
18. Biceps femoris muscle
19. Tensor fasciae latae
20. Gluteus medius muscle
21. Vastius lateralis muscle
22. Gastronemus muscle
23. Semitendinosus muscle
24. Semimembranosis muscle
25. Adductor muscle
26. Rectus femoris muscle
27. Vastius medialis muscle
28. Sartorius muscle
29. Pectineus muscle
30. Gracilis muscle
DIGESTIVE SYSTEM
Objectives:
1. Identify and describe the functions of the main organs of the digestive system.
2. Gain an appreciation of the spatial relationships of the many organs and structures that contribute to the digestion of food and the nourishment of the body's cells.
The digestive system of mammals consists of the alimentary canal (mouth, oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus) and other associated structures/organs/glands (salivary glands, gall bladder, liver, pancreas).
The cavity behind the teeth and gums is the oral cavity. Note the papillae on the tongue. These provide friction for food handling and contain taste buds. Like all young mammals, fetal pigs have milk teeth (baby teeth) that are later replaced by permanent teeth.
There are 3 pairs of salivary glands (Fig. 2). Of these, we will view only the mandibular (the parotid is rather diffuse and the sublingual is too difficult to get to). To view the mandibular gland you must remove the skin and muscle tissue from one side of the face (cheek) and neck of your pig. You’ll need to dig through subcutaneous fat, connective tissue, and the parotid salivary gland in order to see it. The mandibular gland is a large, well-defined circular salivary gland just posterior to the masseter. Don’t confuse it with the small oval lymph nodes in the region. Keep an eye out for the facial nerve that runs posteriorly across the masseter. Try to find the parotid duct that carries saliva to the corner of the mouth. This duct can be moved surgically to empty out at the eyes. Until relatively recently, the standard "cure" for dogs whose lacrimal (tear) glands failed to produce the watery component of tears was to move the parotid duct up! The saliva producing glands are:
• Parotid gland: a large dark triangular gland overlying part of the masseter muscle (also note the facial nerve that runs across the dorsal part of the masseter).
• Mandibular gland: under the parotid gland. Not to be confused with the small oval lymph nodes in the region.
• Sublingual gland: long, slender, and difficult to locate (so don't bother).
• Salivary glands produce prodigious amounts of saliva (>1 l/day in humans). Saliva contains:
o water for moistening food
o mucus (mucin) for lubricating food and binding it into a bolus
o salivary amylase to start the breakdown of starch
o bicarbonate to buffer acidic food in the mouth
o antibacterial agents to kill bacteria in the mouth
With scissors, carefully cut through the tissue and bone starting at the corners of the mouth and back toward the ears (keeping the roof of the mouth intact) until the lower jaw can be dropped and the oral (buccal) cavity exposed (Fig. 3).
Find the following structures:
• hard palate: has ridges; separates the oral cavity from the nasal cavities
• soft palate: soft because there is no bone underneath (nasopharynx lies above it)
• buccal cavity: from opening of mouth to the base of the tongue
• pharynx: (throat) common passageway for digestive and respiratory system
• esophagus: tube connecting oral cavity to stomach. Swallowing can be initiated voluntarily, but thereafter it is a reflex controlled by a brain region.
• glottis: the opening to the larynx
• epiglottis: the flap that covers the glottis during swallowing
• Eustachian tubes: may be visible on each side of the pharynx.
Internal Anatomy of Digestive System
As you prepare to open up your pig, remember that most internal organs, including the digestive system, are located in the body cavity, or coelom. A large muscular structure, the diaphragm, divides the mammalian body cavity into the thoracic cavity and the abdominal (peritoneal) cavity. The thoracic cavity is further divided into a pericardial cavity (heart) and two pleural cavities (lungs). Epithelial membranes line these cavities and cover the surface of all organs. Names of the epithelial linings are determined by their location. The word "parietal" refers to the wall of the body, and the word "visceral" in this case refers to organs within those cavities.
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Figures 2, 3, and 4. Salivary glands/neck region (Fig 2), oral cavity (Fig 3), and incision guide (Fig 4).
For example:
• visceral peritoneum: covers organs of the peritoneal cavity
• parietal peritoneum: lines peritoneal cavity
• visceral pericardium: covers surface of heart
• parietal pericardium: lines pericardial cavity
• visceral pleura: covers surface of lungs
• parietal pleura: lines pleural cavity
Coelomic fluid fills the space between membrane layers. This moisture acts as a lubricant, allowing organs some degree of easy movement. The organs are connected to each other and to the inner body wall by thin sheets of connective tissue called mesenteries, which suspend the organs and provide bridges for blood vessels, nerves, and ducts.
Examine the neck, thoracic, and abdominal regions of your pig (Fig. 5). First find the thymus gland, which partially covers the anterior portion of the heart and extends along the trachea to the larynx. The thymus plays an important role in the development and maintenance of the immune system – this is where white blood cells mature into antibody-producing T-lymphocytes.
Immediately beneath the thymus in the neck is the thyroid gland, a small,solid, reddish, oval mass. The thyroid secretes thyroxine, which in mammals influences the metabolic rate of cells, which in turn influences growth and development. Because iodine is necessary for the production of thyroxine, our salt is often iodized. If synthesis of thyroxine declines (e.g. due to a lack of iodine), the anterior pituitary increases the release of thyroid stimulating hormone (TSH). This may stimulate the proliferation of thyroid cells, but if there is no iodine, thyroxine production will not increase, which causes additional TSH release. The thyroid also produces calcitonin, a hormone that stimulates osteoblasts to lay down bone. The consequence of this activity is a surprisingly rapid decline in blood calcium levels. If blood Ca levels drop too low, or if extra Ca is needed, the parathyroid glands release parathormone. The parathyroid is not a discrete organ in mammals – parathyroid tissue is embedded in the thyroid. Parathormone raises blood Ca levels by activating osteoclasts, by stimulating Ca resorption in the kidney, and by activating vitamin D to enhance absorption of Ca from food.
In the neck find the trachea and use it as a landmark to locate the esophagus. Make a small incision in the esophagus in the throat and insert a blunt probe anteriorly; note where it emerges in the oral cavity.
Insert the blunt probe through this incision posteriorly toward the stomach (you will need to move the liver to one side to fully expose the stomach). Note that the esophagus penetrates the diaphragm before entering the stomach. Cut open the stomach lengthwise with your scissors. The contents of a fetus's digestive tract is called meconium, composed of a variety of substances including bile stained mucus, amniotic fluid, sloughed epithelial cells, and hair. Clean out the stomach and note the folds (rugae). What role might the rugae play? Many glands that secrete pepsinogen and hydrochloric acid are embedded in the wall of the stomach.
Two muscular rings (smooth muscle), the cardiac (closer to the heart) and the pyloric sphincter (adjoining the small intestine), control the movement of food through the stomach.
The majority of digestion and absorption takes place in the small intestine. It is composed of the duodenum, the jejunum, and the ileum, the latter two being difficult to distinguish. The duodenum, into which bile and enzymes from the gall bladder and pancreas enter, passes posteriorly and then curves to the left. The coils of the small intestine are held together by mesenteries. A rule of thumb is that the small intestine in both pigs and humans (omnivores) is about five times the length of the body. Note the lymph nodes embedded in the mesenteries. These nodes filter pathogens from the lymph.
Cut a 0.5 cm section of the small intestine, slit it lengthwise, and place it in a clear shallow dish filled with water. Examine it with a dissecting microscope. How does the inner surface appear? Locate the villi, the absorptive projections on the inner surface. A microscopic view of the villi shows microvilli, which further enhance their absorptive capacity (surface area). Villi contain both capillaries and lacteals. What are lacteals and into what system do they empty?
Locate the caecum, a small blind-ended sac found at the juncture of the ilium and the colon (large intestine). This juncture is also the site of the ileocecal valve. Feel for it by rolling the junction between your index finger and thumb. In the pig, the caecum houses bacterial symbionts that help break down cellulose (a major component of plants) – much in the same way that gut protozoans in termites allow the termites to eat wood. Many herbivorous mammals (pigs, horses, rodents, rabbits) use "hindgut fermentation" in the caecum to digest cellulose. One clade of ungulates, the "ruminants" (camels, giraffes, deer, sheep, cattle) use "foregut fermentation". Ruminants have a multi-chambered stomach in which cellulose breakdown takes place. This breakdown is aided by their ability to regurgitate the contents of their fermentation chamber back into their mouth for further mechanical breakdown (i.e., chewing cud). In humans the caecum is known as the appendix and is not used in digestion. Although the human appendix contains some lymphatic tissue, its function is poorly understood and it can be removed without any harmful effects. So why haven't we lost our appendix completely? Recent evidence suggests that the smaller it gets, the more likely it is to get obstructed, inflamed, and infected (appendicitis). Too large an appendix is wasteful, too small is dangerous. Barring a mutation that eliminates it completely, we are stuck with a slightly wasteful, occasionally dangerous tradeoff. Evolution is not about perfection.
Figure 5 (next page). Views of the internal organs of the fetal pig. A. Neck, thorax, and abdomen, as they appear after just opening, with no disturbance. B. Close-up of neck region. C. Close-up view of thorax. D. Abdomen, view of intestines.
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The colon (large intestine) can be divided into three major regions: ascending, coiled, and descending. The colon runs from the caecum to the rectum. As with the small intestine, examine a small piece of colon under a dissecting scope. How does the internal surface compare with that of the small intestine? The colon functions to absorb water for compaction of the feces. Just past the rectum is the anus, the site of the final muscles of the alimentary canal, the anal sphincter.
Other Associated Organs
The liver, the largest organ in the abdominal cavity, has a multitude of functions, most of which are underappreciated. For example, in the fetus, blood cell production takes place in the liver as well as the bone marrow. In the adult, the liver:
• Synthesizes bile, plasma proteins (prothrombin, fibrinogen, albumin), lipids, and cholesterol.
• Stores vitamins, iron, and glycogen.
• Converts glucose to glycogen, glucose to fat, glycogen to glucose, lactic acid to glycogen, excess amino acids into carbohydrates and fats (producing ammonia in the process), and ammonia (a toxic nitrogenous waste) to urea (a less toxic form).
• Recycles hemoglobin components (and excretes bile pigments).
• Detoxifies chemicals, pollutants, and poisons.
The gall bladder, a small, usually greenish sac which lies on the underside of the right central lobe of the liver, stores bile secreted by the liver. Bile from the liver enters the common bile duct via the hepatic duct; bile from the gall bladder enters via the cystic duct. Pick away at the surrounding tissue to find these structures. Bile is composed of bile salts (which emulsify fats (breaks them into small droplets) in the duodenum) and bilirubin, which is a bile pigment. Bilirubin is a byproduct of the breakdown of hemoglobin from old red blood cells, which takes place in the liver and spleen.
Locate the pancreas, an elongate granular mass between the stomach and the small intestine. Actually the pancreas consists of two lobes: one that runs transversely and another than runs longitudinally along the duodenum. The pancreas secretes digestive enzymes and other substances into the small intestine via the pancreatic duct (which you will not be able to see). Remember, the pancreas is an endocrine as well as an exocrine organ. Endocrine glands have no ducts; they secrete their products (hormones) directly into capillaries. Hormones act as chemical signals to mediate other physiological processes. Scattered throughout the exocrine tissue of the pancreas are small islands of endocrine tissue (Islets of Langerhans). Although these islets are too small to see with the naked eye, they are extremely important. They secrete insulin, glucagon, and somatostatin directly into the tiny blood vessels that run through the pancreas. Insulin and glucagon lower and raise blood glucose levels, respectively, and somatostatin regulates levels of both insulin and glucagon.
The spleen is a long, flat, red-brown organ which lies across the stomach. It is not part of the digestive system and is actually the largest organ of the lymphatic system. It stores and releases red bloods cells into the bloodstream, recycles old red blood cells from circulation, and aids in the development of white blood cells. Despite all of these important functions, your spleen can be removed with few ill effects. What organs pick up the slack?
Think about it
1. Saliva contains water (to moisten food), mucus (to lubricate food), salivary amylase (to break down starch), bicarbonate (to buffer acids in food), and antibacterial agents. Why might these last three components be necessary when the stomach is the next destination anyway?
2. Everyone knows different parts of the tongue are especially sensitive to different tastes. But why should we devote tongue space to bitterness?
3. In humans, the uvula hangs as a pendant from the posterior end of the soft palate. During swallowing, it lifts upward and closes off the nasopharynx. Why is this important?
4. Is diarrhea a defense strategy to rid your body of pathogens or a way for intestinal pathogens to spread to others (still occurs in less developed countries with no sewage treatment)?
5. In olden days, coal miners often suffered from rickets, a disease characterized by brittle bones. Miners rarely see the sun, a "source" of vitamin D. What's going on here?
6. Would you expect carnivores to have longer or shorter intestines than herbivores?
7. What happens if the contents of the colon pass too rapidly through the colon? too slowly?
8. What are some possible advantages and disadvantages of foregut and hindgut fermentation?
RESPIRATORY SYSTEM
Objectives
1. Identify and describe the function of the main organs and structures in the respiratory system.
2. Describe the movement of air into and out of the lungs.
3. Apply this knowledge to organismal adaptive strategies and problems in human physiology.
The respiratory system is responsible for bringing a fresh supply of oxygen to the blood stream and carrying off excess carbon dioxide. In mammals, air enters the body through the external nares and enters the nasal cavities dorsal to the hard palate. As air passes through these convoluted cavities, it is humidified and warmed to body temperature and dust is caught in the mucus of the membranes that line the cavities. Air moves from here into the nasopharynx, where it passes through the glottis into the larynx. Carefully cut the soft palate longitudinally to examine the nasopharynx of your specimen.
The larynx is a hard-walled chamber composed of cartilaginous tissue. In the course of hominid evolution, the larynx has moved downward (caudally). As a result, human vocalizations tend to come out of the mouth, where the tongue can manipulate them. In chimps, the larynx is higher in the throat, with the result that vocalizations are very nasal (and thus less controllable and understandable). Our descended larynx comes with a price – it makes choking on food far more likely. Interestingly, human babies retain an elevated larynx. It makes baby talk difficult, but it also allows babies to nurse and breathe at the same time.
Slit the larynx longitudinally to expose the vocal cords. The vocal cords are elastic ridges that stretch across the space within the larynx. When air passes over the vocal cords during exhalation, the cords vibrate and produce sound. In adult humans, laryngitis results from viral infection of the vocal cords. They swell and regular speech is difficult to impossible.
Read the following information about the respiratory system. However, do not attempt to identify structures other than the trachea until you have exposed the heart and its major vessels (see Circulatory System further below).
The trachea, distinguished by its cartilaginous rings (incomplete on the dorsal side), divides into the two bronchi (singular bronchus), which enter the lungs and divide into bronchioles (don’t try to find the bronchi until you’ve finished examining the heart and its major vessels). Bronchioles terminate in alveoli, where gas exchange takes place.
The right lung typically consists of four lobes and the left of two or three. How many does your pig have? The lungs in your fetal pig are small and fairly solid because they have never been inflated. Inflation causes lungs to have a spongy appearance. Note the position of the diaphragm in relation to the lungs. Contraction of the diaphragm enlarges the thoracic cavity and pulls air into the lungs. Remember that only mammals have a true muscular diaphragm; other terrestrial vertebrates use a variety of methods to inflate their lungs.
Examine the lungs and note the pleural membranes (one lining the inner surface of the pleural cavity and the other covering the outer surface of the lung). As mentioned earlier, the intrapleural space is filled with fluid. This fluid allows the membranes to slide freely across each other, much like two wet panes of glass (easy to slide, hard to separate), and allows them to maintain contact. This ensures that the lungs will inflate when the thoracic cavity expands as a result of diaphragmatic contraction or expansion of the rib cage.
When neonatal mammals inhale for the first time, their lungs inflate. When they then exhale, the lungs don’t deflate all the way. That’s because pulmonary surfactants reduce the surface tension of water (just like soap does – you can float a bottlecap on water until you add a surfactant like soap). In this case the water is in the form of a film that coats each and every alveolus. If it weren’t for these surfactants, the surface tension of this layer would collapse the delicate alveoli – causing the lungs to “collapse” after each breath. This surfactant is produced by the lungs during the last part of pregnancy.
Think about it
1. Why does the trachea have cartilaginous rings?
2. Why is it important for air to be moist when it enters the lungs? Many desert mammals have extremely convoluted nasal cavities. How might these large and complex nasal cavities conserve water during exhalation?
3. When you catch a cold, you get a runny nose. Is snot your body’s way of combating a viral invader, or is the virus simply using you to reproduce and spread itself? The common cold generally doesn't land you in bed: is this evidence of you're own abilities to "fight" the virus, or is the virus manipulating you to maximize its exposure to uninfected individuals?
4. What is the function of the eustachian tubes?
CIRCULATORY SYSTEM
Objectives
1. Identify and describe the function of the main organs and structures in the circulatory system.
2. Trace the flow of blood through the pulmonary and systemic circuits.
3. Describe how the circulatory and respiratory systems work together to bring about the integrated functioning of the body.
4. Understand portal circulation.
5. Understand mammalian fetal circulation from a mechanical, physiological, and evolutionary perspective.
The circulatory (or cardiovascular) system is responsible for transporting nutrients, gases, hormones, and metabolic wastes to and from individual cells. Actually, the loading and unloading take place in capillaries. Oxygen is added to the blood (and carbon dioxide removed) in the capillaries of the lungs. In the capillaries of the small intestine, nutrients are added to the blood, while in the capillaries of the kidneys the blood is cleansed of various metabolic wastes and excess ions.
In mammals, the circulatory system is divided into a pulmonary circuit, which involves blood flow to and from the lungs, and the systemic circuit, which involves blood flow to and from the rest of the body. Your pig has been doubly injected (red for arteries, blue for veins). However, note that in reality, arteries and veins are defined by the direction of blood flow, not by the oxygen content of the blood contained therein.
1. The Heart (Fig. 6)
You may remove as much thymus as you need to in order to view the heart. Carefully remove the pericardial sac from the heart. In living animals, the pericardial cavity is filled with fluid that acts as a shock absorber to protect the heart from injury. Identify the coronary artery and coronary vein lying in the diagonal groove between the 2 ventricles. These vessels supply and drain the heart (the heart is a muscle and as such has the same requirements of any other organ). When the coronary artery becomes obstructed, a heart attack may occur. It is the coronary arteries that are "bypassed" in coronary bypass surgery. Note that the atria have external flaps, known as auricles.
In an adult mammal (fetal circulation will be discussed below), deoxygenated blood flows into the right atrium from the anterior and posterior vena cavae. It then makes the following circuit: right ventricle, pulmonary trunk, pulmonary artery, lungs, pulmonary vein, left atrium, left ventricle, aortic arch, aorta, and on into the systemic circulation. On the heart model, trace this path and find the above as well as the following structures:
• right atrioventricular valve
• atrioventricular valve
• right semilunar valve (between right ventricle and pulmonary trunk)
• left semilunar valve (between left ventricle and aorta)
• papillary muscles: support chordae tendinae
• chordae tendinae: support AV valves, preventing eversion
2. Major veins of the systemic circulation, anterior to the heart (Fig. 7a)
Following the path of deoxygenated blood, find the external jugular vein, which drains the head and neck, and the internal jugular vein, which drains the brain. Note the vagus nerve running between the right common carotid artery and the internal jugular vein (the vagus nerve is responsible for slowing the heart, constricting bronchi, and stimulating the stomach and gallbladder). The jugular veins meet with the subclavian vein to form the brachiocephalic vein. The right and left brachiocephalic veins join to form the anterior (cranial) vena cava. Note, however, that the mass of veins (and arteries) anterior to the heart may not look exactly like what you see in the figure. For example, do the external and internal jugulars join before reaching the brachiocephalic? Does your pig even have a subclavian vein? or do the subscapular (from the shoulder) and axillary (from the arm) veins empty straight into the brachiocephalic vein? How substantial is the brachiocephalic vein? or do the subclavian and jugulars empty straight into the vena cava? Make sure you examine other pigs to appreciate the variability of these vessels.
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Fig. 6. The heart and major arteries and veins.
3. Major arteries of the systemic circulation, anterior to the heart (Fig. 7b)
Viewing the major thoracic arteries may require moving (but not removing) some of the thoracic veins (attempt the former before resorting to the latter since you will see them on the lab practical). Like the veins, however, there is a great deal of variation in the branching patterns of the brachiocephalic trunk and the left subclavian artery. The first large vessel that branches from the aortic arch is the brachiocephalic trunk. This artery soon branches into the right subclavian and the common carotid arteries (as well as sending vessels along the inner and outer walls of the rib cage). The subclavian arteries carry blood to the forelimbs, the carotid arteries carry blood to the head. The carotid branches into an internal carotid, which goes to the brain, and the external carotid, which goes to the face. In desert-dwelling ungulates, the internal carotid forms an arterial "capillary" bed (rete) over the nasal passages and then reforms the carotid artery and delivers blood to the brain. Because the nasal passages represent the intersection of hot dry outside air and moist internal body surfaces, a great deal of evaporative cooling takes place there. Instead of expending energy (and water) to cool their entire bodies, these mammals can allow their bodies to heat up to brain-damaging temperatures while their brain's blood stays cool.
The second large vessel that branches from the aortic arch is the left subclavian artery. Note how the branching of the arteries is less symmetrical than that of the veins.
4. Major arteries of the systemic circulation, posterior to the heart (Figs. 8, 9)
Move the internal organs to view the pig’s left kidney area. Pick away the connective tissue to expose the aorta just below the diaphragm and find the coeliac artery. It branches off the aorta to supply the stomach, spleen, and liver. Huh? but the coeliac is so tiny! So the liver, the largest
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Figure 7. A. Major veins anterior to the heart. B. Major arteries of systemic circulation anterior to the heart.
organ in the body, is supplied by a mere branch of a rather small artery! Well, it's more complicated than that. First, the liver also gets blood from the hepatic portal vein (see below). “But that's a vein" you say. And you are correct. But so much blood flows through it ... and the intestines aren't always a super metabolically active organ, so the liver can benefit from it (and it certainly benefits nutrient-wise). The other trick is that despite its size, the liver is not particularly metabolically active. At any given time, only a small proportion of its cells are doing anything. So that, plus the fact that the liver is really weird in having sinusoids rather than proper capillaries, allows it to work as it does.
Just posterior to the coeliac artery, you will find the cranial (superior) mesenteric artery, which supplies the pancreas and small intestine. Watch out! Make sure you find the crescent-shaped adrenal gland before you go digging for the cranial mesenteric artery. Don’t worry. Your pig has two, so you can look at the adrenal on the right later. Also note the lobe of the pancreas situated just ventral to the cranial mesenteric.
At the kidneys, short renal arteries supply blood to the kidneys. At the caudal end of the abdominal cavity, you can see several branches of the aorta. The external iliac arteries are the main arteries of the hindlimbs. The tiny internal iliac arteries, which supply the rectum and hip, can be found where the aorta branches to form the two umbilical arteries.
5. Major veins of the systemic circulation, posterior to the heart (Figs. 8, 9, 10)
In the lower abdominal cavity, find where the external iliac vein and internal iliac vein join to form the common iliac vein. The right and left common iliac veins then join to form the posterior vena cava. Find the renal veins.
6. The hepatic portal system (Figs. 8, 9)
In a normal circulatory pathway, blood takes the following path: artery – capillary bed – vein. In a portal system, the blood travels in the following manner: artery – capillary bed – portal vein – capillary bed – vein. Portal systems are found in many different parts of the body and carry blood from the capillaries of one organ to the capillaries of another organ. In the case of the hepatic portal system, nutrient-rich blood from the mesenteric veins flow into a single mesenteric vein, which joins with the lienogastric (gastrosplenic) vein from the spleen and stomach and becomes the hepatic portal vein. This vein now carries blood to the liver, where it breaks into a second capillary bed. Here the products of digestion pass into liver cells. This ensures that the liver has "first shot" at toxins from the diet as well as glucose, amino acids, and lipids. Capillaries in the liver then converge into the hepatic veins, which empty into the caudal vena cava for transport back to the heart. If the intake of toxins (such as alcohol) exceeds the liver's ability to filter them from the blood, the excess enters the general circulation and on to other organs (like the brain).
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Figure 8. Illustration of hepatic portal system depicting associated veins and organs.
7. Fetal Circulation
The amniotic egg is characterized by a yolk sac, a chorion, an allantois, and an amnion, and was one of the keys to the success of the early reptiles (and their descendents – the modern reptiles, birds, and mammals). In a standard amniotic egg (think chicken egg), the yolk sac provides the nutrition for growth, the amnion provides a watery medium to float in, the allantois provides a sac to contain and isolate nitrogenous wastes, and the chorion surrounds all and is the means by which oxygen diffuses in to the embryo. Live birth and placentas have evolved multiple times within the Amniota. This is just a fusion of the chorion and amnion which lies against a highly vascularized uterine wall. Nutrients and oxygen diffuse from maternal capillaries, across these two thin membranes, and into fetal capillaries on the fetal side of the chorion-amnion barrier. Carbon dioxide and nitrogenous wastes diffuse out of fetal capillaries and across into maternal blood. Fetal and maternal blood never mix (this is why mothers and their children can have different blood types). In pigs, nutrients and gases must diffuse across maternal capillary walls, the uterine tissue, the chorion, and finally the fetal capillary walls. In humans (and other primates), the uterine wall and maternal capillaries break down, forming open blood sinuses. Thus in humans, fetal capillaries are separated from sloshing maternal blood by only a thin chorionic layer. Keep in mind that fetal tissues are not as well oxygenated as maternal tissues.
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Figures 9 and 10. 9. Hepatic portal system. 10A. Fetal and renal circulation. 10B. Posterior circulation.
The umbilical vein carries oxygen- and nutrient-rich blood from the fetal side of the placenta to the fetus. However, this relatively well-oxygenated blood mixes with deoxygenated fetal blood before it enters fetal arterial circulation. The first site of mixing is within the liver. The umbilical vein enters the liver and its sinusoids, but as a result of the increasing blood volume as the fetus develops, essentially clears a path through the liver tissue. The resulting channel is known as the ductus venosus. Oxygenated venous blood from the liver (from the hepatic portal system as well as hepatic veins) is mixed in the ductus venosus and continues on to the caudal (posterior) vena cava. Here it mixes with deoxygenated blood from the rest of the body on its way to the heart. Finally, within the right atrium, this blood is once more mixed with deoxygenated blood, this time from the cranial (anterior) vena cava.
How do mammalian fetal tissues survive in such a low oxygen environment? The answer lies in their red blood cells. Fetuses have a different kind of hemoglobin than do adult mammals. Fetal hemoglobin has a higher affinity for oxygen than does adult hemoglobin; it is still able to pick up oxygen molecules when environmental oxygen levels are very low (levels at which maternal hemoglobin is shedding oxygen). On the flip side, fetal hemoglobin holds oxygen until tissues are on the verge of oxygen deprivation.
In the fetus, the pulmonary circuit is not functional and is therefore bypassed. About half of the blood entering the right atrium flows directly into the left atrium via the foramen ovale (it is not necessary for you to find this small opening, but you can try). From here it moves into the left ventricle, then the aorta, and out into the systemic circulation. The remainder of the blood entering the right atrium flows into the right ventricle and out to the pulmonary trunk. However, instead of flowing on to the pulmonary artery and the lungs, this blood bypasses the pulmonary artery and goes through the ductus arteriosus into the aorta, where it enters systemic circulation.
So fetal tissues never have the benefit of contact with highly oxygenated blood. And actually, it’s worse than that. Because maternal blood doesn’t give up all that much oxygen to the placenta. So how do mammalian fetal tissues survive in such a low oxygen environment? The answer lies in their red blood cells. Fetuses have a different kind of hemoglobin than do adult mammals. Fetal hemoglobin has a higher affinity for oxygen than does adult hemoglobin; it is still able to pick up oxygen molecules when environmental oxygen levels are very low (levels at which maternal hemoglobin is shedding oxygen). On the flip side, fetal hemoglobin holds oxygen until tissues are on the verge of oxygen deprivation.
In late fetal life, the foramen ovale becomes smaller relative to the rest of the heart and the lumen of the ductus arteriosus narrows, forcing more blood through the pulmonary circuit. At birth, decrease of blood flow through the right atrium and the decreased resistance within the pulmonary circuit has several effects. First, pressure between the two atria equalizes, closing the flaps of the foramen ovale and allowing it to seal itself. Second, the decreased resistance of the pulmonary circuit directs blood in the pulmonary trunk toward the lungs. This causes some of the blood leaving the heart through the aorta (blood that has already been partly aerated by having passed through the lungs) to re-enter the pulmonary artery by the ductus arteriosus and pass through the lungs a second time. Double circulation lasts only a day or so, after which the ductus arteriosus contracts and fills with connective tissue.
So…to review… the most oxygenated blood is in the umbilical vein. It mixes (rather counterintuitively) with the very deoxygenated blood of the posterior vena cava. Thus the posterior vena cava just posterior to the heart is more oxygenated than the anterior vena cava just anterior to the heart. The aorta is thus less oxygenated than the anterior portion of the posterior vena cava! Even more strange, the aorta sends a great deal of rather oxygenated blood straight back to the placenta (via the umbilical arteries). You’d think the fetus would send deoxygenated vena cava blood to the placenta to get oxygen, but nope! It sends oxygenated aorta blood! Remember though, the blood pressure in the vena cava is essentially zero … and that capillary beds work better when there is a blood pressure differential between the arterial and venous sides. The aorta provides that blood pressure.
Think about it
1. Why are the larger arteries white rather than red? Why are the large veins nevertheless blue?
2. The dual circulation of mammals is reflected not only in separate pulmonary and systemic circulatory systems, but also in the four-chambered heart. Fish blood leaves the heart, goes to the gills for oxygenation, and then continues out to the body. How many atria do fish have? How many ventricles?
3. Some salamanders that "breathe" (exchange gases) through their damp skins have lost their lungs. Do you think their hearts have one or two atria?
4. What might happen if the ductus arteriosus fails to close completely in the days after birth? The foramen ovale?
5. The condition known as jaundice (yellow skin and eyes) is a result of a build-up of bilirubin and is usually a sign of liver malfunction. Newborn human infants often go through a period of fetal jaundice in which they turn yellow. This usually reflects not a liver malfunction, but rather the destruction of huge numbers of red blood cells. Why would newborns be cashing in so many red blood cells? In serious cases, neonates are often put under special lights that promote the breakdown of bilirubin. However, recent evidence demonstrates that bilirubin is a potent anti-oxidant. Why would a neonate need so many anti-oxidants?
6. Is the blood within a fetus’s hepatic portal vein nutrient rich?
7. Why are so many alcoholics signed up for liver transplants?
UROGENITAL SYSTEM
Objectives
1. Identify and describe the function of the excretory system of the fetal pig, noting differences between the sexes and noting structures shared with the reproductive system.
2. Identify and describe the function of the reproductive systems of male and female fetal pigs and trace the pathway of sperm and egg from their origin out of the body.
Excretory System
The bean-shaped kidneys (Fig. 11) perform two functions. First, they continuously remove metabolic wastes from the blood (primarily urea resulting from the metabolism of amino acids in the liver). Second, they monitor and adjust the composition of the blood (particularly water and salts) so that the cells of the body are bathed in a fluid of constant composition. Although the kidneys are situated below the diaphragm, they are actually located outside the peritoneal cavity (dorsal to the parietal peritoneum, the membrane that lines the abdominal cavity). Carefully cut one of the kidneys in half longitudinally (slice it as though you were separating the two halves of a lima bean). Within the kidney, the ureter expands to form a funnel-shaped chamber called the renal pelvis. The dark kidney tissue that you see extending into the renal pelvis is known as medullary tissue (medulla). The outermost portion of the kidney is called the cortex. The cortex contains glomeruli, Bowman’s capsules, proximal convoluted tubules, and distal convoluted tubules. The medulla contains the loops of Henle and the collecting ducts. The medulla is characterized by high solute concentration, so when "pre-urine" flows down the loops of Henle, water flows out of the loops and into the medullary tissue. The net result of this (and a few other processes of the medulla) is that the "urine" becomes increasingly concentrated. In humans, the kidneys filter 1500 liters of blood a day, producing only about 1.5 liters of urine in that time.
Near (but usually not actually on) the anterior/medial edge of each kidney lies a narrow band of light colored tissue, the adrenal gland (adrenal means "near or adjacent to the renal [kidney])". The adrenals may be difficult to see, especially in smaller pigs. Despite it's subtle appearance, the adrenal gland is one of the most bizarre and important glands in the body. The cortex is epithelial in origin; the medulla is neural! In fact, the cortex and medulla are not even united in some vertebrates. The outer layer of the adrenal cortex secretes aldosterone, an important hormone for water balance. The middle cortex produces glucocorticoids like cortisol. These “stress hormones” have a variety of effects ranging from carbohydrate balance to immunosuppression. The inner cortex produces androgens, even in females. These androgens are involved in the growth spurt, development of pubic hair during puberty in girls. The adrenal medulla, being of neural origin, produces norepinephrine and epinephrine (aka adrenaline), which are neurotransmitters. When released, these adrenal hormones produce the fight or flight response, mobilize glucose, and increase heart rate.
The renal pelvis of each kidney drains into a coiled tube called the ureter. The ureters lead from the kidney to the urinary bladder, where urine is temporarily stored. Note the unusual shape (elongated) and location (between the umbilical arteries) of the urinary bladder in your fetal pig. In fact it extends into the umbilical cord! Urine produced by the fetus actually bypasses the urethra (the tube that transports urine from the bladder to the outside of the body). If a fetus urinated in an adult manner, the amnionic sac would soon be fouled with toxic nitrogenous wastes (urea is toxic). Instead, urine produced by the fetus proceeds from the bladder through the allantoic duct and to the allantois (a special sac for nitrogenous wastes). But remember that most nitrogenous wastes are transported to the placenta via the umbilical arteries. However, even in reptiles and birds, the allantois takes on a dual function. In addition to storing nitrogenous wastes, it fuses with the chorion to create a vascularized membrane that mediates gas exchange. In mammals, this latter diffusion function takes place in conjunction with the placenta, as does nutritional exhange and waste removal. In both pigs and humans, the allantoic duct collapses at birth and urine flows from the bladder into the urethra.
To follow the urethra to the urogenital opening, you will have to also examine the reproductive system, as they are linked together. Examine the urogenital system in your pig. Then examine a pig of the opposite sex. You are responsible for both male and female anatomy.
To examine the urethra and the reproductive structures fully, you will need to carefully cut through the pelvis (pubic bone or pubis) of your pig. Don’t make this cut without consulting me. Make sure you keep your cut slightly to the left or right of the midline to avoid cutting important structures.
Female Reproductive System (Fig. 11)
In the female, the opening of the urogenital sinus / vaginal vestibule lies directly ventral to the anus. It is bounded laterally by low folds, the labia, which come together ventrally to form a protruding genital papilla. The clitoris, a small body of erectile tissue on the ventral portion of the urogenital sinus, may be visible. The clitoris is homologous (similar in structure and developmental origin) to the male penis. In the male, the tissues of the penis develop around and enclose the urethra, while in the female the urethra opens posteriorly to the clitoris.
[pic]
Figure 11. Kidneys, excretory system, and female reproductive system.
Within the body of the female, the urethra is bound by connective tissue to the vagina. Gently separate this tissue. The vagina and the urethra join together about 1 cm from the exterior body opening to form the urogenital sinus / vaginal vestibule. This structure is not present in adult females--separate external vaginal and urinary openings begin to develop after birth as the urogenital sinus shrinks. How might this occur?
Trace the vagina anteriorly to the cervix, a slightly constricted region of tissue which leads to the uterus (did you know that 99% of cervical cancers in humans are due to viral infection?). The cervix acts as a sphincter to separate the vagina from the uterus. It's usually closed. In fact, the female mammalian reproductive system has many safeguards against sexually transmitted disease: an acidic vagina, antibacterial mucus, and lots of white blood cell activity. Why are such safeguards especially important in humans? The uterine body branches anteriorly into two uterine horns (pigs and many other mammals have a bicornate uterus; humans have a simplex uterus). Another feature of uterine horns is the production of litters (incidentally, pigs are the only ungulates that produce litters). Trace the uterine horns to the oviducts, where fertilization normally takes place. These tubes are much smaller than the horns and lie extremely close to the ovaries. The ovaries are the sites of egg production and the source of female sex hormones, estrogen and progesterone. Every egg (actually primary oocyte) that a female pig (or human) will ever produce is already present in the ovary at the time of birth.
Male Reproductive System (Fig. 12)
Bear in mind that the testes, the site of sperm and testosterone production, are found in the scrotum in older fetuses, but may remain undescended within the body cavity in younger fetuses. The following instructions/discussion assumes descended testes.
First, make a midline incision into the scrotum. Pull out the two elongated bulbous structures covered with a transparent membrane. This membrane is actually an outpocketing of the abdominal wall. The gubernaculum is the white cord that connects the posterior end of the testes to the scrotum wall. It grows more slowly than the surrounding tissues and thus "pulls" the testes into the scrotum.
Cut through the tunica vaginalis to expose a single testis and locate the epididymis, a tightly coiled tube along one side. Sperm produced in the testis mature in the epididymis until ejaculation. Unlike females, male mammals are not born with a lifetime supply of gametes. Sperm are produced only after puberty, but then continue to be produced for the rest of the life of the male. Cells within the testis (but not those that give rise to sperm) are responsible for the production of testosterone. Evidence suggests that sperm may not be recognized as “self” by the immune system and must therefore be protected. Not only is there a blood/testis barrier (just like there is a blood/fetus barrier in females), but also the immunosuppressive characteristics of testosterone are no accident. The testosterone produced within the testes by the interstitial cells that physically surround the spermatogenic cells provide a strong defense.
The slender elongated structure that emerges from each testis is the spermatic cord. It goes through the inguinal canal (actually an opening in the abdominal wall connecting the abdominal cavity to the scrotal cavity). It is through this canal that the testes descend. The spermatic cord consists of the vas deferens (plural vasa deferentia), the spermatic nerve, and the spermatic artery and vein. The vasa deferentia are severed in a vasectomy.
Expose the full length of the penis and its juncture with the urethra. Make an incision with a scalpel through the muscles in the midventral line between the hindlegs until they lie flat. Carefully remove the muscle tissue and pubic bone on each side until the urethra is exposed. With a blunt probe, tear the connective tissue connecting the urethra to the rectum, which lies dorsal to it.
Locate the seminal vesicles on the dorsal surface of the urethra where the two vasa deferens enter. The seminal vesicles are responsible for 60% of the volume of the seminal fluid. They release fructose to provide energy for the swimming sperm and prostaglandins and clotting factors to aid in the mass movement of the ejaculate up the female reproductive tract.
Situated between the bases of the seminal vesicles is the prostate gland. This gland produces bicarbonate, an alkaline substance, to neutralize the acidic environment of the vagina. The bulbourethral (Cowper's) glands lie on either side of the juncture of the penis and urethra--their precise function is poorly understood, though they also produce an alkaline solution. The urethra joins the penis just posterior to the Cowper's glands. The retractable penis extends through the tissue of the "flap" that holds the bladder to the urogenital opening. Use your finger to feel the penis within the flap. Carefully pick away the tissue in this area to separate the penis.
Think about it
1. Would you expect to find corpa lutea on the ovaries of your pig? Why or why not?
2. Why do most male mammals have testes in an external sac (scrotum)? (Hint: there is some evidence that men that wear briefs produce fewer viable sperm than men that wear boxers). Why might some mammals pull their testes back into their body in the non-breeding season?
3. The spermatic artery and vein don't just traverse the same canal. They are so closely associated with one another that they exhibit countercurrent properties – not only do they run side by side through the inguinal canal, they also form a rete by which venous blood is warmed and arterial blood is cooled. Why is this necessary?
4. What is the difference between the ejaculate of a vasectomized man and a man who has not undergone this procedure? Do vasectomized men continue to produce testosterone, and if so, by what path does it get into the general circulation?
5. Some human males develop an inguinal hernia, a condition in which part of the intestine drops through the inguinal canal into the scrotum. Pigs and other quadrupeds do not develop inguinal hernias. Why not?
6. Although you may not be able to distinguish it, the thoracic cavity of your pig contains brown fat. This is a special type of adipose tissue that, when metabolized, produces a great deal of heat (it’s chock full of mitochondria). Birth triggers the metabolism of brown fat in mammalian neonates. Why does a newborn mammal need such a heat source?
7. The rectus abdominus muscle is the main muscle of the abdominal region. Until the early 1980's, women who gave birth via Caesarian section were opened up with a longitudinal cut down the midline of their abdomen. Since then there has been a shift for C-section incisions to cut across the bottom of the belly (i.e. right to left). Cutting a muscle perpendicular to the “grain” is much more damaging to it than a cut along the length of the muscle (“with the grain”). Why is it that obstetricians now cut in this seemingly more damaging direction?
Works Cited:
Lab 19-24:
Persons, M (2004). Invertebrate Zoology Lab Manual. Selinsgrove: Susquehanna University.
Lab 25:
Stanback, M (2005). Fetal Pig Anatomy. Retrieved May 10, 2008, from Davidson University Biology Department Web site:
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