Development



Development

Gradual modification of anatomical structures and physiological characteristics from fertilization to maturity

Differentiation

Creation of different types of cells required in development

Occurs through selective changes in genetic activity

As development proceeds, some genes are turned off, others are turned on

Fertilization

Also called conception

When development begins

Embryological Development

Occurs during first 2 months after fertilization

Study of these events is called embryology

Fetal Development

Begins at start of ninth week

Continues until birth

Prenatal Development

Embryological and fetal development stages

Postnatal Development

Commences at birth

Continues to maturity when aging process begins

Inheritance

Transfer of genetically determined characteristics from generation to generation

Genetics

Study of mechanisms responsible for inheritance

Fertilization

Fertilization

Fusion of two haploid gametes, each containing 23 chromosomes

Produces zygote containing 46 chromosomes

Spermatozoon

Delivers paternal chromosomes to fertilization site

Travels relatively large distance

Is small, efficient, and highly streamlined

Gamete

Provides

Cellular organelles

Inclusions

Nourishment

Genetic programming necessary to support development of embryo for a week

Occurs in uterine tube within a day after ovulation

Secondary oocyte travels a few centimeters

Spermatozoa must cover distance between vagina and ampulla

Capacitation

Must occur before spermatozoa can fertilize secondary oocyte

Contact with secretions of seminal glands

Exposure to conditions in female reproductive tract

Hyaluronidase

Enzyme breaks down bonds between adjacent follicle cells

Allows spermatozoon to reach oocyte

Acrosin

Is a proteolytic enzyme

Is required to reach oocyte

Acrosomal Caps

Release hyaluronidase and acrosin

Penetrate corona radiata, zona pellucida, toward oocyte surface

Oocyte Activation

Contact and fusion of cell membranes of sperm and oocyte

Follows fertilization

Oocyte completes meiosis II, becomes mature ovum

Polyspermy

Fertilization by more than one sperm

Prevented by cortical reaction

Cortical Reaction

Releases enzymes that

Inactivate sperm receptors

Harden zona pellucida

Female Pronucleus

Nuclear material remaining in ovum after oocyte activation

Male Pronucleus

Swollen nucleus of spermatozoon

Migrates to center of cell

Amphimixis

Fusion of female pronucleus and male pronucleus

Moment of conception

Cell becomes a zygote with 46 chromosomes

Fertilization is complete

Cleavage

Series of cell divisions

Produces daughter cells

Differentiation

Involves changes in genetic activity of some cells but not others

Gestation

Induction

Cells release chemical substances that affect differentiation of other embryonic cells

Can control highly complex processes

Gestation

Time spent in prenatal development

Consists of three integrated trimesters, each 3 months long

Gestation

First Trimester

Period of embryological and early fetal development

Rudiments of all major organ systems appear

Second Trimester

Development of organs and organ systems

Body shape and proportions change

By end, fetus looks distinctively human

Third Trimester

Rapid fetal growth and deposition of adipose tissue

Most major organ systems are fully functional

The First Trimester

Cleavage

Sequence of cell divisions begins immediately after fertilization

Zygote becomes a pre-embryo, which develops into multicellular blastocyst

Ends when blastocyst contacts uterine wall

Implantation

Begins with attachment of blastocyst to endometrium of uterus

Sets stage for formation of vital embryonic structures

Placentation

Occurs as blood vessels form around periphery of blastocyst and placenta develops

Placenta

Complex organ permits exchange between maternal and embryonic circulatory systems

Supports fetus in second and third trimesters

Stops functioning and is ejected from uterus after birth

Embryogenesis

Formation of viable embryo

Establishes foundations for all major organ systems

Most dangerous period in prenatal life

40% of conceptions produce embryos that survive past first trimester

Blastomeres

Identical cells produced by cleavage divisions

Morula

Stage after 3 days of cleavage

Pre-embryo is solid ball of cells resembling mulberry

Reaches uterus on day 4

Blastocyst

Formed by blastomeres

Hollow ball with an inner cavity

Known as blastocoele

Trophoblast

Outer layer of cells separate outside world from blastocoele

Cells responsible for providing nutrients to developing embryo

Inner Cell Mass

Clustered at end of blastocyst

Exposed to blastocoele

Insulated from contact with outside environment by trophoblast

Will later form embryo

Implantation

Occurs 7 days after fertilization

Blastocyst adheres to uterine lining

Trophoblast cells divide rapidly, creating several layers

Cellular Trophoblast

Cells closest to interior of blastocyst

Syncytial Trophoblast

Outer layer

Erodes path through uterine epithelium by secreting hyaluronidase

Ectopic Pregnancy

Implantation occurs outside of uterus

Does not produce viable embryo

Can be life threatening

Lacunae

Trophoblastic channels carrying maternal blood

Villi

Extend away from trophoblast into endometrium

Increase in size and complexity until day 21

Amniotic Cavity

A fluid-filled chamber

Inner cell mass is organized into an oval sheet two layers thick

Superficial layer faces amniotic cavity

Deeper layer is exposed to fluid contents of blastocoele

Gastrulation

Formation of third layer of cells

Cells in specific areas of surface move toward central line

Known as primitive streak

Primitive Streak

Migrating cells leave surface and move between two layers

Creates three distinct embryonic layers, or germ layers

Ectoderm: consists of the superficial cells that did not migrate into interior of inner cell mass

Endoderm: consists of cells that face blastocoele

Mesoderm: consists of poorly organized layer of migrating cells between ectoderm and endoderm

Embryonic Disc

Oval, three-layered sheet

Produced by gastrulation

Will form body of embryo

Rest of blastocyst will be involved in forming extraembryonic membranes

Formation of the Extraembryonic Membranes

Support embryological and fetal development

Yolk sac

Amnion

Allantois

Chorion

Yolk Sac

Begins as layer of cells spread out around outer edges of blastocoele to form complete pouch

Important site of blood cell formation

Amnion

Combination of mesoderm and ectoderm

Ectodermal layer enlarges and cells spread over inner surface of amniotic cavity

Mesodermal cells create outer layer

Continues to enlarge through development

Amniotic Fluid

Contained in amniotic cavity

Surrounds and cushions developing embryo or fetus

Allantois

Sac of endoderm and mesoderm

Base later gives rise to urinary bladder

Chorion

Combination of mesoderm and trophoblast

Blood vessels develop within mesoderm

Rapid-transit system for nutrients that links embryo with trophoblast

First step in creation of functional placenta

Chorionic Villi

In contact with maternal tissues

Create intricate network within endometrium carrying maternal blood

Body Stalk

Connection between embryo and chorion

Contains distal portions of allantois and blood vessels that carry blood to and from placenta

Yolk Stalk

Narrow connection between endoderm of embryo and yolk sac

Decidua Capsularis

Thin portion of endometrium

No longer participates in nutrient exchange and chorionic villi in region disappear

Decidua Basalis

Disc-shaped area in deepest portion of endometrium

Where placental functions are concentrated

Decidua Parietalis

Rest of the uterine endometrium

No contact with chorion

Umbilical Cord

Connects fetus and placenta

Contains allantois, placental blood vessels, and yolk stalk

Blood Flow to Placenta

Through paired umbilical arteries

Returns in single umbilical vein

The Endocrine Placenta

Synthesized by syncytial trophoblast, released into maternal bloodstream

Human chorionic gonadotropin (hCG)

Human placental lactogen (hPL)

Placental prolactin

Relaxin

Progesterone

Estrogens

Human Chorionic Gonadotropin (hCG)

Appears in maternal bloodstream soon after implantation

Provides reliable indication of pregnancy

Pregnancy ends if absent

Helps prepare mammary glands for milk production

Stimulatory effect on other tissues comparable to growth hormone (GH)

Placental Prolactin

Helps convert mammary glands to active status

Relaxin

A peptide hormone secreted by placenta and corpus luteum during pregnancy

Increases flexibility of pubic symphysis, permitting pelvis to expand during delivery

Causes dilation of cervix

Suppresses release of oxytocin by hypothalamus and delays labor contractions

Embryogenesis

Body of embryo begins to separate from embryonic disc

Body of embryo and internal organs start to form

Folding, differential growth of embryonic disc produces bulge that projects into amniotic cavity

Projections are head fold and tail fold

Organogenesis

Process of organ formation

The Second and Third Trimesters

Second Trimester

Fetus grows faster than surrounding placenta

Third Trimester

Most of the organ systems become ready

Growth rate starts to slow

Largest weight gain

Fetus and enlarged uterus displace many of mother’s abdominal organs

Pregnancy and Maternal Systems

Developing fetus is totally dependent on maternal organ systems for nourishment, respiration, and waste removal

Maternal adaptations include increases in

Respiratory rate and tidal volume

Blood volume

Nutrient and vitamin intake

Glomerular filtration rate

Uterus and mammary glands increase in size

The Second and Third Trimesters

Progesterone

Released by placenta

Has inhibitory effect on uterine smooth muscle

Prevents extensive, powerful contractions

Opposition to Progesterone

Three major factors

Rising estrogen levels

Rising oxytocin levels

Prostaglandin production

The Second and Third Trimesters

False Labor

Occasional spasms in uterine musculature

Contractions not regular or persistent

True Labor

Results from biochemical and mechanical factors

Continues due to positive feedback

Labor Contractions

Begin in myometrium

Parturition is forcible expulsion of fetus

Contractions

Begin near top of uterus, sweep in wave toward cervix

Strong, occur at regular intervals, increase in force and frequency

Change position of fetus, move it toward cervical canal

Labor

Dilation Stage

Begins with onset of true labor

Cervix dilates

Fetus begins to shift toward cervical canal

Highly variable in length, but typically lasts over 8 hours

Frequency of contractions steadily increases

Amniochorionic membrane ruptures (water breaks)

Expulsion Stage

Begins as cervix completes dilation

Contractions reach maximum intensity

Continues until fetus has emerged from vagina

Typically less than 2 hours

Delivery

Arrival of newborn infant into outside world

Placental Stage

Muscle tension builds in walls of partially empty uterus

Tears connections between endometrium and placenta

Ends within an hour of delivery with ejection of placenta, or afterbirth

Accompanied by a loss of blood

Episiotomy

Incision through perineal musculature

Needed if vaginal canal is too small to pass fetus

Repaired with sutures after delivery

Cesarean Section (C-section)

Removal of infant by incision made through abdominal wall

Opens uterus just enough to pass infant’s head

Needed if complications arise during dilation or expulsion stages

Premature Labor

Occurs when true labor begins before fetus has completed normal development

Newborn’s chances of surviving are directly related to body weight at delivery

Immature Delivery

Refers to fetuses born at 25–27 weeks of gestation

Most die despite intensive neonatal care

Survivors have high risk of developmental abnormalities

Premature Delivery

Refers to birth at 28–36 weeks

Newborns have a good chance of surviving and developing normally

Forceps Delivery

Needed when fetus faces mother’s pubis instead of sacrum

Risks to infant and mother are reduced if forceps are used

Forceps resemble large, curved salad tongs

Used to grasp head of fetus

Breech Birth

Legs or buttocks of fetus enter vaginal canal first instead of head

Umbilical cord can become constricted, cutting off placental blood flow

Cervix may not dilate enough to pass head

Prolongs delivery

Subjects fetus to severe distress and potential injury

Dizygotic Twins

Also called fraternal twins

Develop when two separate oocytes were ovulated and subsequently fertilized

Genetic makeup not identical

70% of twins

Monozygotic Twins

Identical twins

Result either from

Separation of blastomeres early in cleavage

Splitting of inner cell mass before gastrulation

Genetic makeup is identical because both formed from same pair of gametes

Conjoined Twins

Siamese twins

Genetically identical twins

Occurs when splitting of blastomeres or of embryonic disc is not completed

Rates of Multiple Births

Twins in 1 of every 89 births

Triplets in 1 of every 892 (7921) births

Quadruplets in 1 of every 893 (704,969) births

Postnatal Life

Five Life Stages

Neonatal period

Infancy

Childhood

Adolescence

Maturity

Neonatal Period: extends from birth to 1 month

Infancy: 1 month to 2 years of age

Childhood: 2 years until adolescence

Adolescence: period of sexual and physical maturation

Senescence: process of aging that begins at end of development (maturity)

Developmental Stages

Neonatal period, infancy, childhood, and adolescence

Two major events occur

Organ systems become fully operational

Individual grows rapidly and body proportions change significantly

Pediatrics

Medical specialty focusing on postnatal development from infancy to adolescence

Neonate

Newborn

Neonatal Period

Transition from fetus to neonate

Systems begin functioning independently

Respiratory

Circulatory

Digestive

Urinary

Colostrum

Secretion from mammary glands

Ingested by infant during first 2–3 days

Contains more proteins and less fat than breast milk

Many proteins are antibodies that help ward off infections until immune system is functional

Mucins present inhibit replication of rotaviruses

As production drops, mammary glands convert to milk production

Breast Milk

Consists of water, proteins, amino acids, lipids, sugars, and salts

Also contains large quantities of lysozymes—enzymes with antibiotic properties

Milk Let-Down Reflex

Mammary gland secretion triggered when infant sucks on nipple

Continues to function until weaning, typically 1–2 years

Infancy and Childhood

Growth occurs under direction of circulating hormones

Growth hormone

Suprarenal steroids

Thyroid hormones

Growth does not occur uniformly

Body proportions gradually change

Puberty is a period of sexual maturation and marks the beginning of adolescence

Generally starts at age 12 in boys, age 11 in girls

Three major hormonal events interact

Hypothalamus increases production of GnRH

Circulating levels of FSH and LH rise rapidly

Ovarian or testicular cells become more sensitive to FSH and LH

Hormonal changes produce sex-specific differences in structure and function of many systems

Adolescence

Begins at puberty

Continues until growth is completed

Maturity (Senescence )

Aging

Reduces functional capabilities of individual

Affects homeostatic mechanisms

Sex hormone levels decline at menopause or male climacteric

Geriatrics

Medical specialty dealing with problems associated with aging

Trained physicians, or geriatricians

Inheritance

Nucleated Somatic Cells

Carry copies of original 46 chromosomes present in zygote

Genotype

Chromosomes and their component genes

Contain unique instructions that determine anatomical and physiological characteristics

Derived from genotypes of parents

Phenotype

Physical expression of genotype

Anatomical and physiological characteristics

Inheritance

Homologous Chromosomes

Members of each pair of chromosomes

23 pairs carried in every somatic cell

At amphimixis, one member of each pair is contributed by spermatozoon, other by ovum

Autosomal Chromosomes

22 pairs of homologous chromosomes

Most affect somatic characteristics

Each chromosome in pair has same structure and carries genes that affect same traits

Sex Chromosomes

Last pair of chromosomes

Determine whether individual is genetically male or female

Karyotype

Entire set of chromosomes

Locus

Gene’s position on chromosome

Alleles are various forms of given gene

Alternate forms determine precise effect of gene on phenotype

Homozygous

Both homologous chromosomes carry same allele of particular gene

Simple Inheritance

Phenotype determined by interactions between single pair of alleles

Heterozygous

Homologous chromosomes carry different allele of particular gene

Resulting phenotype depends on nature of interaction between alleles

Strict Dominance

Dominant allele expressed in phenotype, regardless of conflicting instructions carried by other allele

Recessive Allele

Expressed in phenotype only if same allele is present on both chromosomes of homologous pair

Incomplete Dominance

Heterozygous alleles produce unique phenotype

Codominance

Exhibits both dominant and recessive phenotypes for traits

Penetrance

Percentage of individuals with particular genotype that show “expected” phenotype

Expressivity

Extent to which particular allele is expressed

Teratogens

Factors that result in abnormal development

Punnett Square

Simple box diagram used to predict characteristics of offspring

Polygenic Inheritance

Involves interactions among alleles on several genes

Cannot predict phenotypic characteristics using Punnett square

Linked to risks of developing several important adult disorders

Suppression

One gene suppresses other

Second gene has no effect on phenotype

Complementary Gene Action

Dominant alleles on two genes interact to produce phenotype different from that seen when one gene contains recessive alleles

Sources of Individual Variation

During meiosis, maternal and paternal chromosomes are randomly distributed

Each gamete has unique combination of maternal and paternal chromosomes

Genetic Recombination

During meiosis, various changes can occur in chromosome structure, producing gametes with chromosomes that differ from those of each parent

Greatly increases range of possible variation among gametes

Can complicate tracing of inheritance of genetic disorders

Crossing Over

Parts of chromosomes become rearranged during synapsis

When tetrads form, adjacent chromatids may overlap

Translocation

Reshuffling process

Chromatids may break, overlapping segments trade places

Genomic Imprinting

During recombination, portions of chromosomes may break away and be deleted

Effects depend on whether abnormal gamete is produced through oogenesis or spermatogenesis

Chromosomal Abnormalities

Damaged, broken, missing, or extra copies of chromosomes

Few survive to full term

Produce variety of serious clinical conditions

Mutation

Changes in nucleotide sequence of allele

Spontaneous Mutations

Result of random errors in DNA replication

Errors relatively common, but in most cases error is detected and repaired by enzymes in nucleus

Errors that go undetected and unrepaired have potential to change phenotype

Can produce gametes that contain abnormal alleles

Carriers

Individuals who are heterozygous for abnormal allele but do not show effects of mutation

Sex Chromosomes

X Chromosome

Considerably larger

Have more genes than do Y chromosomes

Carried by all oocytes

Y Chromosome

Includes dominant alleles specifying that the individual will be male

Not present in females

Sperm

Carry either X or Y chromosome

Because males have one of each, can pass along either

Genes that affect somatic structures

Carried by X chromosome

Inheritance does not follow pattern of alleles on autosomal chromosomes

Human Genome Project

Goal was to transcribe entire human genome

Has mapped thousands of human genes

Genome

Full complement of genetic material

Karyotyping

Determination of individual’s complete chromosomal complement

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download