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Chapter 6 The Integumentary System

1 Terminology

1 Integument

The skin alone is known as the integument.

2 Integumentary system

The integumentary system includes the skin and its accessory organs.

1 Accessory organs

1 Hair

2 Nails

3 Cutaneous glands

3 Dermatology

The scientific study and medical treatment of the integumentary system is called dermatology.

2 Overview of skin

1 Largest organ

In adults, the skin covers about 1.5-2.0 square meters and accounts for about 15% of body weight.

2 Two layers of skin

1 Epidermis

Stratified squamous epithelium.

2 Dermis

Deeper connective tissue layer.

3 The hypodermis is not part of the skin

Below the dermis, often studied with the skin.

4 Thick and thin skin

Most skin is 1-2mm thick but can range from about 0.5mm (eyelids) to 6mm (between the shoulder blades).

The difference is due mainly to variation in the thickness of the dermis, but thick and thin skin is characterized on the basis of the thickness of the epidermis, especially the stratum corneum.

1 Thick skin

The palms, soles of the feet and corresponding areas of the hands have epidermal layers that are about 0.5mm. Thick skin has sweat glands but no hair follicles.

2 Thin skin

The rest of the body is covered with thin skin, which has an epidermis of about 0.1mm thick, and a thin stratum corneum. It possesses hair follicles, sebaceous glands, and sweat glands.

3 The skin and subcutaneous tissue

1 Functions of the skin

1 Resistance to trauma and infection

1 Keratin

The epidermal cells are packed with the protein keratin and the cells are linked with desmosomes. This produces a durable tissue.

2 Acid mantle

Skin is relatively dry and slightly acid, both of which inhibit bacterial growth.

2 Other barrier functions

1 Water barrier

Slows down the absorption of water from the environment and loss of water from underlying tissues.

2 Ultraviolet (UV) light barrier

Inhibits the absorption of energetic radiation by the deeper tissue layers in which cell division is taking place.

3 Vitamin D synthesis

The first step in a complicated sequence of events in the synthesis of vitamin D. The liver and kidneys are required to complete the process (what is the function of vitamin D?).

4 Sensation

The skin is the most extensive sensory organ. Includes a variety of sensory nerve endings that respond to heat, cold, touch, texture, pressure, vibration, and tissue injury.

5 Thermoregulation

1 Thermoreceptors

In the dermis, these receptors respond to cold and ultimately signal the dermal blood vessels to constrict, reducing the flow of blood and reducing heat loss. Overheating does the opposite, allowing an increase in blood flow to the skin and increase in heat loss.

6 Social functions

Nonverbal communication: expressions of the face.

2 The epidermis

1 Cells of the epidermis

1 Stem cells

These are undifferentiated cells that undergo mitosis and give rise to the keratinocytes of the epidermis. They are found in the deepest layer of the epidermis (stratum basale).

2 Keratinocytes

The majority of the cells in the epidermis are keratinocytes. They synthesize keratin.

3 Melanocytes

Found in the stratum basale with the stem cells. Synthesize the pigment melanin, which is taken up by the keratinocytes in the vicinity. Melanin protects the DNA of dividing cells from UV light by absorbing it.

4 Tactile (Merkel) cells

Receptors for the sense of touch. Found in the stratum basale and are associated with a dermal sensory neuron.

5 Dendritic (Langerhans) cells

Found in the stratum spinosum and stratum granulosum. They are phagocytic cells that originate in the bone marrow. They identify and consume microbes and some foreign molecules and communicate with the immune system to mount an immune response.

2 Layers of the epidermis

Below are listed from deep to superficial, which corresponds to youngest to oldest (dead) keratinocytes.

1 Stratum basale

Consists mainly of stem cells and keratinocytes resting on a basement membrane. Some melanocytes and tactile cells. Stem cells give rise to keratinocytes that migrate towards the skin surface to replace lost (dead) epidermal cells.

2 Stratum spinosum

Several layers of keratinocytes. Deepest layers of keratinocytes retain mitotic capability, more superficial layers lose ability to divide. They produce more and more keratin as they are pushed towards the surface.

Dendritic cells also found in this layer.

3 Stratum granulosum

Layers of flattened keratinocytes, which are loaded with keratin. Forms a barrier between superficial and deep cell layers, which prevents nutrients from diffusing to superficial layers.

4 Stratum lucidum

Thin translucent zone superficial to the stratum granulosum seen only in thick skin. Keratinocytes are devoid of organelles and nuclei at this point and densely packed with keratin. They don't stain well as a result and appear as a clear layer in tissue sections.

5 Stratum corneum

Many layers of dead, scaly, keratinized cells that form a durable and expendable layer.

3 The life history of a keratinocyte

Keratinocytes are constantly shed as the dead layers from the stratum corneum. They are replaced by the dividing keratinocytes deep in the stratum basale. Mitosis ceases when the keratinocytes are two or three cell layers away from the deepest cells of the stratum basale.

New keratinocytes formed by mitosis push the older ones towards the surface. Takes about 30 to 40 days for keratinocytes to reach the surface and flake off. Injured skin speeds up this process.

Keratinocytes grow flatter and more filled with keratin as they move outward.

Tight junctions are formed between keratinocytes at the level of the stratum granulosum and stratum spinosum producing an epidermal water barrier.

3 The dermis

Found beneath the epidermis, this is mainly a connective tissue layer. Composed mainly of collagen, but has some elastic fibers.

Fibroblasts and some other cells are found here, but they are sparce.

Well supplied with blood vessels, sweat glands, sebaceous glands, and nerve endings.

The hair follicles and nail roots are found in the dermis.

Smooth muscles associated with the hair follicles erect the hair shaft and produce "goose bumps".

1 Dermal papillae

Extensions of the dermis that project into the epidermis, producing a wavy boundary between the two layers.

1 Epidermal ridges

The raised projections of the epidermis above the dermal papillae. On the fingertips these produce the fingerprints.

2 Two zones

1 Papillary layer

A thin zone of loosely organized connective tissue in or near the dermal papillae allows mobility of immune cells. Rich in blood vessels.

2 Reticular layer

Deeper and thicker layer of the dermis. Consists of dense irregular connective tissue. Thicker bundles of collagen provides structural support for dermis.

4 The hypodermis

Beneath the skin, also called the subcutaneous layer or superficial fascia. Highly vascular.

1 Subcutaneous fat

The hypodermis generally has more adipose tissue than the dermis and collects in specific areas as subcutaneous fat. Acts as an insulating layer and energy storage.

5 Skin color

1 Normal skin colors

1 Pigments

The most significant factor in skin color is the presence of melanin and its variants. Produced by the melanocytes and accumulates in keratinocytes.

1 Melanin

Two forms of melanin.

1 Eumelanin

Brownish-black pigment.

2 Pheomelanin

Reddish-yellow pigment

2 Hemoglobin

Where there is an abundance of skin capillaries, the hemoglobin will confer color to the skin.

3 Carotene

Yellow pigment that accumulates in the stratum corneum and subcutaneous fat.

2 Abnormal skin colors

1 Cyanosis

Blue color in the skin resulting from a deficiency of oxygen in the circulating blood. De-oxygenated hemoglobin is a different color than oxygenated blood and through the skin, de-oxygenated blood appears blue. Anything that interferes with the association of hemoglobin with oxygen can result in cyanosis: cold (slow circulation means de-oxygenated blood retained in tissues), lung disease, choking, etc.

2 Erythema

Abnormal redness of the skin resulting from increased circulation to the skin (exercise, emotional states) or from increased permeability of skin capillaries (as in sunburn).

3 Pallor

Pale or ashen color resulting from deficient blood flow. Results from emotional stress, low blood pressure, circulatory shock, cold, anemia.

4 Albinism

Genetic lack of melanin.

5 Jaundice

Yellowing of the skin and whites of eyes (sclera). Results from high levels of bilirubin in the blood. Bilirubin is a breakdown product of heme, from hemoglobin, and will accumulate in skin if there is a rapid destruction of RBCs or liver malfunction.

6 Bronzing

Addison's disease (an endocrine disorder that results in a deficiency of adrenal glucocorticoid hormones) produces hyperpigmentation.

7 Hematoma

Bruising, results from a mass of clotted blood showing through the skin.

6 Skin markings

1 Friction ridges

Finger prints. Found in primates, prevent slippage from surfaces and from objects being held.

2 Flexion lines

Lines on the surfaces of digits, palms, wrists, elbows etc mark areas where skin folds during flexion. of joints.

3 Freckles and moles

Aggregations of melanocytes. Freckles are flat and moles are elevated. Changes in moles may indicate development of malignancy.

4 Hemangiomas

"Birthmarks" caused by benign tumors of dermal blood capillaries.

4 Hair and nails

Hair and nails are accessory organs of the skin. Hair and nails are composed mostly of dead, keratinized cells. The skin has soft keratin in the stratum corneum, hair and nails have hard keratin, which is more compact and more highly cross-linked.

1 Hair

Also known as a pilus (singular), thus removal of hair is called depilation. Hair has a slender filament of keratinized cells that grow from a hair follicle.

1 Distribution and types

1 Distribution

Found everywhere on the body except soles and palms, ventral and lateral surfaces of fingers and toes, distal segments of fingers, lips, nipples and genitals.

The number of hairs in a given area of skin does not differ much from one person to another or between sexes: differences are mainly due to hair texture and pigmentation.

2 Types

1 Lanugo

Fine, downy, unpigmented appearing on the fetus in the third trimester. Mostly lost by the time of birth.

2 Vellus

Lanugo hair replaced by vellus hair. Similarly fine and unpigmented. Retained to adulthood: about 2/3 hair of females, 1/10 hair of males, all of children's hair except for eyebrows, eyelashes, and scalp hair.

3 Terminal

Longer, coarser, and pigmented. Eyebrows, eyelashes, scalp, axillary and pubic hair, male facial hair and some of male trunk and limb hair.

2 Structure of the hair follicle

1 Bulb

The swelling at the base of the hair follicle where the hair originates. Grows around a bud of vascular connective tissue called the dermal papilla, which provides hair with nutrients.

2 Root

The remainder of the hair within the follicle.

3 Shaft

The hair above the skin surface.

4 Hair receptors

Nerve fibers that surround each follicle and respond to hair movement.

5 Piloerector muscle

Bundle of smooth muscle fibers attached to follicle that make the hair stand on end in response to cold, fear, etc. Probably no function in humans but makes many species look bigger and may provide some insulation advantage.

3 Hair texture and color

The texture of hair is related to the cross-sectional shape of the hair: straight is round, wavy is oval and tightly curled is relatively flat.

Color is due to the presence of pigment granules in the cells of the cortex of the hair shaft.

4 Hair growth and loss

For your information only.

5 Functions of hair

1 Body hair is sensory, not insulating

2 Scalp hair protective, heat retention

3 Indicative of sexual maturity

4 Guard hairs and eyelashes

5 Expressive of emotions with eyebrows

2 Nails

Derivatives of the stratum corneum, hard keratin.

Protect sensory cells in fingertips and act as tools.

5 Cutaneous glands

Five types of glands: merocrine sweat glands, apocrine sweat glands, ceruminous glands, and mammary glands.

1 Sweat glands

1 Merocrine sweat glands

The most numerous sweat glands in skin, produce watery sweat to cool the body. Simple, tubular gland.

2 Apocrine sweat glands

Found near hair follicles, these glands are scent glands that respond to stress and sexual stimulation.

2 Sebaceous glands

Produce and oily secretion called sebum that usually open onto hair follicles. Prevents the hair and skin from drying.

3 Ceruminous glands

Found only in the external ear canal. Their secretion combines with sebum to produce earwax, which protects the delicate tissues and is anti-bacterial.

4 Mammary glands

Milk-producing glands that develop within the female breast during pregancy and lactation only. They are modified apocrine glands.

6 Skin disorders

For your information only. Reviewed in class if there is time.

1 Skin cancer

2 Burns

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