The skin is also known as the integument; the integumentary system includes the skin and its accessory organs such as hair, nails, and glands; dermatology is the study of the integumentary system. (p. 188)
B. The skin is the largest organ of the body and consists of the outer epidermis and the deeper dermis; the hypodermis underlies this latter layer but is not a true part of the skin. (p. 188) (Fig. 6.1)
C. Thick skin covers the palms, soles of the feet, and surfaces of the fingers and toes and contains sweat glands; thin skin covers the rest of the body and contains hair follicles and sebaceous glands as well as sweat glands. (p. 188)
D. The skin provides six important functions. (pp. 188-189)
1. Resistance to trauma and infection.
a. The skin resists and recovers from trauma quickly.
b. Epidermal cells are packed with keratin and linked with desmosomes that increase durability and impermeability.
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c. The skin is slightly acidic, with a pH of 4-6, which is antibacterial.
2. Other barrier functions.
a. The skin is a barrier to water moving either direction.
b. The epidermis blocks UV radiation.
c. The skin is a barrier to some harmful chemicals but is permeable to some drugs and poisons.
Insight 6.1 Transdermal Absorption
3. Vitamin D synthesis; vitamin D is needed for bone development and maintenance.
a. The skin contains a variety of nerve endings for heat, cold, touch, texture, pressure, vibration, and tissue injury.
b. Most sensory receptors are abundant on the face, palms, fingers, soles, nipples, and genitals; relatively few receptors are on the back and in skin overlying joints.
5. Thermoregulation; thermoreceptors in the skin transmit signals to the brain that can lead to vasoconstriction of dermal blood vessels to prevent heat loss or vasodilation to increase it.
6. Nonverbal communication
a. Facial expression in humans is the result of complex skeletal muscles that pull on skin of the face.
b. The general appearance of skin, hair, and nails is also important in social acceptance and emotional state.
E.. The epidermis is a keratinized stratified squamous epithelium, and like other epithelia, lacks blood vessels and depends on diffusion for nutrients and waste removal. (pp. 189-192)
1. The epidermis is composed of five types of cells. (Fig. 6.3)
a. Stem cells are undifferentiated cells that divide to give rise to keratinocytes; stem cells are found in the deepest layer (stratum basale).
b. The majority of epidermal cells are keratinocytes, which synthesize keratin.
c. Melanocytes also occur only in the stratum basale; they synthesize the pigment melanin.
i. Melanocytes shed melanin-containing fragments from the tips of branching processes.
ii. The keratinocytes phagocytize these fragments and accumulate melanin on the "sunny side" of the nucleus.
d. Tactile (Merkel) cells are the receptors for the sense of touch; found in the stratum basale, they are associated with an underlying dermal nerve fiber.
e. Dendritic (Langerhans) cells, which are microphages that migrate from bone marrow, are found in the stratum spinosum and stratum granulosum where they stand guard against foreign pathogens.
2. The cells of the epidermis are arranged in four to five strata.
a. The stratum basale is the innermost layer resting on the basement membrane.
i. It consists of a single layer of cuboidal to low columnar stem cells and keratinocytes.
ii. Melanocytes and tactile cells are scattered among these.
b. The stratum spinosum consists of several layers of keratinocytes.
i. As cells are pushed upward by cell division, they lose the ability to divide and produce more and more keratin fibers.
ii. The keratin fibers cause the cells to flatten, so the higher one looks, the flatter the cells appear.
iii. Keratinocytes are firmly attached to each other by desmosomes, and when exposed to fixatives, the cells shrink but the desmosomes remain intact, giving the cells a spiny appearance (spinosum).
iv. Epidermal keratinocytes are also bound by tight junctions.
c. The stratum granulosum consists of three to five layers of flat keratinocytes-more in thick skin than in thin skin; dark-staining keratohyalin granules give the layer its name.
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d. The stratum lucidum is a thin, translucent zone external to the stratum granulosum and is seen only in thick skin; the keratinocytes are densely packed with eleidin.
e. The outermost stratum corneum consists of up to 30 layers of dead, scaly keratinized cells that form the surface layer.
3. A keratinocyte's life cycle begins in the stratum basale and ends as household dust.
Insight 6.2 Dead Skin and Dust Mites (Fig. 6.4)
a. Mitosis requires oxygen and nutrients, and epidermal cells acquire these from blood vessels in the dermis.
b. New keratinocytes push the older ones toward the surface; once a keratinocyte migrates more than two or three cells away from the dermis, mitosis ceases.
c. In 30 to 40 days, a keratinocyte makes its way to the skin surface and flakes off.
i. The speed is slower in old age and faster in skin that has been injured or stressed.
ii. Mechanical stress or rubbing results in calluses or corns.
iii. As keratinocytes are pushed upward, their cytoskeleton increases, they become flatter, and they produce membrane-coating vesicles.
d. In the stratum granulosum, three developments occur.
i. Keratinocytes undergo apoptosis.
ii. The keratohyalin granules release a substance that converts intermediate filaments to keratin.
iii. The membrane-coating vesicles release a lipid mixture that waterproofs the surface of the cell.
e. An epidermal water barrier forms between the stratum granulosum and the stratum spinosum that prevents dehydration; cells above the barrier die because they are cut off from nutrients below.
f. The stratum corneum consists of layers of dead cells that exfoliate as tiny specks called dander.
F. The dermis is a connective tissue layer beneath the epidermis; it is composed mainly of collagen but contains elastic and reticular fibers, fibroblasts, and other cells of fibrous connective tissue. (pp. 192-194)
1. Hair follicles and nail roots are embedded in the dermis.
2. Piloerector muscles associated with hair follicles are responsible for "goosebumps" and other skin contraction in response to stimuli.
3. Skeletal muscles attached to dermal collagen fibers produce facial expressions.
4. The boundary between epidermis and dermis is usually wavy; upward waves are called dermal papillae and downward waves are epidermal ridges.
a. On the fingertips, the wavy boundary produces the friction ridges responsible for fingerprints.
5. The two zones of dermis are the papillary layer and the reticular layer. (Fig. 6.5)
a. The papillary layer is a thin zone in and near the dermal papillae that is loosely organized and allows mobility of leukocytes; it is rich in small blood vessels.
b. The reticular layer is deeper and thicker and consists of dense irregular connective tissue with thicker collagen bundles; tearing of the collagen from stretching produces striae or stretch marks.
G. The hypodermis, or subcutaneous tissue, has an indistinct boundary but contains more areolar and adipose tissue; it binds the skin to the underlying tissues. (p. 194) (Table 6.1)
1. The hypodermis is the target for subcutaneous injections because it is highly vascularized and absorbs drugs quickly.
2. Subcutaneous fat is hypodermis composed primarily of adipose tissue.
a. Subcutaneous fat serves as an energy reservoir and provides thermal insulation.
b. The subcutaneous fat averages about 8% thicker in women than in men and varies with age.
H. Skin color is produced by pigment molecules. (pp. 194-196) (Fig. 6.6)
1. The most significant factor is melanin, produced by melanocytes and accumulated in keratinocytes.
a. The two forms of melanin are brownish-black eumelanin and reddish-yellow pheomelanin, which contains sulfur.
b. People with different skin colors have the same number of melanocytes, but in dark-skinned people, these cells produce greater quanitities of melanin, and the melanin breaks down more slowly.
c. In dark-skinned people, the melanin is also more spread out through the keratinocyte cell, while in light-skinned people, the melanin remains clumped near the nucleus.
d. The amount of melanin in the skin also varied with exposure to UV radiation, which stimulates melanin synthesis.
e. Variations in ancestral exposure to UV radiation and the resulting natural selection accounts for geographic and ethnic differences in skin color today.
Insight 6.3 The Evolution of Skin Color
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2. Other factors in skin color are hemoglobin and carotene.
a. Hemoglobin, the red pigment of blood, imparts reddish to pinkish hues to the skin.
b. Carotene, a yellow pigment, can become concentrated in the stratum corneum and subcutaneous fat.
3. The skin may exhibit abnormal colors of diagnostic value.
a. Cyanosis is a blue color to the skin resulting from deficiency of oxygen, which turns hemoglobin to a more violet color; it may result from blockage of airways, respiratory arrest, and cold weather that slows blood flow.
b. Erythema is abnormal redness of the skin caused by increased blood flow, such as during exercise, emotions such as anger and embarrassment, and sunburn.
c. Pallor is a pale or ashen color that occurs when blood flow through the skin is diminished; it may be caused by low blood pressure, shock, severe anemia, or emotional stress.
d. Albinism is a genetic lack of melanin that results in white hair, pale skin, and pink eyes; it is generally caused by mutations in the pathway by which tyrosine becomes melanin.
e. Jaundice is yellowing of the skin and whites of the eyes resulting from high levels of bilirubin in the blood; if liver function is compromised, bilirubin can accumulate.
f. A hematoma, or bruise, is a mass of clotted blood showing through the skin; bruising is usually due to accidental trauma but may also occur in hemophilia, physical abuse, or metabolic or nutritional disorders.
I. Skin markings include creases, lines, ridges, and patches of pigmentation. (p. 196-197)
1. Friction ridges are the markings on fingertips responsible for fingerprints; these help us and other primates hold on to objects; they remain unchanged for life and are unique for every individual.
2. Flexion lines (flexion creases) mark sites where the skin folds during flexion of the joints.
3. Freckles are flat melanized patches that vary with heredity and exposure to the sun.
4. A mole (nevus) is an elevated patch of melanized skin, often with hair; these are harmless but should be watched for changes that might mean malignancy.
5. Birthmarks (hemangiomas) are patches of discolored skin cause by benign tumors of the blood capillaries.
a. Capillary hemangiomas (strawberry birthmarks) usually develop about a month after birth; about 90% disappear by the age of 5 or 6 years.
b. Cavernous hemangiomas are flatter and duller; about 90% disappear by the age of 9 years.
c. A portwine stain is flat and pinkish to dark purple, and remains for life.
II. Hair and Nails (pp. 197-202)
A. Hair, nails, and cutaneous glands are accessory organs of the skin; the hair and nails are composed mostly of hard keratin rather than soft keratin. (p. 197)
B. A hair is also known as a pilus; it is a slender filament of keratinized cells that grows from a hair follicle. (p. 197-201) (Fig. 6.7)
1. Hair is found almost everywhere on the body; exceptions are the palms and soles, ventral and lateral surfaces of finger and toes, distal segment of the fingers, and the lips, nipples, and parts of the genitals.
2. Over the course of a lifetime, a person grows three different kinds of hair.
a. Lanugo is fine, downy, unpigmented hair that appears on the fetus in the last 3 months of development.
b. Vellus is fine, pale hair that replaces lanugo by the time of birth; it is the body hair of children and comprises two-thirds of the body hair of women and one-tenth of the body hair of men.
c. Terminal hair is longer, coarser, and pigmented, and forms the eyebrows and eyelashes, covers the scalp, and after puberty forms the axillary and pubic hair, the male facial hair, and some hair on the trunk and limbs.
3. Structurally, a hair has three zones along its length and three layers in cross section.
a. The bulb is a swelling at the base where the hair originates in the dermis or hypodermis.
b. The root is the remainder of the hair within the hair follicle.
c. The shaft is the portion of the hair above the skin surface.
d. The only living cells of a hair are in and near the bulb, which grows around a bud of vascular connective tissue called the dermal papilla.
i. The dermal papilla provides the hair with nutrition.
ii. Immediately above the papilla is a region of mitotically active cells, the hair matrix, which is the hair's growth center.
e. In cross section, the innermost layer of the hair is the medulla, a core of loosely arranged cells and air spaces; it is most prominent in thick hairs such as those of the eyebrows.
f. The cortex, external to the medulla, constitutes most of the hair; it consists of several layers of elongated keratinized cells.
g. The cuticle, the outermost layer of a hair, is composed of layers of very thing, scaly cells that overlap like shingles with their free edges directed upward.
i. Cells lining the follicle are like shingle facing the opposite direction and interlock with the hair cuticle to resist pulling. (Fig. 6.7)
4. The hair follicle is a diagonal tube that extend into the dermis and even the hypodermis; it has two principle layers and associated nerve and muscle fibers.
a. The epithelial root sheath lies adjacent to the hair root; it forms a bulge toward the deep end, which is a source of stem cells for follicle growth.
b. The connective tissue root sheath is derived from the dermis; it surrounds the epithelial sheath and is somewhat denser that adjacent dermal connective tissue.
c. Nerve fibers called hair receptors entwine each follicle and respond to hair movements such as when a hair is touched.
d. Piloerector muscles (pilomotor muscles or the arrector pili) are bundles of smooth muscle cells extending from dermal collagen fibers to the connective tissue root sheath of the follicle; the sympathetic nervous system controls contraction of these muscles in response to cold, fear, or other stimuli. (Fig. 6.8)
5. Hair texture is related to the cross-sectional shape of the hair.
a. A straight hair is round.
b. A wavy hair is oval.
c. A tightly curly hair is relatively flat.
6. Hair color is due to pigment granules in the cells of the cortex.
a. A black or brown hair is rich in eumelanin.
b. Red hair has a slight amount of eumelanin and a higher concentration of pheomelanin.
c. Blond hair has some pheomelanin but very little eumelanin.
d. Gray and white hair results from absence of melanins in the cortex and the presence of air in the medulla.
7. A hair's growth cycle includes three stages: anagen, catagen, and telogen. (Fig. 6.9)
a. In the anagen stage, stem cells from the bulge in the follicle multiply and travel downward, pushing the dermal papilla deeper into the skin and forming the epithelial root sheath.
i. Root sheath cells above the papilla form the hair matrix.
ii. Sheath cells in the hair matrix transform into hair cells, which synthesize keratin and then die as they are pushed upward.
iii. The new hair grows up the follicle, often alongside an old club hair from the previous cycle.
iv. At any given time, about 90% of scalp follicles are in anagen phase.
b. In the catagen phase, mitosis in the hair matrix ceases and sheath cells below the bulge die; the hair is now known as a club hair.
i. The follicle shrinks and the dermal papilla is drawn up toward the bulge.
ii. Club hairs are easily pulled out by brushing, and the hard club can be felt at the hair's end.
c. When the papilla reaches the bulge, the hair goes into a resting period called the telogen phase; a club hair may fall out during catagen, telogen, or as it is pushed out by a new hair in the next anagen phase.
d. Hair grows fastest from adolescence to about age 40, when an increasing percentage of follicles are in catagen and telogen phases.
e. Thinning of the hair, or baldness, is called alopecia, aand it occurs to some degree in both sexes.
f. Pattern baldness is the condition in which hair is lost from specific regions of the scalp rather than thinning uniformly; it results from genetic and hormonal influences.
i. The gene responsible has two alleles, one of which causes patterned hair growth.
ii. The baldness allele is dominant in males and expressed only in the presence of a certain level of testosterone.
g. Excessive or undesirable hairiness is called hirsutism; it tends to run in families and is related to increased testosterone.
h. Hair and nails do not continue to grow after death, cutting hair does not make it grow faster, and emotional stress cannot make the hair turn white overnight.
8. In humans, hair of the trunk and limbs is probably vestigial (has no present purpose).
a. Hair on the scalp prevents heat loss from the head and also protects the scalp from burning.
b. The beard, pubic hair, and axillary hair signals sexual maturity.
c. Stout guard hairs (vibrissae) guard the nostrils and ear canals and prevent foreign particles from entering easily.
d. Eyelashes shield the eyes from windblown debris and act as a screen when we are squinting.
e. The eyebrows are often presumed to keep sweat or debris out of the eyes, but their more important function may be in nonverbal communication through facial expression.
C. Fingernails and toenails are clear, hard derivatives of the stratum corneum composed of very thin, dead, scaly cells packed densely together and filled with parallel fibers of hard keratin. (pp. 201-202) (Fig. 6.10)
1. Flat nails are one of the distinguishing characteristics of primates.
2. The hard part of the nail is the nail plate, the nail body, and the nail root.
a. The nail plate includes the free edge overhanging the tip of the finger or toe.
b. The nail body is the visible attached part of the nail.
c. The nail root extends underneath the overlying skin.
3. The surrounding skin rises a bit above the nail as a nail fold, separated from the margin of the nail plate by a nail groove; the nail groove and space under the free edge collect dirt and bacteria.
4. The skin underlying the nail plate is the nail bed.
a. The epidermis of the nail bed is called the hyponychium.
b. At the nail's proximal end, its stratum basale thickens to constitute a growth zone called the nail matrix; mitosis here accounts for the growth of the nail.
c. The thickness of the nail matrix obscures the underlying dermal blood vessels, creating an opaque white crescent, the lunule.
d. A narrow zone of dead skin, the eponychium (cuticle) also overhangs this proximal end of the nail.
5. The appearance of fingertips and nails can be used in medical diagnosis.
a. Swollen or clubbed fingertips may indicate long-term oxygen deficiency.
b. Dietary deficiencies sometimes can be seen in the nails, such as iron deficiency, which may cause nails to become flat or concave.
III. Cutaneous Glands (pp. 202-204) (Table 6.2)
A. Sweat glands, or sudoriferous glands, are of two kinds. (pp. 202-204)
1. Merocrine (eccrine) sweat glands, the most numerous glands of the skin, produce watery perspiration. (Fig. 6.11)
a. They are especially abundant on palms, soles, and forehead, but are widely distributed as well.
b. Each merocrine gland is a simple tubular gland with a twisted coil in the dermis or hypodermis and an undulating or coiled duct leading to a sweat pore on the surface.
c. The duct is lined by stratified cuboidal epithelium in the dermis and by keratinocytes in the epidermis.
d. The deep end of the gland contains secretory cells along with specialized myoepithelial cells that respond to sympathetic nervous system stimulation and squeeze perspiration up the duct.
e. Sweat begins as a protein-free filtrate of the blood plasma produced in the secretory portion of the gland.
i. Most sodium chloride is reabsorbed as the secretion passes through the duct, but potassium ions, urea, lactice acid, ammonia, and some sodium chloride is excreted.
ii. On average, sweat is 99% water and has a pH of 4-6.
f. Sweat glands secrete about 500 mL per day of insensible perspiration, which does not produce noticeable wetness of the skin.
g. Sweating with wetness of the skin is called diaphoresis, and a person may lose as much as a liter of perspiration an hour.
i. Excessive sweating can cause circulatory shock.
2. Apocrine sweat glands occur in the groin, anal region, axilla, and areola, and in mature mailes, the beard area, but they are absent or sparse in the axillary region of Koreans and Japanese.
a. They produce secretions in the same was as do merocrine glands, but their secretory part has a much larger lumen.
b. The ducts of apocrine sweat glands lead into nearby hair follicles rather than onto the skin surface.
c. Apocrine sweat is thicker and more milky than merocrine sweat because it contains more fatty acids.
d. Apocrine sweat glands are scent glands that respond especially to stress and sexual stimulation; they do not develop until sexual maturity.
i. Apocrine sweat does not have a disagreeable odor, but if trapped by clothing, bacteria degrade the secretion and release fatty acids with a rancid odor.
ii. Disagreeable body odor is called bromhidrosis and most often reflects poor hygiene.
iii. In many mammals apocrine scent glands are associated with specialized tufts of hair; in humans, these glands are found mainly in regions covered by pubic hair, axillary hair, and beard.
iv. The role of these glands is thought to be production of pheromones that influence physiology or behavior of members of the same species.
B. Sebaceous glands produce an oily secretion called sebum; they are flask shaped with short ducts that usually open into a hair follicle. (p. 204) (Fig. 6.11)
1. These are holocrine glands with little visible lumen; their secretion consists of broken-down cells replaced by mitosis.
2. Sebum keeps the skin and hair from becoming dry, brittle, and cracked.
C. Ceruminous glands are found only in the external ear canal, where their secretion combines with sebum and dead epidermal cells to form earwax. (p. 204)
1. They are simple, coiled, tubular glands with ducts to the skin surface.
2. Cerumen keeps the eardrum pliable, waterproofs the canal, kills bacterial, and coats the guard hairs.
D. Mammary glands and breasts (mammae) are often thought to be the same, but breasts are present in both sexes and rarely contain more than small traces of mammary gland. (p. 204)
1. The mammary glands are milk-producing glands that develop with the female breast only during pregnancy and lactation.
2. Mammary glands are modified apocrine sweat glands that produce a richer secretion and channel it through ducts to a nipple.
IV. Skin Disorders (pp. 204-208) (Table 6.3)
A. Skin is not only the most vulnerable organ to injury and disease but is also the one place where we most likely notice anything out of the ordinary. (p. 204)
B. Skin cancer is induced by UV radiation from the sun and occurs most often on the head and neck, where exposure is greatest. (p. 205-207)
1. Skin cancer is most common in fair-skinned people and the elederly.
2. The popularity of sun tanning has caused an increase in skin cancer among hounger people.
3. Although a common cancer, skin cancer is one of the easiest to treat and has a high survival rate when detected and treated early.
Insight 6.4 UVA, UVB, and Sunscreens
4. There are three types of skin cancer named for the epidermal cells from which they originate: basal cell carcinoma, squamous cell carcinoma, and malignant melanoma. (Fig. 6.12)
a. Basal cell carcinoma is the most common but least dangerous type, originating in the stratum basale; on the surface the lesion first appears as a small, shiny bump that later develops a central depression and a beaded "pearly" edge.
b. Squamous cell carcinoma arises from cells of the stratum spinosum and appears most often on the scalp, ears, lower lip, or back of the hand; the lesion has a raised, reddened, scaly appearance later forming a concave ulcer with raised edges. This cancer can metastasize to lymph nodes and be lethal.
c. Malignant melanoma arises from melanocytes, often in a preexisting mole; it is the most deadly form.
i. If malignant melanoma metastasizes, which it does quickly, it is usually fatal.
ii. The average person lives only 6 months from diagnosis, and only 5% to 14% of patients survive for 5 years.
iii. The greatest risk factor is a family history of the disease.
iv. About two-thirds of cases in men result from an oncogene called BRAF, which in women has been linked to some breast and ovarian cancers.
v. Malignant melanoma can be recognized by the ABCD rule: Asymmetry, Border irregularity, Color, and Diameter (greater than 6 mm).
C. Burns are the leading cause of accidental death and can result from fires, spills, hot bathwater, sunlight, ionizing radiation, strong acids and bases, or electrical shock. (pp. 207-208)
1. Burn deaths result primarily from fluid loss, infection, and the toxic effects of eschar (the burned, dead tissue).
2. Burns are classified according to the depth of tissue involvement. (Fig. 6.13)
a. First-degree burns involve only the epidermis and have symptoms of redness, slight edema, and pain; they heal in a few days and seldom leave scars-most sunburns are first-degree burns.
b. Second-degree burns involve the epidermis and part of the dermis but leave at least some of the dermis intact; first- and second-degree burns are known as partial-thickness burns.
i. A second-degree burn may be red, tan, or white and is blistered and very painful.
ii. Healing may take from 2 weeks to several months and may leave scars.
iii. The epidermis regenerates by division of epithelial cells in the hair follicles and sweat glands and around the edges of the lesion.
iv. Some sunburns and many scalds are second-degree burns.
c. In third-degree burns, also called full-thickness burns, the epidermis, dermis, and often some deeper tissue are completely destroyed.
i. Third-degree burns often require skin grafts because no dermis remains.
ii. If a third-degree burn is left to heal on its own, contracture due to abnormal connective tissue fibrosis and severe disfigurement may result.
d. Fourth-degree burns extend all the way through the skin and subcutaneous tissue into the underlying muscles, tendons, or bones.
3. The two most urgent considerations in treating a burn patient are fluid replacement and infection control.
a. A patient can lose several liters of water, electrolytes, and protein each day from the burned area.
i. Up to 75% of the blood plasma may be lost within a few hours, potentially leading to circulatory shock and cardiac arrest, the principle cause of death; intravenous fluid must be administered to compensate.
ii. A severely burned patient may also require thousands of calories daily to compensate for protein loss and the demands of tissue repair.
b. Infection is controlled by keeping the patient in a germ-free environment and administering antibiotics.
i. Eschar is sterile for the first 24 hours, but then quickly becomes infected.
ii. Its removal, called debridement, is essential to infection control.
Insight 6.5 Skin Grafts and Artificial Skin
Connective Issues: Integumentary system interactions
Additional information on topics mentioned in Chapter 6 can be found in the chapters listed below.
Chapter 5: Types of connective tissue
Chapter 5: Types of sweat glands
Chapter 5: Healing of cuts and injuries to the skin
Chapter 16: Sense organs of the skin
Chapter 28: Anatomy and physiology of mammary glands
Chapter 29: Aging of the skin