1. The epidermis is stratified squamous epithelium.

2. The epidermis is cornified with the protein keratin.

3. Stratum Basale (p. 203; Fig. 7.3)

4. Stratum Spinosum (p. 204)

5. Stratum Granulosum (p. 204)

6. Stratum Lucidum (p. 205)

7. Stratum Corneum (p. 205)

1. The dermis is primarily composed of fibroconnective tissue, with blood vessels, nerve endings, sweat glands, hair follicles, and nail roots.

2. The papillary layer of the dermis is a zone of areolar tissue adjacent to the epidermis, projecting upward as dermal papillae. Blood vessels extend close to the epidermis within the papillae, and the large number of white blood cells within this layer helps prevent infection.

3. The reticular layer contains dense irregular connective tissue. Stretching of the skin causes tears in this area, called linea albicantes, or stretch marks.

1. The hypodermis (subcutaneous tissue, or superficial fascia) is composed of areolar or adipose tissue between the skin and underlying muscle.

2. The hypodermis is 8% thicker in females.

1. Skin color is due to the presence of blood vessels close to the surface, and the presence of melanin.

2. Melanin is produced by melanocytes in response to exposure to UV light. Heredity determines the extent of melanin production.

3. The skin can take on other colors: carotene imparts a yellowish caste; the skin can appear bluish during cyanosis; stress or shock can cause pallor; those that lack the ability to produce melanin are albinos; erythema is abnormal redness of the skin; jaundice makes the skin appear yellow; and a hematoma (bruise) gives a purplish caste.

1. Skin markings include hemiangiomas (birthmarks), moles (nevus), freckles, friction ridges, and flexion creases.

1. Keratin and epidermal desmosomes make skin a tough barrier to penetration and injury. The dry habitat of the skin, and the acid mantle of sweat are unfavorable to microbial growth.

2. The skin is impervious to water and a barrier to ultraviolet radiation.

1. UV radiation can penetrate into the dermis where it reaches the steroid dehydrocholesterol found in blood. The steroid is converted to cholecalciferol that the liver and kidneys turn into an active form of vitamin D.

1. The blood receives 1-2% of its oxygen through the skin.

2. Many toxic compounds, as well as fat-soluble vitamins, can be absorbed through the skin.

1. A variety of types of sensory nerve endings are present in skin.

1. The skin functions as an adjustable radiator to regulate body temperature.

2. When cold, dermal blood vessels constrict, thus retaining heat in the body core.

3. When hot, dermal blood vessels dilate, radiating heat to the surroundings. Also, perspiration allows evaporative cooling.

1. The condition of the skin and hair, and the presence of skin markings greatly influences social interactions among individuals.

1. Almost all areas of the skin have hair, except lips, nipples, palms and soles, and portions of the fingers and genitals.

2. Lanugo, a fine, downy, unpigmented hair covers the fetus before birth.

3. Fine, unpigmented hair is otherwise called vellus, and covers most of the body.

4. Terminal hair is coarser, longer, and pigmented. It can be found on the head and face (men), and in axillary and pelvic regions after puberty.

1. The hair itself is divided into the swollen base, where the hair originates, the root (the remainder of the hair within its follicle), and the shaft, the portion above the skin.

2. In cross section, the core of the hair (the medulla) is made up of loosely arranged cells and air spaces; the cortex is densely packed keratinized cells; and the cuticle is a layer of overlapping scaly cells.

3. The color of hair is due to the relative abundance of keratin. Red hair also contains trichosiderin.

4. The hair follicle dips into the dermis and has two layers.

5. The epithelial root sheath is an extension of the epidermis.

6. The connective tissue root sheath, derived from the dermis, surrounds the epidermal sheath.

7. Also associated with the follicle are hair receptors, and an arrector pili muscle.

1. The growth of hair is due to mitosis in cells in the stratum basale of the epidermal root sheath.

2. Thinning of the hair (alopecia) is usually age-related, but may be influenced by disease, nutrition, emotional trauma, radiation, or chemotherapy.

3. Pattern baldness is more common in males.

1. Most human hair is probably vestigial, but hair on the head helps retard heat loss and protect from the sun's rays.

2. Tufts of hair otherwise indicate individual identity, sex, or age.

3. Hairs at the entrance to the nose and ears, as well as the eyelashes, have a protective function.

4. Eyebrows may serve to enhance facial expression.

1. Nails are clear derivatives of the stratum corneum.

2. Flat nails of humans allow for greater dexterity.

3. The nail matrix is the growth zone at the proximal end of the nail.

4. The nail plate is the nail itself.

1. Sudoriferous (sweat) glands are the most numerous cutaneous glands.

2. Merocrine sweat glands produce watery perspiration to cool the body.

3. Apocrine sweat glands occur in the groin, axilla, and areola as well as on the faces of males. Some population differences in their distribution can be found. These are scent glands that respond to stress and sexual stimulation.

1. Sebaceous glands produce an oily sebum to moisturize the skin and hair.

1. Ceruminous glands are found only in the external ear canal where they produce cerumen (earwax).

1. Both men and women have breasts; it is only during pregnancy and lactation that women develop mammary gland tissue within the breasts capable of producing and secreting milk.

2. Mammary glands are modified apocrine sweat glands.

1. At puberty, the skin of women thickens somewhat. Otherwise, it contains more blood vessels than that of men, thus women blush and bleed more easily.

2. After age 40, skin degenerative changes (senescence) become more noteworthy.

1. With increasing age and UV exposure, the development of skin cancer becomes more likely.

2. Skin cancer is the most common of all cancers, with basal cell carcinoma (arising from the stratum basale) the most frequent skin cancer. It appears as a raised bump with a central depression, and seldom metastasizes.

3. Squamous cell carcinoma arises from the stratum spinosum usually on the scalp, ears. lips, or hands. This cancer is readily curable, but can metastasize and turn deadly if left untreated.

4. The deadliest, but least common, form of skin cancer is malignant melanoma. It usually arises from melanocytes in a preexisting mole. It is most likely in those who suffered severe sunburns as children, especially redheads.

1. Burns are the leading cause of accidental death.

2. Burns are classified according to the depth of tissue involvement. First-degree burns involve only the epidermis; second-degree burns involve the upper dermis; and third-degree burns, also called full-thickness burns, destroy the skin and sometimes deeper tissue.

3. With severe burns, fluid and electrolyte replacement, and infection control (and use of debridement) are essential.