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Eccrine gland Apocrine gland Hair follicle maturation, keratinization, and shedding takes approximately 4 weeks. The cells of the basal layer are intermingled with melanocytes, which produce melanin. The number of melanocytes is approximately equal in all people. Differences in skin color are related to the amount and type of melanin produced, as well as to its dispersion in the skin.

Beneath the epidermis is the dermis, which is the dense connective tissue stroma forming the bulk of the skin. The dermis is bound to the overlying epidermis by finger-like projections that project upward into the corresponding recesses of the epidermis. In the dermis, blood vessels branch and form a rich capillary bed in the dermal papillae. The deeper layers of the dermis also contain the hair follicles with their associated muscles and cutaneous glands. The dermis is supplied with sensory and autonomic nerve fibers. The sensory nerves end either as free endings or as special end organs that mediate pressure, touch, and temperature. The autonomic nerves supply the arrector pili muscles, blood vessels, and sweat glands.

The third layer of the skin is the subcutaneous tissue, which is composed largely of fatty connective tissue. This highly variable adipose layer is a thermal regulator, as well as a protection for the more superficial skin layers from bone prominences.

The sweat glands, hair follicles, and nails are termed skin appendages. The evaporation of water from the skin by the sweat glands provides a thermoregulatory mechanism for heat loss. Figure 8-2 illustrates the types of sweat glands.

Within the skin, there are 2 to 3 million small, coiled eccrine glands. The eccrine glands are distributed over the body surface and are particularly profuse on the forehead, axillae, palms, and soles. They are absent in the nail beds and in some mucosal surfaces. These glands are capable of producing more than 6 L of watery sweat in 1 day. The eccrine glands are controlled by the sympathetic nervous system.

The apocrine glands are larger than the eccrine glands. The apocrine glands are found in close association with hair follicles but tend to be much more limited in distribution than are the eccrine glands. The apocrine glands occur mostly in the axillae, the areolae, the pubis, and the perineum. They reach maturity only at puberty, secreting a milky, sticky substance. Apocrine glands are adrenergically mediated and appear to be stimulated by stress.

The sebaceous glands are also found surrounding hair follicles. The sebaceous glands are distributed over the entire body;the largest glands are found on the face and upper back. They are absent on the palms and soles. Their secretory product, sebum,

Nail matrix

Paronychial space

Nail matrix

Paronychial space

Nail bed

Proximal nail fold Lunula

Figure 8-3 Structural relations of the nail: cross section and from above.

Lovibond's angle Nail plate

Nail bed

Proximal nail fold Lunula

Lateral nail fold

Figure 8-3 Structural relations of the nail: cross section and from above.

Sebaceous gland

Arrector pili muscle

Hair follicle' Hair matrix

Figure 8-4 The hair follicle and its surrounding structures.

Sebaceous gland

Arrector pili muscle

Hair follicle' Hair matrix

Figure 8-4 The hair follicle and its surrounding structures.

is discharged directly into the lumen of the hair follicle, where it lubricates the hair shaft and spreads to the skin surface. Sebum consists of sebaceous cells and lipids. The production of sebum depends on gland size, which is directly influenced by androgen secretion.

Nails protect the tips of the fingers and toes against trauma. They are derived by keratin-ization of cells from the nail matrix, which is located at the proximal end of the nail plate. The nail plate consists of the nail root embedded in the posterior nail fold, a fixed middle portion, and a distal free edge. The whitish nail matrix of proliferating epithelial cells grows in a semilunar pattern. It extends outward past the posterior nail fold and is called the lunula. The structural relationships of the nail are shown in Figure 8-3.

A hair shaft is a keratinized structure that grows out of the hair follicle. Its lower end, called the hair matrix, consists of actively proliferating epithelial cells. The cells at this end of the follicle, along with those of the bone marrow and gut epithelium, are the most rapidly growing dividing cells in the human body. This is the reason that chemotherapy causes hair loss, along with anemia, nausea, and vomiting. Visible hair is present over the entire body surface except on the palms, soles, lips, eyelids, glans penis, and labia minora. In apparently hairless areas, the hair follicles are small, and the shafts produced are microscopic. Hair follicles show conspicuous morphologic and functional heterogeneity. Follicles and their developing shafts differ from location to location in shaft length, color, thickness, curl, and androgen sensitivity. Some follicles, those in the axilla and inguinal areas, are very sensitive to androgens, whereas others in the eyebrow are insensitive. The arrector pili muscles attach to the follicle below the opening of the sebaceous gland. Contraction of this muscle erects the hair and causes ''goose bumps.'' The structure of a hair follicle is shown in Figure 8-4.

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Hair Loss Prevention

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