Hair follicles and their associated sebaceous glands (pilosebaceous glands), eccrine glands, apocrine glands, and finger and toenails are all considered skin appendages. Hair follicles are found everywhere within the skin except for the soles of the feet, the palms of the hand, the red portion (vermilion border) of the Hps, and the external genitalia. All are formed from fetal epidermal cells. Hair differs markedly in its prominence from place to place over the body. Delicate primary hair is found on the fetus; secondary hair or down covers the adult forehead; terminal hair ordinarily blankets the scalp and is found as pubic and axillary (underarm) hair (2). A hair (hair shaft) emerges from & follicle, as shown in Figure 1. Each follicle is set within the skin at a slight angle. Each consists of concentric layers of cellular and noncellular components, and each is anchored to the surrounding connective tissue by an individual strand of smooth muscle, the arrector pilorum. Contraction of this muscle causes the hair to stand upright, merely raising goose pimples on human skin. The hair shaft is formed continuously by cell division, differentiation, and compaction within the bulb (base) of each active hair follicle, a process that is completed deep in the follicle. Hair, like stratum corneum, is thus a compact of fused, keratinized cells. Collectively, hair follicles occupy about one-thousandth of the skin's surface with about 100 follicles per square centimeter of skin (9,14), a factor that sets a limit on the role that follicular orifices can play as a route of penetration. Each hair follicle possesses one or more flask-like sebaceous glands (Figure 1). These have ducts that vent into the open space surrounding the hair shaft just below the skin's surface. The cells of sebaceous glands, sebocytes by name, are programmed to divide, differentiate, and die. Before they die and disintegrate, they pack themselves full of lipid-containing vesicles. The residue left behind at their death is mixed with other follicular debris below the follicular orifice to form an oily substance called sebum. Sebum is then forced upward around the hair shaft and onto the skin surface. The follicular outlets for sebum exhibit diameters ranging from 200 to 2000 |im (2 mm) depending on body location (10). Glands with the largest openings are found on the forehead, face, nose, and upper back. These large follicles contain an almost microscopic hair, if they contain one at all, and are therefore referred to as sebaceous follicles.
Eccrine glands (salty sweat glands) are found over the entire body except the genitalia and lips. They appear as tubes extending from the skin surface all the way to the footings of the dermis. Here the tube coils into a ball roughly 100 |im in diameter (Figure 1) (10). By anatomical count, there are between 150 and 600 glands per square centimeter of body surface depending on body site (15). They are particularly concentrated in the palms and soles, attaining densities in these locations well in excess of 400 glands/cm2. However, since many of these glands remain dormant, estimates of their numbers are appreciably lower if based on actual sweating units. Each gland has an approximately 20 |im diameter orifice at surface of the skin from which its secretions are spilled. In total, these glandular openings represent approximately one-ten-thousandth of the skin's surface (9). Eccrine sweat is a dilute (hypotonic), slightly acidic (pH «5.0 due to traces of lactic acid) aqueous solution of salt. Its secretion is stimulated when the body becomes overheated through warm temperatures or exercise. Evaporation of the water of the sweat cools the body's surface and thus the body. Since the gland is innervated by the autonomic nervous system, eccrine sweating is also stimulated emotionally (the clammy handshake).
Apocrine glands are found only in the axillae (armpits), in the anogenital region, and around the nipples. Along with other secondary sexual characteristics, these glands develop at puberty. We know that they are innervated emotionally and through concupiscence. In the mature female, they exhibit cyclical activities in harmony with the menstrual cycle. Like eccrine glands, they are coiled tubular structures, but the coils are roughly 10 times larger. They extend well into the subcutaneous layer beneath the skin (2,10). Each gland is paired with a hair follicle; its secretion is vented into the sebaceous duct of the follicle beneath the surface of the skin. Because this secretion is minor in amount and combines with sebum before reaching the skin's surface, its exact chemical makeup remains an enigma. What is not a mystery is that bacterial decomposition of the secretion is responsible for human body odor.
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