PILOSEBACEOUS UNIT

Part of “CHAPTER 101 – HIRSUTISM, ALOPECIA, AND ACNE“

An understanding of the clinical conditions associated with hyperandrogenism requires some background information about the pilosebaceous unit (PSU), the common structure in skin that gives rise to hair and sebaceous glands. PSUs and hair are distributed over virtually the entire body except the palms and soles. If the sebaceous component of the PSU is prominent, the hair is merely vellus—soft, fine, and unpigmented hair that may remain unrecognized (Fig. 101-6). If the pilary component is prominent, the terminal hair is differentiated from vellus hair by its darker color, greater length, and coarseness.

FIGURE 101-6. Changes occurring after several generations of hair cycles that result in the transition from terminal to vellus hair. During puberty, under the influence of androgens, some of the hair changes from vellus to terminal, especially in the axillary and pubic regions. However, after puberty, for uncertain reasons, an increase in the normally low level of 5α-reductase activity in the scalp may induce a transition from terminal to vellus hair, leading eventually to alopecia. (Modified from Montagna W, Parakkal PF. The structure and function of skin, 3rd ed. New York: Academic Press, 1974:250.)

Before puberty, the predominant body hair is vellus. During puberty, some of the vellus hair normally is transformed into terminal hair, particularly in the pubic and axillary regions. After puberty, terminal hair undergoes normal cyclic changes, the control of which is only partly understood.

The characteristics and distribution of body hair differ greatly among women and are strongly influenced by ethnic and racial factors (i.e., Native Americans, Asians, fair-skinned whites, and some blacks have less hair). Body hair is also influenced by immediate genetic factors; family members frequently have similar hair characteristics. Elderly women often have increased facial hair, which may be associated with a diminution in pubic and axillary hair. Body hair is more noticeable in women with dark hair.

The attitude toward body hair varies among different societies and individuals. In some societies, a relatively large amount of body hair in women is admired; in others, it is considered unattractive. Similarly, the psychology and immediate environment of a woman may alter markedly her attitude toward a degree of body hair that most women would not consider excessive, but which she finds alarming or intolerable.

GROWTH PHASES OF HAIR

Anagen is the growth phase of hair; the length of each phase of growth varies according to body site. This process is somewhat influenced by the hormonal environment. A primary feature of anagen hair is its pigmentation and medullary component (Fig. 101-7). After the growth phase, the transitional catagen phase ensues, in which the club-shaped bulb moves distally, eventually releasing the dermal papilla and becoming inactive. A resting stage, telogen, follows until the hair is shed and active follicular growth begins again.

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