Androgens are required for sexual hair and sebaceous gland development. However, pilosebaceous unit (PSU) growth and differentiation require the interaction of androgen with numerous other biological factors. The pattern of PSU responsiveness to androgen is determined in the embryo. Hair follicle growth involves close reciprocal epithelial-stromal interactions that recapitulate ontogeny; these interactions are necessary for optimal hair growth in culture. Peroxisome proliferator-activated

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... ors (PPARs) and retinoids have recently been found to specifically affect sebaceous cell growth and differentiation. Many other hormones such as GH, insulin-like growth factors, insulin, glucocorticoids, estrogen, and thyroid hormone play important roles in PSU growth and development. The biological and endocrinological basis of PSU development and the hormonal treatment of the PSU disorders hirsutism, acne vulgaris, and pattern alopecia are reviewed. Improved understanding of the multiplicity of factors involved in normal PSU growth and differentiation will be necessary to provide optimal treatment approaches for these disorders. FIG. 3. Homeobox (HOX) gene clusters. The four sets of homeobox genes are organized in tandem on four different chromosomes. Previous nomenclature of the HOX genes is shown in parentheses, and the chromosomal location in mice is likewise shown in parentheses. The genes are numbered according to their anterior-posterior sequence and are expressed in a "lock-step" manner with genes in the 3Ј-end of the clusters being transcribed earlier in embryonic development than genes in the 5Ј-ends of the clusters. Anticipated HOX genes are represented by unnumbered boxes. HOX genes thought to be important for PSU morphogenesis include A4, A5, C4, C6, C8, and D4. [Adapted with permission from R. L. Rosenfield and D. Deplewski: Am J Med 98:805-885, 1995 (1a) © FIG. 4. The hair growth cycle. Hair follicles progress through repetitive cycles of growth, from anagen (active phase of growth which is the longest phase in the hair cycle), through catagen (shortening of the hair follicle), to telogen (resting phase of the hair cycle), after which the club hair is shed, and the follicle begins a new hair cycle again. 366