TAILIEUCHUNG - Chapter 050. Hirsutism and Virilization (Part 2)

Hair Follicle Growth and Differentiation Hair can be categorized as either vellus (fine, soft, and not pigmented) or terminal (long, coarse, and pigmented). The number of hair follicles does not change over an individual's lifetime, but the follicle size and type of hair can change in response to numerous factors, particularly androgens. Androgens are necessary for terminal hair and sebaceous gland development and mediate differentiation of pilosebaceous units (PSUs) into either a terminal hair follicle or a sebaceous gland. In the former case, androgens transform the vellus hair into a terminal hair; in the latter, the sebaceous component proliferates and. | Chapter 050. Hirsutism and Virilization Part 2 Hair Follicle Growth and Differentiation Hair can be categorized as either vellus fine soft and not pigmented or terminal long coarse and pigmented . The number of hair follicles does not change over an individual s lifetime but the follicle size and type of hair can change in response to numerous factors particularly androgens. Androgens are necessary for terminal hair and sebaceous gland development and mediate differentiation of pilosebaceous units PSUs into either a terminal hair follicle or a sebaceous gland. In the former case androgens transform the vellus hair into a terminal hair in the latter the sebaceous component proliferates and the hair remains vellus. There are three phases in the cycle of hair growth 1 anagen growth phase 2 catagen involution phase and 3 telogen rest phase . Depending on the body site hormonal regulation may play an important role in the hair growth cycle. For example the eyebrows eyelashes and vellus hairs are androgeninsensitive whereas the axillary and pubic areas are sensitive to low levels of androgens. Hair growth on the face chest upper abdomen and back requires greater levels of androgens and is therefore more characteristic of the pattern typically seen in men. Androgen excess in women leads to increased hair growth in most androgen-sensitive sites except in the scalp region where hair loss occurs because androgens cause scalp hairs to spend less time in the anagen phase. Although androgen excess underlies most cases of hirsutism there is only a modest correlation between androgen levels and the quantity of hair growth. This is due to the fact that hair growth from the follicle also depends on local growth factors and there is variability in end-organ sensitivity. Genetic factors and ethnic background also influence hair growth. In general dark-haired individuals tend to be more hirsute than blonde or fair individuals. Asians and Native Americans have relatively sparse hair in

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