Hair Discussions - Hair Growth

At the base of the follicle is the papilla, which is the "hair manufacturing plant." The papilla is fed by the blood-stream which carries nourishment to produce new hair. Male hormones or androgens regulate hair growth. Pubic and axillary (armpit) hair are particularly androgen-sensitive and grow at lower androgen levels than hair on the chest or legs. In boys, most pubic hair is grown by age 15, followed by the development of armpit hair two to three years later. In girls, too, an increase in androgens at puberty triggers growth of pubic and armpit hair. Scalp hair, not directly androgen-responsive, is influenced by local amounts of a testosterone derivative, dihydrotestosterone.

Hair follicles initially form in utero. No new follicles are created after birth, and none are lost in adult life. The first hair to be produced by the fetal hair follicles is Lanugo hair, which is fine, soft, and unpigmented. This is usually shed in about the eighth month of gestation.

The first postnatal hair is vellus hair, which is fine, soft, usually unpigmented, and seldom more than 2 cm long. Vellus hair remains on the so-called hairless regions of the body, such as the forehead and balding scalp. At puberty, the vellus hair in some areas is replaced by terminal hair, which is longer, coarser, and pigmented. Growth starts in the pubic region; then the eyelashes and eyebrows become thicker. Axillary hair and male facial hair appear about two years after growth of pubic hair begins. Body hair continues to develop long after puberty, stimulated by male hormones that paradoxically, also cause terminal hair to be replaced by vellus hair when balding begins.