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Androgenic hair, commonly known as body hair, is the terminal hair that develops on the human body during and after puberty. It is differentiated from the head hair and less visible vellus hair, which are much finer and lighter in color. The growth of androgenic hair is related to the level of androgens (male hormones) in the individual. Due to a normally higher level of androgen, men tend to have more androgenic hair than women. From childhood onward, regardless of sex, vellus hair covers almost the entire area of the human body. Exceptions include the lips; the backs of the ears; the palms of hands; the soles of the feet; certain external genital areas; the navel; and scar tissue. The density of hair – i.e. the number of hair follicles per unit area of skin – varies from person to person. In many cases, areas on the human body that contain vellus hair will begin to produce darker and thicker body hair, such as the first growth of beard hair on a male adolescent's previously smooth chin. Androgenic hair follows the same growth pattern as the hair that grows on the scalp, but with a shorter anagen phase and longer telogen phase. While the anagen phase for the hair on one's head lasts for years, the androgenic hair growing phase lasts a few months.〔(Hair Growth: EMedicine Otolaryngology and Facial Plastic Surgery)〕 The telogen phase for body hair lasts close to a year.〔 This shortened growing period and extended dormant period explains why the hair on the head tends to be much longer than other hair found on the body. Differences in length seen in comparing the hair on the back of the hand and pubic hair, for example, can be explained by varied growth cycles in those two regions. The same goes for differences in body hair length seen in different people, especially when comparing men and women. == Development and growth == (詳細はtestosterone and its derivatives, with different areas on the body having different sensitivity. As androgen levels increase, the rate of hair growth and the weight of the hairs increase. Genetic factors determine both individual levels of androgen and the hair follicle's sensitivity to androgen, as well as other characteristics such as hair colour, type of hair and hair retention. Rising levels of androgen during puberty cause vellus hair to transform into terminal hair over many areas of the body. The sequence of appearance of terminal hair reflects the level of androgen sensitivity, with pubic hair being the first to appear due to the area's special sensitivity to androgen. The appearance of pubic hair in both sexes is usually seen as an indication of the start of a person's puberty. There is a sexual differentiation in the amount and distribution of androgenic hair, with men tending to have more terminal hair in more areas. This includes facial hair, chest hair, abdominal hair, leg hair, arm hair, and foot hair. Women retain more of the less visible vellus hair, although leg, arm, and foot hair can be noticeable on women. It is not unusual for women to have a few terminal hairs around their nipples as well. In the later decades of life, especially after the 5th decade, there begins a noticeable reduction in body hair especially in the legs. The reason for this is not known but it could be due to poorer circulation, lower free circulating hormone amounts or other reasons. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Androgenic hair」の詳細全文を読む スポンサード リンク
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