Chapter 2 57 significant loss of functioning melanocytes in the hair follicle, and usually begins anywhere from the mid-30s to the mid-40s. Even though hair graying is one of the most typical and common signs of aging, premature hair graying as well as other hair color disorders, which are often caused by congenital pigmentation defects, are also psychologically disturbing to certain extent.67 The major cause of graying has been identified to be a depletion of the melanocyte stem cells in the bulge, as demonstrated by Bcl2-deficient mice and various MITF spontaneous mutations. Damages to the hair follicle pigmentary unit by reactive oxygen species probably also plays an important role in graying.68 Aging of Human Hair Follicles The density of scalp hair decreases with age and the total hair follicle number reduces by 0.22% per year. The growth rate of scalp hair in women slows down with age and the diameter of scalp hair shafts also decreases with age. The trichogram of normal healthy adults demonstrates an increasing percentage of telogen hair with age. The average proportion of telogen hair on the scalp rises from about 12–16% at ages 16–21 to 20–24% at age 41–45.69 The duration of the anagen phase shortens with aging.70 (For more on hair aging, please see Chapter 14.) Hair Disorders The hair follicle undergoes repeated cycles of regression and regeneration. The HS in human anagen grows at a rate of approximately 0.35 mm per day.71 In telogen phase, the mature HS forms a white nodule at the proximal end (club hair) and eventually sheds from the follicle, leading to physiological hair loss before the hair follicle re-enters anagen stage and repeats the hair cycle. Normally 50–150 scalp hairs are lost per day.38 In humans, hair follicles in different hair cycle stages distribute randomly on the scalp, with an anagen to telogen ratio around 9:1. Therefore, this normal shedding of the HS is usually unnoticeable and the total hair amount remains constant. There is a seasonal variation in the