Histological features of the human hair follicle vary greatly during the growth cycle. All hair, terminal and vellus, goes through a growth phase (anagen), a regression or transitional phase (catagen), and a resting phase (telogen).
Anagen hair bulbs are located in the subcutaneous fat, catagen bulbs are in the dermis, and telogen bulbs are in the mid-to-upper dermis. The anagen phase for vellus hairs is much shorter than that for terminal hairs.
The hair growth cycle in different regions of the body varies, and the duration of anagen, catagen, and telogen phases differs according to body site. While the entire cycle duration for the eyelashes is 4 months, the scalp cycle requires about 3-4 years to complete.
In the normal human scalp, the anagen phase duration averages 2-3 years (occasionally much longer); the catagen phase, 2-3 weeks; and the telogen phase, approximately 3 months.
Hair located on other body sites (eg, eyebrows, trunk, extremities) is characterized by longer telogen phases (up to 9 months) and shorter anagen periods (4-7 months). The catagen phase remains constant at 3-4 weeks.
Absent a disease state, 85-90% of all scalp hair follicles are in the anagen phase, 10-14% are in the telogen, and only 1-2% are in the catagen. Estimates place daily scalp hair shedding at about 100 hairs per day.
Scalp hair grows at an average rate of 0.4 mm/day (approximately 0.5 in/month). At any particular time, not all human hair follicles in any given anatomical location are in identical growth stages; some are in the anagen, while others are in the telogen or catagen. This phenomenon is referred to as the mosaic pattern and is in contrast to some animals whose follicular units all are synchronized. This explains some animals’ ability to shed winter coats when the ambient temperature becomes warm, while humans do not go bald during the follicular rest period.