Hair Physiology

The document discussed the development and structure of hair follicles, which first appeared on the body around the 8th week of intrauterine life and on the scalp by the 7th month. Hair, composed of keratin, grows from follicles that extend from the skin's surface into the dermis. Keratin is rich in cysteine and contains almost all amino acids. Hair growth occurs in three stages: the anagen stage (growing phase) lasting 3-5 years, the catagen stage (transition phase) lasting a few weeks, and the telogen stage (resting or shedding phase) lasting 2 months.