Hair Follicles

Hair follicles are distributed over the entire body, except the palms and the soles. The hair follicle is a stratum corneum-lined sac that extends into the dermis. At the base of the follicle is the hair bulb, where cells divide and differentiate to form the hair shaft - a strand of tightly packed keratinized cells.

There are two types of hair shafts. The vellus hair is a fine, soft, wispy, barely noticeable hair. Terminal hairs are thicker, longer, and darker. On some body areas, terminal hairs are hormone-dependent and do not appear until puberty. The scalp, eyebrows, axillae, and pubic regions are covered with terminal hairs.

Hair growth in each follicle is cyclical. There is an active growing phase anagenen). The follicle grows one hair for 2-6 years, and then growth stops. The follicle goes into a resting phase for about 4 months before the hair is shed and the cycle begins again. An average adult sheds about 100 hairs a day from a head of 120,000 hairs.

A hair erector muscle is connected to each hair follicle. When this muscle contracts in response to cold or fear the follicles become erect, producing "goose bumps."

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