Introduction

Hair originates in the hair follicles, which are located in the dermis, the skin layer just below the surface layer. Hair follicles are present everywhere on the surface of the body except the lips, palms of the hands, and soles of the feet. New hair is made in the hair bulb at the base of the hair follicle. Living cells in the hair bulb multiply and push upward. These cells rapidly dehydrate, die, and compact into a dense, hard mass that forms the hair shaft. The hair shaft, which is made up of dead protein, is covered by a delicate covering (cuticle) composed of platelike scales.

Hair is colored by the pigment melanin, which is also responsible for skin color. Human hair colors come from two types of melanin: eumelanin in black or brown hair and pheomelanin in auburn or blond hair.

Hair grows in cycles. Each cycle consists of a long growing phase followed by a short resting phase. At the end of the resting phase, the hair falls out and a new hair starts growing in the follicle, beginning the cycle again. Eyebrows and eyelashes have a growing phase of 1 to 6 months. Scalp hairs have a growing phase of 2 to 6 years. Normally, about 100 scalp hairs reach the end of the resting phase each day and fall out.

Hair growth is regulated by male hormones (androgens, such as testosterone and dihydrotestosterone), which are present in both men and women, although in different amounts. Testosterone stimulates hair growth in the pubic area and underarms. Dihydrotestosterone stimulates beard hair growth.

Hair disorders include excessive hairiness (hirsutism), hair loss, and ingrown beard hairs. Although most hair disorders are not serious or life threatening, they are often perceived as major cosmetic issues that require treatment.