Hair in humans is largely an ornamental rather than a protective appendage. It is distributed in varying amounts over the surface of the body; the amount and patterns of distribution depend on race, sex, and age. Hair is most abundant on the scalp, in the armpits, and in the pubic region. It is totally absent from the palms and soles.
The hair follicle, which produces and supports the hair, is an epithelial tube extending deep into the dermis. At its deep end the follicle expands into the hair bulb. The bulb surrounds a connective tissue papilla that contains nerve endings and blood vessels.
Hair formation occurs in the matrix, the layer of epithelial cells on top of the papilla, which is comparable to the stratum germinativum of the epidermis. The matrix produces the hair root, a column of growing and dividing epithelial cells that become keratinized as they move upward. The hair shaft is a cylinder of keratinized cells, continuous with the root, whose free end protrudes above the surface of the skin. The shaft consists of the medulla (an inner layer of loosely packed cells and air spaces), the cortex (a middle layer of tightly packed, heavily keratinized cells), and the cuticle (an outer layer of cells whose overlapping shingle like arrangement prevents erupted hairs form matting). The growing hair receives melanin granules from melanocytes located over the tip of the dermal papilla. Hair colour is determined by the amount and aggregation of melanin granules in the cortex and the amount of air spaces in the medulla.
Hair follicles lie diagonal to the surface of the skin. The erector pili muscle, a bundle of smooth muscle fibers attached to the hair follicle run obliquely from the greater angle of the follicle to the papillary layer of the dermis. Contraction of this muscle in response to cold, fear, or anger straightens the hair and elevates the skin surrounding the hair. Also, contraction of the erector pili muscle causes the release of sebum into the hair follicle by squeezing the sebaceous gland between the muscle and follicle wall.