(har )

1. A keratinized, threadlike outgrowth from the skin of mammals.
2. Collectively, the threadlike outgrowths that form the fur of animals or grow on the human body.

Hair in each part of the body has a definite period of growth, after which it is shed. In the adult human there is a constant gradual loss and replacement of hair. Hair of the eyebrows lasts only 3 to 5 months; that of the scalp lasts 2 to 5 years. Baldness or alopecia results when replacement fails to keep up with hair loss. It may be hereditary or due to pathologic conditions, e.g., infections or irradiation injury. Cytotoxic agents used in cancer chemotherapy may cause temporary loss of hair.

SEE: alopecia.

A hair is a thin, flexible shaft of cornified cells that develops from a cylindrical invagination of the epidermis, the hair follicle. Each consists of a free portion or shaft (scapus pili) and a root (radix pili) embedded within the follicle. The shaft consists of three layers of cells: the cuticle or outermost layer; the cortex, forming the main horny portion of the hair; and the medulla, the central axis. Hair color is due to pigment in the cortex.

SEE: illus..

auditory h.The stereocilia of a specialized epithelial cell. These are present in the ear in the spiral organ of Corti, concerned with hearing; and in the crista ampullaris, macula utriculi, and macula sacculi, concerned with equilibrium.

bamboo h.Sparse, brittle hair with bamboo-like nodes. The nodes are partial fractures of the hair shaft, caused by atrophy of the hair. SYN: clastothrix; trichorrhexis nodosa.

beaded h.Hair with swellings and constrictions in the hair shaft caused by monilethrix.

burrowing h.A hair that grows horizontally under the skin, causing a foreign body reaction.

gustatory h.Taste hair

ingrown h.A hair that reenters the skin, causing a foreign body reaction.

kinky h.Short, sparse, tightly twisted hair that may be poorly pigmented.

lanugo h.

SEE: lanugo.

moniliform h.Monilethrix.

moth-eaten h.Hair with patchy areas of baldness and poorly defined borders. This type of alopecia is one of the cutaneous hallmarks of syphilis.

pubic h.Hair superior to the pubes, appearing at the onset of sexual maturity.

SEE: mons veneris; mons pubis.

sensory h.Specialized epithelial cells with hairlike processes.

tactile h.Hair that is receptive to touch or contact.

taste h.Any of several fine hairlike processes that extend from the ends of gustatory cells in a taste bud. They project through the inner pore of a taste bud. SYN: gustatory hair.

terminal h.The long, coarse, pigmented hair of the adult.

twisted h.Congenitally deformed hair that is short, brittle, and coiled.