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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.
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.
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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

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