hairs

Hair refers to a filamentous keratinous structure produced by hair follicles in the skin. Each hair consists of a shaft that protrudes above the surface and a root embedded in a follicle beneath the epidermis. Hair is made of keratin, produced by matrix cells in the hair bulb.

The hair shaft has three concentric layers: medulla, cortex, and cuticle. The follicle includes the hair bulb, which contains the dermal papilla and rapidly dividing matrix cells that supply new keratinocytes. The follicle is associated with sebaceous glands and an arrector pili muscle, which can raise hair in response to stimuli.

There are two main types of human hair: vellus hair, which is fine and lightly pigmented, and terminal hair, which is coarser and often pigmented. Terminal hair is prevalent on the scalp, face, and other regions and is influenced by sex hormones.

Hair growth occurs in cycles. The anagen (growth) phase lasts years in scalp hair, followed by a shorter catagen (transition) phase and a telogen (resting) phase. On average, scalp hair grows about 0.3-0.4 mm per day, with daily shedding of a small number of hairs.

Functions include protection from ultraviolet radiation, insulation, and sensing light touch via hair follicle receptors. Hair color results from melanin produced by melanocytes in the hair matrix and transferred to keratin.

Abnormal hair patterns include alopecia (hair loss), hirsutism (excess terminal hair in androgen-dependent areas), and hypertrichosis (excessive growth of hair in non-androgenic areas).