Hair

Hair is a filamentous structure made primarily of keratin that grows from follicles in the skin. In humans, hair covers most of the body, with notable exceptions such as the palms of hands, soles of feet, lips, nipples, and parts of the genital area. Hair patterns and density vary by genetics, age, sex, and environmental factors.

A hair fiber consists of the shaft, which projects above the skin, and the root, which resides within a hair follicle. The follicle extends from the epidermis into the dermis and often includes a sebaceous gland and, in mammals, arrector pili muscles. The hair shaft has three layers: the cuticle (outer scale-like cells), the cortex (pigmented or unpigmented cells providing strength and color), and the central medulla in some hairs. Hair growth originates in the hair bulb, where matrix cells divide and form the hair shaft, while the papilla supplies blood and nutrients.

Hair undergoes a growth cycle that includes anagen (active growth), catagen (transition), and telogen (rest). Growth rates vary by body site but are typically around 0.3–0.6 millimeters per day. Some body hair is vellus (fine and pale) and replaces by terminal hair in response to hormones or aging.

Hair serves protective, insulating, and sensory roles, and participates in camouflage and social signaling in many species. Common human hair disorders include alopecia (hair loss), hirsutism (excess hair in women and children), and hypertrichosis. Hair care involves hygiene, trimming, and, when desired, dyeing or styling; damage from chemicals, heat, or traction can affect texture and growth.