hårstråets

Hårstrået, or the hair strand, is a keratinized filament that grows from a hair follicle located in the skin of mammals, including humans. The portion that extends above the skin is the hair shaft; the portion below is the follicle. The visible shaft consists of dead cells filled with keratin. In humans, the diameter of a hair strand typically falls within roughly 50 to 100 micrometers, varying by body site and individual.

The hair shaft has three concentric structural zones. The outermost layer is the cuticle, a sheet of flattened, overlapping cells that protect the inner layers. Beneath lies the cortex, which contains keratin filaments and most of the pigment melanin that determines hair color. In many hairs there is a central medulla, a loosely organized core that may be absent in finer hairs. The arrangement of these layers influences strength, elasticity, and how the hair reflects light.

Hair growth is cyclical, comprising anagen (growth), catagen (transitional), and telogen (resting) phases. Anagen duration varies by body site and individual, with scalp hair often growing for several years. Catagen is a brief transition, followed by telogen, during which the hair rests before a new cycle begins and a replacement shaft grows. During the anagen phase, living cells in the hair follicle produce the shaft; once the strand emerges from the skin, it is composed of non-living, keratinized cells.

Functions of hair include protection, insulation, and sensory input. Variation in color, thickness, and curl results from genetic factors that affect keratin structure and melanin distribution. External factors such as chemical treatments and environmental exposure can alter the cuticle and overall hair health.