hairstrand

Hair strand, or hair shaft, is the visible portion of a hair that extends from a hair follicle through the skin surface. It is composed of keratinized cells that have died as they move away from the follicle, and it remains non-living while above the skin. The strand has three concentric layers: the cuticle, cortex, and medulla. The outer cuticle consists of overlapping scale-like cells that protect the inner layers. The cortex forms the main body and contains keratin and the pigment melanin, which determines hair color. The medulla, when present, is a looser core of cells that may be air-filled.

Hair shaft length is determined by the growth cycle of the follicle, including anagen (growth), catagen (transition), and telogen (rest). On average, scalp hair grows about 0.3–0.4 millimeters per day. The strand's diameter varies, typically around 50–100 micrometers, and color depends on melanin concentration and type: eumelanin yields brown to black tones, while pheomelanin yields red to yellow tones.

Texture and shape—straight, wavy, or curly—are influenced by the follicle's geometry and the arrangement of keratin within the cortex. The hair strand provides protection, reduces heat loss, and contributes to tactile sensing, though much of its function is passive. Damage to the cuticle from chemical processing or heat can weaken or break the strand. As people age, melanin production often decreases, leading to gray or white strands.

Because the strand is keratinized and non-living, microscopic examination of hair shafts is used in dermatology and forensic science to assess hair type, condition, and exposure history.