Hair

Each hair has a shaft projecting above the skin and a root within the follicle, ending in the expanded bulb. In cross-section the shaft has three concentric layers: an inner medulla of loosely arranged cells and air spaces (present only in thick terminal hairs), a cortex of tightly packed, hard-keratinised cells that forms the bulk of the hair and contains the pigment, and an outer cuticle of overlapping, shingle-like cells pointing towards the tip. Hard keratin (rich in cystine disulfide bonds) makes hair strong and durable.

Three types occur. Lanugo is fine, unpigmented fetal hair, mostly shed by birth. Vellus hair is short, fine and lightly pigmented and covers most of the body. Terminal hair is long, coarse and pigmented: scalp, eyebrows and eyelashes, and — after puberty under androgens — axillary, pubic and (in males) beard and body hair. Colour depends on the proportion of eumelanin (black-brown) and pheomelanin (red-yellow) transferred from bulb melanocytes; greying reflects depletion of follicular melanocyte stem cells.

The adult has ≈5 million follicles, ≈100 000 on the scalp. Scalp hair grows ≈0.35 mm/day (≈1 cm/month), and 50–100 hairs are normally shed each day. Because hair incorporates substances from the blood as it grows, segmental analysis can reveal past exposure to drugs and toxins such as arsenic.