Scalp hair

Function

Hair is keratin extruded from follicles — and scalp hair is its showpiece: 100,000 follicles cycling through anagen (2–7 years of growth), catagen (regression) and telogen (rest and shed). 50–100 strands fall daily as the cycle turns.

Beyond insulation and UV shielding, hair's follicles are stem-cell sanctuaries — the bulge region's cells re-epithelialise wounds — and social signalling of the most powerful kind.

Blood supply

Dermal papilla capillary loops feed each follicle.

Nerve supply

Sympathetic fibres piloerect; follicular receptors sense the lightest deflection.

Clinical relevance

Androgenetic alopecia

DHT-sensitive follicles miniaturise in a pattern — the commonest hair loss, treatable with finasteride and minoxidil.

Alopecia areata

Autoimmune attack on anagen follicles — sudden smooth patches; usually reversible.