Hair Follicle

The hair follicle is a small, tunnel-shaped organ embedded in the skin that produces hair. Together with its attached sebaceous (oil) gland and a tiny arrector pili muscle, it forms the pilosebaceous unit. Follicles cover nearly the entire body surface except the palms, soles, and lips, and they originate from the epidermis but extend down into the dermis, sometimes reaching the subcutaneous fat.

Structure and Location

A follicle has three main segments. The upper infundibulum runs from the skin surface to the opening of the sebaceous gland; the middle isthmus contains the bulge region where epithelial stem cells reside; and the lower segment ends in the expanded hair bulb. At the base of the bulb sits the dermal papilla, a cluster of connective tissue cells fed by tiny blood vessels that nourish the growing hair. Surrounding the papilla are rapidly dividing matrix cells, which act as the germ cells that build the hair shaft. The follicle is wrapped in inner and outer root sheaths that guide and shape the emerging hair. The small arrector pili muscle attaches to the follicle and contracts to produce "goosebumps."

The Hair Growth Cycle

Follicles cycle continuously through four phases: anagen (active growth, lasting 2 to 6 years on the scalp), catagen (a short regression phase), telogen (rest), and exogen (shedding of the old hair). Only during anagen is the deep hair bulb fully formed. Because each follicle cycles independently, humans shed hair gradually rather than all at once.

Clinical Relevance

Disruptions of the follicle cause many common conditions. Androgenetic alopecia (male- and female-pattern baldness) reflects hormone-driven miniaturization of follicles, while telogen effluvium shifts too many follicles into the shedding phase after stress or illness. Clogged or inflamed follicles produce folliculitis and contribute to acne. The follicle's stem cells are also a key source of cells for wound healing. This content is educational, not medical advice.