The hair follicle is a structure of the skin from which hair grows. There are hair follicles all over the skin, with the exception of the lips, palms of the hands, and soles of the feet. Hair follicles grow hair by packing old cells together.
The hair follicle is supplied with one or more sebaceous glands, which provide sebum, an oily substance that helps lubricate the hair and skin. Apocrine sweat glands also help lubricate the hair follicles of the armpits, groin, and areolae. Areas that have thicker hair growth, such as the scalp, have more sebaceous glands. In addition the the sebaceous gland, the hair follicle is provided with the arrector pili, a bundle of muscle fibers that creates goose bumps when contracted. Hair follicles also have stem cells at their base, which contribute to regular hair growth.
The base of the hair follicle is called the papilla. It consists of connective tissue and a capillary loop, or tiny blood vessel. The papilla is surrounded by the hair matrix, which consists of epithelial cells and melanocytes. The epithelial cells divide very quickly, causing regular hair growth, while the melanocytes provide pigment, and are responsible for hair color.
The hair follicle is surrounded by a protective root sheath, consisting of the external and internal root sheath. The internal root sheath, in turn, has three layers: the innermost internal cuticle, the medial Huxley's layer, and the outermost Henle's layer. The internal cuticle is continuous with the outermost layer of the hair fiber. The hair fiber also has three layers: the cuticle, the intermediate cortex, and the inner medulla.
Hair growth takes place in four-phase cycles. Anagen is the active growth phase, the length of which varies greatly between people and individual hair follicles. Anagen lasts for two to seven years on the human scalp, but for only months on the eyebrows. The growth phase is followed by catagen, a brief transition phase lasting for approximately two to four weeks.
After catagen, the telogen phase, a resting phase, begins. Hairs in the telogen phase are dead and are called club hairs. Many club hairs are shed from the body daily. The telogen phase lasts about three weeks for hairs on the human scalp. The final stage of the hair follicle cycle is exogen, a shedding phase in which one of many hairs that may arise from a single follicle is shed.