Hair follicle

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Structure and layers of the skin.

The hair follicle is a skin appendage located deep in the dermis of the skin.

Its function is to produce hair and enclose the hair shaft. A hair follicle consists of two main layers, an inner (epithelial) root sheath and an outer (fibrous) root sheath. At the base of the hair follicle is the hair bulb, which houses the dermal papillae and hair matrix cells. The hair bulb and inner epithelial layer generates hair. More specifically, the hair matrix keratinocytes proliferate to form the growing hair shaft while the dermal papilla regulates the hair’s growth and the inner root sheath provides a tube shaped channel for the hair. The outer fibrous layer surrounds the hair follicle. It is formed from dermal connective tissue, supplies blood to the epithelial sheath and holds undifferentiated hair stem cells. The cells of the hair follicle actively change during the growth cycle of a hair.

Each hair follicle is associated with a specialized smooth muscle, several sebaceous glands and free nerve endings. The arrector muscle of hair, or arrector pili muscle, acts to make the hair ‘stand on end’ when we have ‘goose bumps’ from feeling cold or scared. Sebaceous glands release an oily substance (sebum) into the hair follicle. Sebum keeps the hair soft and acts as a bactericide. Nerve endings associated with hair follicles provide sensory mechanoreceptor input relating to hair movement and deformation.

Hair has a number of important functions. For example, it provides a protective cushion layer for skin, protects against ultraviolet light, shields eyes, filters air, is involved in thermoregulation and sensation, and is associated with secondary sexual characteristics.