Accessory Structures of the Skin: Hair and Hair Follicles

The hair shaft contains three concentric layers. The outermost cuticle consists of heavily keratinized flat cells that protect the inner layers. The middle cortex has several layers of elongated cells. The innermost medulla contains large irregular cells forming the central core of the hair structure.

The hair papilla is a nipple-shaped structure deep within the hair bulb containing capillaries and nerve endings from the dermis. Its capillaries nourish the developing hair, while the hair matrix—composed of germinal cells surrounding the papilla—is responsible for the formation and growth of new hair cells.

The hair shaft is the visible part of hair extending above the skin surface and not anchored to the follicle. The hair root lies below the epidermis, anchored within the hair follicle, and extends deep into the dermis to the hair bulb where growth occurs.

Hair follicles contain sensory nerve endings called hair follicle receptors at the hair bulb base. Hair is extremely sensitive to air movement and environmental disturbances, making it far more responsive than the skin surface alone. This heightened sensitivity helps detect insects and potentially damaging substances on the skin.

The arrector pili is a smooth muscle attached to each hair root that contracts in response to sympathetic nervous system signals. This contraction makes the external hair shaft stand up, trapping a layer of air for insulation and thermoregulation. In humans, this response appears visibly as goose bumps.

Hair provides multiple protective functions across different body regions. Head hair protects the skull from sun exposure. Nasal, ear, and eyelash hair trap dust particles containing allergens and microbes, defending against pathogens. Eyebrow hair prevents sweat and other particles from dripping into and irritating the eyes.

The hair follicle is teardrop-shaped and composed of epidermal cells and cellular matrix. It includes the outer root sheath present only on the hair bulb, the inner root sheath extending about halfway up the hair shaft through the dermis, and the hair matrix as the innermost layer surrounding the hair shaft base within the bulb.