Hair consists of two distinct structures:
- follicle—the living part located under the skin
- hair shaft—fully keratinized nonliving part above the skin surface.
The arrector pili muscle takes place between the hair bulge area and dermo-epidermal junction. Above the insertion of the arrector pili muscle, sebaceous glands and, in some certain regions, apocrine glands are opened into the follicle.
The hair shaft consists of three layers:
- cuticle
- cortex
- medulla
Flat and square-shaped cuticle cells adhere tightly to the cortex cells proximally. Peripheric movements of cuticle cells make the direction of the distal free edge upward and cause extensive overlapping. These imbrications are crucial. By interlocking with the cuticle cells of the inner root sheath, they contribute to the follicular anchorage of the growing hair. These imbricated surfaces also facilitate the removal of dirt and desquamated cells from the scalp. The cuticle has also important protective properties and barrier functions against physical and chemical insults.
