At the site of a fully-grown hair, the epidermis invaginates to form an external root sheath - a cord of cells - projecting downwards into the dermis. At its deepest part, a terminal expansion forms - the papilla - into which dermal connective tissue and blood vessels migrate. The combination of epidermal cells and connective tissue is called the hair follicle. The follicle is surrounded by a connective tissue layer that is used to adhere hair to an associated arrector pili muscle.
The hair forms upwards from the papilla region of the dermis. Here, in the keratogenous zone, a circumferential layer of specialized epidermal cells equivalent to the stratum basale, the internal root sheath, differentiate to produce keratin. Internal to this sheath, cells become progressively more filled with condensed keratin on their route to the surface. New cells, dividing in the papilla, push older cells upwards. Hence, the keratin shaft of a hair forms and grows.
Ultrastructurally, the hair consists of three layers, from innermost out: medulla, cortex and cuticle.
One third of the way deep to the skin surface, the external root sheath grows tangentially to form a sebaceous gland.
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