Hair follicles, which are downward growths in the dermis, are found all over the body except on the lips, soles and palms. Cells move up the follicle from the hair bulb at its base, changing in structure to form a hair. Tiny muscles (the erector pili) attached to hair follicles cause hairs to stand erect when we are cold, trapping heat near the surface of the skin to warm us. Aggression or fright also contracts these muscles, leading to the appearance of ‘goose bumps’.
In the next section, we look in more detail at the structure of the hair.
The main function of hair is protection. It guards the scalp from cold temperatures, injury and the sun’s rays; the eyes from foreign particles; and the ear canal and nostrils from insects and inhaled particles. Each hair is composed of a shaft, most of which projects above the surface of the skin, and the root below.
Indian Head Massage for Hair Photo Gallery
The shaft of coarse hairs, such as those on the scalp, consists of three parts: the medulla, the cortex and the cuticle.
The medulla is composed of large, loosely connected cells containing keratin and air spaces. Reflection of light through air spaces within the medulla determines the visible sheen and colour tone of hair. The cortex, which forms the bulk of the hair, is composed of long cells containing keratin and the pigment melanin, determining hair colour. This pigment can vary from brown-black to yellow-red. Hair turns grey and eventually white as pigment glands slow their production and eventually shut down. The outer cuticle consists of a single layer of thin, flat, keratinised cells overlapping like slates on a roof. These cells point upwards and interlock with the scale-like cells that line the inner root sheath of the follicle, which point downwards, securing the hair in place.
The hair root, which penetrates into the dermis and in some cases into the subcutaneous layer, also contains a medulla, cortex and cuticle, continuous with the hair shaft. Surrounding the root is the hair follicle, which consists of several layers:
The inner root sheath is composed of three layers, the innermost of which interlocks with the hair cuticle, keeping the hair in place.
The outer root sheath is a downward continuation of the epidermis. Near the surface it contains all the epidermal layers, while at the base of the follicle it contains only basal cells.
The base of each hair follicle is enlarged into an onion-shaped bulb, containing an indentation (papilla) filled with loose connective tissue and with blood capillaries, which nourish the growing hair. Hair growth originates from the central area of the bulb, the matrix, an area of active growth where cells undergo mitosis. These cells are then pushed upwards, where they fill with keratin to produce a hair that eventually projects from the open end of the follicle when older hairs are shed. Electrolysis and laser treatments destroy the hair bulb to prevent re-growth.