Each hair follicle has a sebaceous gland part way along its length. Sebaceous glands are simple branched alveolar glands which are found all over the body, except on the palms and soles. They are larger on the face, neck and upper chest and smaller on the limbs and trunk. The gland releases sebum onto the hair surface, thus lubricating it and the skin surface. The central cells of the alveoli accumulate oily lipids until they become so engorged that they burst, the gland is thus made up of cells that must ‘die’ as they release their secretory product. The cells are replaced by underlying cells.
Sebaceous glands are stimulated by hormones, particularly androgens, and therefore are key to the development of acne (see
Acne).
Attached to the hair follicle is a bundle of muscle called the arrector pili muscle. These contract when an individual is cold and pull the hair into an upright position. This causes goosebumps and is one of the body’s ways of conserving heat. The erect hair traps warm air next to the skin surface. This biological phenomenon was much more effective when humans were hairier, now body hair is sparse and fine, this mechanism for keeping warm is relatively ineffective.
