SEBACEOUS GLAND

small oil-producing gland present in the skin of mammals. Sebaceous glands are usually attached to hair follicles and release a fatty substance, sebum, into the follicular duct and thence to the surface of the skin. The glands are distributed over the entire body with the exception of the palms of the hands and the soles of the feet; they are most abundant on the scalp and face. The sebaceous gland secretes a mixture of fats (triglycerides, wax esters, squalene, and cholesterol) and cellular debris, which is discharged as sebum through the sebaceous duct connecting the gland to the hair follicle. Sebum helps to form the slightly greasy surface film of the skin; it thus helps keep the skin flexible and prevents the skin's loss of absorption of excessive amounts of water. Sebaceous glands begin to form from embryonic hair follicles during the fourth month of gestation and are large and well-developed at birth. They shrink during childhood but enlarge again with the onset of puberty; the maturation of the glands seems to be dependent on circulating levels of male hormones, in particular testosterone. The sebaceous glands are involved in the development of the common adolescent skin disorder known as acne vulgaris. Acne occurs when the outlet from the gland to the surface of the skin is plugged, allowing sebum to accumulate in the follicle and sebaceous duct. The chemical breakdown of triglycerides in the sebum, possibly by bacterial action, releases free fatty acids, which in turn trigger an inflammatory reaction producing the typical lesions (pimples) of acne.