Pilosebaceous unit

Sebaceous glands are microscopic exocrine glands in the skin that secrete an oily or waxy matter, called sebum, to lubricate and waterproof the skin and hair of mammals. In humans, they occur in the greatest number on the face and scalp, but also on all parts of the skin except the palms of the hands and soles of the feet. The type of secretion of the sebaceous glands is referred to as holocrine. In the eyelids, meibomian glands, also called tarsal glands, are a type of sebaceous gland that secrete a special type of sebum into tears. Areolar glands surrounding the female nipples are another example of sebaceous gland. Fordyce spots are ectopic (misplaced) sebaceous glands found usually on the lips, gums and inner cheeks, and genitals. Several related medical conditions involve sebum—including acne, sebaceous cysts, hyperplasia, and sebaceous adenoma. These are usually attributable to overactive sebaceous glands, which produce excess sebum. Sebaceous glands are found throughout all areas of the skin, except the palms of the hands and soles of the feet. There are two types of sebaceous gland, those connected to hair follicles, and those that exist independently. Sebaceous glands are found in hair-covered areas, where they are connected to hair follicles. One or more glands may surround each hair follicle, and the glands themselves are surrounded by arrector pili muscles, forming a pilosebaceous unit. The glands have an acinar structure (like a many-lobed berry), in which multiple glands branch off a central duct.The glands deposit sebum on the hairs, and bring it to the skin surface along the hair shaft. The structure consisting of hair, hair follicle, arrector pili muscles, and sebaceous gland is an epidermal invagination known as a pilosebaceous unit. Sebaceous glands are also found in hairless areas (glabrous skin) of the eyelids, nose, penis, labia minora, the inner mucosal membrane of the cheek, and nipples. Some sebaceous glands have unique names. Sebaceous glands on the lip and mucosa of the cheek, and on the genitalia, are known as Fordyce spots, and glands on the eyelids are known as meibomian glands. Sebaceous glands of the breast are also known as Montgomery's glands. Sebaceous glands are first visible from the 13th to the 16th week of fetal development, as bulgings off hair follicles. Sebaceous glands develop from the same tissue that gives rise to the epidermis of the skin. Overexpression of the signalling factors Wnt, Myc and SHH all increase the likelihood of sebaceous gland presence. The sebaceous glands of a human fetus secrete a substance called vernix caseosa, a waxy, translucent white substance coating the skin of newborns. After birth, activity of the glands decreases until there is almost no activity during ages 2–6 years, and then increases to a peak of activity during puberty, due to heightened levels of androgens.