A sebaceous gland or oil gland is a microscopic exocrine gland in the skin that opens into a hair follicle to secrete an oily or waxy matter, called sebum, which lubricates the hair and skin of mammals. In humans, sebaceous glands 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. In the eyelids, meibomian glands, also called tarsal glands, are a type of sebaceous gland that secrete a special type of sebum into tears. Surrounding the female nipples, areolar glands are specialized sebaceous glands for lubricating the nipples. Fordyce spots are benign, visible, sebaceous glands found usually on the lips, gums and inner cheeks, and genitals.
Structure
Location In humans, 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 follicles, arrector pili muscles and sebaceous glands, 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.
Development 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 two–six years, and then increases to a peak of activity during puberty, due to heightened levels of androgens.
Function Relative to keratinocytes that make up the hair follicle, sebaceous glands are composed of huge cells (sebocytes) with many large vesicles that contain the sebum. These cells express Na+ and Cl− ion channels, ENaC and CFTR (see Fig. 6 and Fig. 7 in reference). Sebaceous glands secrete the oily, waxy substance called sebum (Latin for 'fat, tallow') that is made of triglycerides, wax esters, squalene, and metabolites of fat-producing cells. Sebum lubricates the skin and hair of mammals. Sebaceous secretions in conjunction with apocrine glands also play an important thermoregulatory role. In hot conditions, the secretions emulsify the sweat produced by the eccrine sweat glands and this produces a sheet of sweat that is not readily lost in drops of sweat. This is of importance in delaying dehydration. In colder conditions, the nature of sebum becomes more lipid, and in coating the hair and skin, rain is effectively repelled. Sebum is produced in a holocrine process, in which sebocyte cells within the sebaceous gland rupture and disintegrate as they release the sebum and the cell remnants are secreted together with the sebum. The cells are constantly replaced by mitosis at the base of the duct.
Sebum Sebum is secreted by the sebaceous gland in humans. It is primarily composed of triglycerides (≈41%), wax esters (≈26%), squalene (≈12%), and free fatty acids (≈16%). The composition of sebum varies between species. Wax esters and squalene are unique to sebum and not produced as final products anywhere else in the body. Sapienic acid is a sebum fatty acid that is unique to humans, and is implicated in the development of acne. Sebum is odorless, but its breakdown by bacteria can produce strong odors. Sex hormones are known to affect the rate of sebum secretion; androgens such as testosterone have been shown to stimulate secretion, and estrogens have been shown to inhibit secretion. Dihydrotestosterone acts as the primary androgen in the prostate and in hair follicles.
Immune function and nutrition Sebaceous glands are part of the body's integumentary system and serve to protect the body against microorganisms. Sebaceous glands secrete acids that form the acid mantle. This is a thin, slightly acidic film on the surface of the skin that acts as a barrier to microbes that might penetrate the skin. The pH of the skin is between 4.5 and 6.2, an acidity that helps to neutralize the alkaline nature of contaminants. Sebaceous lipids help maintain the integrity of the skin barrier and supply vitamin E to the skin. Sebaceous glands actively participate in innate immunity by releasing cytokines like IL-1α, IL-6, and TNF-α.
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