Schematic view of hair follicle and sebaceous gland.
Cross-section of all skin layers. A hair follicle with associated structures. (Sebaceous glands labeled at center left.)
The 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 are found in the greatest number on the face and scalp, and are also found 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. There are several related medical conditions, including acne, sebaceous cysts, hyperplasia and sebaceous adenoma.
The 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, in pilosebaceous units, 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. The glands have an acinar structure similar to a bunch of grapes, 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 muscle, 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 membrane of the cheek, and nipples. Some sebaceous glands have unique names. Sebaceous glands on the lip and mucosa of the cheek 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 decreases until there is almost no activity during ages 2–6, and then increases to a peak of activity during puberty, because of heightened levels of androgens.
Sebaceous glands secrete the oily, waxy substance called sebum (Latin: fat, tallow) that is made of triglycerides, wax esters, squalene, and metabolites of fat-producing cells. Sebum is produced in a holocrine process, in which cells within the sebaceous gland disintegrate as they release the sebum. The cells are constantly replaced by mitosis at the base of the duct.
Sebum secreted by the sebaceous gland is primarily composed of tryglycerides (~41%), wax esters (~26%), and squalene (~12%)., although the composition of sebum varies across species. Wax esters, like squalene, are unique to sebum and not produced 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 odors.
Immune function and nutrition
Sebaceous glands are part of the body's integumentary system and serve to protect the body against germs. Sebaceous lipids make an important contribution in maintaining the integrity of the skin barrier, and express pro-inflammatory and anti-inflammatory properties. Sebum may act as a delivery system for antioxidants, antimicrobial lipids, pheromones, and hydration of the stratum corneum. The insoluble fatty acids contained within sebum have broad antimicrobial activity. Additionally, sebaceous gland secretion provides Vitamin E to the upper layers of facial skin.
Sebaceous secretions in conjunction with apocrine glands also play an important thermoregulatory role. In hot conditions, the secretions emulsify and foment formation of and prevent the loss of sweat drops from the skin. In colder conditions, sebum repels rain from skin and hair.
Unique sebaceous glands
The areolar glands surrounding the nipple in the female breast; volatile compounds secreted from the glands are thought to serve as an olfactory stimulus for the newborn. During pregnancy and lactation these glands, also called Montgomery's glands, become enlarged. Meibomian glands, which secrete a form of sebum called meibum onto the eye, that slows the evaporation of tears. Earwax is partly composed of sebum.
Acne is a very common problem, particularly during puberty in teenagers, and is thought to relate to an increased production of sebum due to hormonal factors. The increased production of sebum can lead to a blockage of the sebaceous gland duct. A result of this is the appearance of a comedo, (commonly called a blackhead or a whitehead) which can then result in a propensity to infection, particularly by the bacteria propionibacterium acnes. This can inflame the comedones which then change into the characteristic acne lesions. Comedones, generally occur on the areas with more sebacious glands, particularly the face, shoulders, upper chest and back. Comedomes may be 'black' or 'white' depending on whether the entire pilosebaceous unit, or just the sebaceous duct, is blocked.
When treated, acne is generally treated with antibiotics or isotretinoin. Isotretinoin reduces the amount of sebum produced by the sebaceous glands. A better treatment is claimed for SMT D002. Another treatment seen to be effective in females is Spironolactone
Other conditions that affect the sebaceous glands include:
- Seborrhoea refers to overactive sebaceous glands, a cause of oily skin  or hair.
- Sebaceous hyperplasia, referring to excessive proliferation of the cells within the glands, and visible macroscopically as small papules on the skin, particularly on the forehead, nose and cheeks.
- Seborrhoeic dermatitis, a severe form of dermatitis effected by changes in the sebaceous glands.
- Sebaceous adenoma, a benign slow-growing tumour which may however in rare cases be a precursor to a cancer syndrome known as Muir-Torre syndrome.
- Sebaceous carcinoma, an uncommon and aggressive cutaneous tumour.
- Sebaceous cyst is a term used to refer to both an epidermoid cyst and a pilar cyst, though neither of these contain sebum, only keratin and do not originate in the sebaceous gland and so are not true sebaceous cysts. A true sebaceous cyst is relatively rare and is known as a steatocystoma.
The word sebaceous – consisting of sebum, was first termed in 1728 and comes from the Latin for tallow.
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