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. Fordyce spots are ectopic (misplaced) sebaceous glands found usually on the lips, gums and inner cheeks. Areolar glands surround the female nipples. Preputial glands also known as Tyson's glands are found in the genitals of both sexes of mice and rats.
There are several related medical conditions, including acne, sebaceous cysts, hyperplasia and sebaceous adenoma. These are usually attributable to overactive sebaceous glands which produce an excess of sebum.
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 (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 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.
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 waterproofs and 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 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 is seen to become more lipid and in coating the hair and skin, rain is effectively repelled.
Sebum is produced in a holocrine process, in which 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 secreted by the sebaceous gland in humans, is primarily composed of triglycerides (~41%), wax esters (~26%), squalene (~12%), and free fatty acids (~16%). 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 strong odors.
Immune function and nutrition
Sebaceous glands are part of the body's integumentary system and serve to protect the body against germs. Sebaceous glands are responsible for the secretion of acids that form the acid mantle. This is a very fine, slightly acidic film on the surface of the skin which acts as a barrier to bacteria, viruses and other potential contaminants that might penetrate the skin. The pH of the skin is between 4.5 and 6.2, and this acidity helps to neutralise the primarily alkaline nature of contaminants.
Sebaceous lipids make an important contribution in maintaining the integrity of the skin barrier, and express both 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.
Unique sebaceous glands
During the last three months of fetal development, it is known that sebaceous glands of the fetus, produce vernix caseosa also referred to as vernix. This is a waxy white substance which coats the skin and serves to protect the skin of the fetus from amniotic fluid.
The areolar glands are in the areola that surrounds the nipple in the female breast. These glands secrete an oily fluid which lubricates the nipple and also secrete volatile compounds that 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, in the eyelids, secrete a form of sebum called meibum onto the eye, that slows the evaporation of tears. It also serves to create an airtight seal when the eyes are closed and its lipid quality also prevents the eyelids from sticking together. The meibomian glands are also known as tarsal glands, Zeis glands and palpebral glands. They attach directly to the follicles of the eyelashes which are arranged vertically within the tarsal plates of the eyelids.
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 sebaceous glands, particularly the face, shoulders, upper chest and back. Comedones may be 'black' or 'white' depending on whether the entire pilosebaceous unit, or just the sebaceous duct, is blocked. Sebaceous filaments which are innocuous build-ups of sebum can often be mistaken for whiteheads.
There are many treatments available for acne from reducing sugars in the diet, to medications that include antibiotics, benzoyl peroxide, retinoids and hormonal treatments. Retinoids reduce the amount of sebum produced by the sebaceous glands. Should the usual treatments fail, the presence of the Demodex mite could be looked for as the possible cause.
Other conditions that involve 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 chronic, usually mild form of dermatitis effected by changes in the sebaceous glands. In newborn infants seborrhoea dermatitis can occur as cradle cap.
- 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, meaning "consisting of sebum", was first termed in 1728 and comes from the Latin for tallow. Sebaceous glands have been documented since at least 1746 by Jean Astruc, who defined them as "the glands which separate the fat".:viii He describes them in the oral cavity, on the head, eyelids and ears, as "universally" acknowledged.,:22–25 viii Astruc describes them being blocked by "small animals" that are "implanted" in the excretory ducts,:64 and attributes their presence in the oral cavity to apthous ulcers, noting "these glands naturally [secrete] a viscous humour, which puts on various colours and consistencies... in its natural state is very mild, balsamic, and intended to wet and lubricate the mouth" :85–86 In The Principles of Physiology 1834, A. Combe noted that the glands were not present in the palms of the hands or soles of the feet.
The preputial glands of mice and rats are large modified sebaceous glands that produce pheromones used for territorial marking. These and the scent glands in the flanks of hamsters have a similar composition to human sebaceous glands, are androgen responsive, and have been used as a basis for study.
Sebaceous adenitis is an autoimmune disease affecting the sebaceous gland, known mainly to occur in dogs, particularly poodles and akitas, where it is thought to be generally autosomal recessively inherited. It has also been described in cats, and one report describes this condition in a rabbit. In these animals, it is a cause of hair loss, although the nature and distribution of the hair loss differs greatly.
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