Asebaceous gland oroil gland[1] is a microscopicexocrinegland in theskin that opens into ahair follicle to secrete an oily or waxy matter, calledsebum, which lubricates the hair and skin ofmammals.[2] In humans, sebaceous glands occur in the greatest number on theface andscalp, but also on all parts of the skin except the palms of thehands andsoles of thefeet. In theeyelids,meibomian glands, also called tarsal glands, are a type of sebaceous gland that secrete a special type of sebum intotears. Surrounding the femalenipples,areolar glands are specialized sebaceous glands for lubricating the nipples.Fordyce spots are benign, visible, sebaceous glands found usually on thelips,gums and innercheeks, andgenitals.
In humans, sebaceous glands are found throughout all areas of the skin, except the palms of thehands andsoles of thefeet.[3] There are two types of sebaceous glands: those connected tohair follicles and those that exist independently.[4]
Sebaceous glands are found in hair-covered areas, where they are connected tohair follicles. One or more glands may surround each hair follicle, and the glands themselves are surrounded byarrector pili muscles, forming a pilosebaceous unit. The glands have anacinar 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 thehair shaft. The structure, consisting of hair, hair follicles, arrector pili muscles, and sebaceous glands, is an epidermal invagination known as apilosebaceous unit.[4]
Sebaceous glands are first visible from the 13th to the 16th week offetal development, as bulgings off hair follicles.[6] Sebaceous glands develop from the same tissue that gives rise to theepidermis of the skin. Overexpression of the signalling factorsWnt,Myc andSHH all increase the likelihood of sebaceous gland presence.[5]
The sebaceous glands of a humanfetus secrete a substance calledvernix caseosa, a waxy, translucent white substance coating the skin ofnewborns.[7] 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 duringpuberty, due to heightened levels ofandrogens.[6]
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.[8] These cells express Na+ and Cl− ion channels,ENaC andCFTR (see Fig. 6 and Fig. 7 in reference[8]).
Sebaceous glands secrete the oily, waxy substance calledsebum (Latin for 'fat,tallow') that is made oftriglycerides,wax esters,squalene, andmetabolites of fat-producing cells. Sebum lubricates the skin and hair of mammals.[9] Sebaceous secretions in conjunction withapocrine glands also play an importantthermoregulatory role. In hot conditions, the secretions emulsify the sweat produced by theeccrine 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.[10][11]
Sebum is produced in aholocrine 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.[12][13] The cells are constantly replaced bymitosis at the base of the duct.[4]
Sebum is secreted by the sebaceous gland in humans. It is primarily composed oftriglycerides (≈41%),wax esters (≈26%),squalene (≈12%), andfree fatty acids (≈16%).[7][14] The composition of sebum varies across species.[14] Wax esters and squalene are unique to sebum and not produced as final products anywhere else in the body.[5]Sapienic acid is a sebum fatty acid that is unique to humans, and is implicated in the development of acne.[15] Sebum is odorless, but its breakdown by bacteria can produce strong odors.[16]
Sebaceous glands are part of the body'sintegumentary system and serve to protect the body against microorganisms. Sebaceous glands secrete acids that form theacid mantle. This is a thin, slightlyacidic film on the surface of theskin that acts as a barrier tomicrobes that might penetrate the skin.[20] ThepH of the skin is between 4.5 and 6.2,[21] an acidity that helps to neutralize thealkaline nature of contaminants.[22] Sebaceous lipids help maintain the integrity of the skin barrier[10][23][24] and supplyvitamin E to the skin.[25] Sebaceous glands actively participate in innate immunity by releasingcytokines like IL-1α, IL-6, and TNF-α.[26]
Theareolar glands are in theareola that surrounds the nipple in the female breast. These glands secrete an oily fluid that 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.[28]
Meibomian glands, in theeyelids, secrete a form of sebum calledmeibum onto theeye, that slows the evaporation oftears.[29] They also serve to create an airtight seal when the eyes are closed, and their lipid quality also prevents the eyelids from sticking together. They attach directly to the follicles of the eyelashes, which are arranged vertically within thetarsal plates of the eyelids.
Fordyce spots, or Fordyce granules, are ectopic sebaceous glands found on the genitals andoral mucosa. They show themselves as yellowish-whitemilia (milk spots).[30]
Earwax is partly composed of sebum produced by glands in the ear canal. These secretions areviscous and have a highlipid content, which provides good lubrication.[31]
Acne is a common occurrence, particularly duringpuberty inteenagers, 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. This can cause acomedo (commonly called ablackhead or awhitehead), which can lead to infection, particularly by the bacteriaCutibacterium acnes. This caninflame 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.[32]Sebaceous filaments—innocuous build-ups of sebum—are often mistaken forwhiteheads.
There are many treatments available for acne from reducing sugars in the diet, to medications that includeantibiotics,benzoyl peroxide,retinoids, and hormonal treatments.[32] Oral retinoids, principallyisotretinoin, reduce the amount of sebum produced by thesebaceous glands via induction ofapoptosis and are among the most effective agents, while topical retinoids produce mildexfoliation and decrease the adhesiveness ofkeratinocytes, thus normalizingkeratinization and reducing the buildup of dead skin cells which can clog pores and lead to microcomedone formation.[33] Should the usual treatments fail, the presence of theDemodex mite could be looked for as the possible cause.[34] However, considering Demodex mites thrive in sebum rich regions, oral isotretinoin should still be efficacious in such cases by destroying their living space.
Other conditions that involve the sebaceous glands include:
Seborrhoea refers to overactive sebaceous glands, a cause of oily skin[5] or hair.[16]
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.[35]
Seborrheic-like psoriasis (also known as "Sebopsoriasis",[37] and "Seborrhiasis") is a skin condition characterized bypsoriasis with an overlapping seborrheic dermatitis.[3]: 193
Sebaceous cyst is a term used to refer to both anepidermoid cyst and apilar 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 asteatocystoma.[39]
Nevus sebaceous, a hairless region or plaque on the scalp or skin, caused by an overgrowth of sebaceous glands. The condition is congenital and the plaque becomes thicker into adulthood.[40]
The wordsebaceous, meaning 'consisting of sebum', was first termed in 1728 and comes from the Latin for 'tallow'.[41] Sebaceous glands have been documented since at least 1746 byJean Astruc, who defined them as "...the glands which separate the fat."[42]: viii He describes them in theoral cavity and on thehead,eyelids, andears, as "universally" acknowledged.[42]: 22–25 viii Astruc describes them being blocked by "small animals" that are "implanted" in the excretory ducts[42]: 64 and attributes their presence in the oral cavity toapthous ulcers, noting that "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".[42]: 85–86 InThe Principles of Physiology 1834, Andrew Combe noted that the glands were not present in the palms of the hands or soles of the feet.[43]
Thepreputial glands ofmice andrats are large modified sebaceous glands that producepheromones used for territorial marking.[5] These and thescent glands in the flanks ofhamsters have a similar composition to human sebaceous glands, are androgen responsive, and have been used as a basis for study.[5] Some species of bat, including theMexican free-tailed, have a specialized sebaceous gland occurring on the throat called a "gular gland".[45] This gland is present more frequently in males than females, and it is hypothesized that the secretions of the gland are used for scent-marking.[46]
Sebaceous adenitis is anautoimmune disease that affects sebaceous glands. It is mainly known to occur indogs, particularlypoodles andakitas, where it is thought to be generallyautosomal recessively inherited. It has also been described in cats, and one report describes this condition in a rabbit. In these animals, it causes hair loss, though the nature and distribution of the hair loss differs greatly.[47]
^abJames, William D.; Berger, Timothy; Elston, Dirk M. (2006).Andrews' Diseases of the Skin: Clinical dermatology. Saunders Elsevier. p. 7.ISBN978-0-7216-2921-6.
^abcdYoung, Barbra; Lowe, James S; Stevens, Alan; Heath, John W; Deakin, Philip J (March 2006).Wheater's Functional Histology (5 ed.). Elsevier Health Sciences. pp. 175–178.ISBN978-0-443-06850-8.
^Wilkinson, P.F.; Millington, R. (1983).Skin (Digitally printed ed.). Cambridge: Cambridge university press. p. 151.ISBN978-0-521-24122-9.
^Monika-Hildegard Schmid-Wendtner; Korting Schmid-Wendtner (2007).Ph and Skin Care. ABW Wissenschaftsverlag. pp. 31–.ISBN978-3-936072-64-8. Retrieved19 June 2012.
^McCulley, JP; Shine, WE (March 2004). "The lipid layer of tears: dependent on meibomian gland function".Experimental Eye Research.78 (3):361–5.doi:10.1016/s0014-4835(03)00203-3.PMID15106913.
^Dorland's (2012).Dorland's Illustrated Medical Dictionary (32nd ed.). Elsevier Saunders. p. 802.ISBN978-0-19-856878-0.
^Roeser, RJ; Ballachanda, BB (December 1997). "Physiology, pathophysiology, and anthropology/epidemiology of human earcanal secretions".Journal of the American Academy of Audiology.8 (6):391–400.PMID9433685.
^abColledge N, Walker B, Ralston S, eds. (2010).Davidson's principles and practice of medicine (21st ed.). Edinburgh: Churchill Livingstone/Elsevier. pp. 1267–1268.ISBN978-0-7020-3085-7.
^Farrell LN, Strauss JS, Stranieri AM (December 1980). "The treatment of severe cystic acne with 13-cis-retinoic acid. Evaluation of sebum production and the clinical response in a multiple-dose trial".Journal of the American Academy of Dermatology.3 (6):602–11.doi:10.1016/S0190-9622(80)80074-0.PMID6451637.
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