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 (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 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 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 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.
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.
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 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 describe 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.
- Dellmann's textbook of veterinary histology (405 pages), Jo Ann Coers Eurell, Brian L. Frappier, 2006, p.29, weblink: Books-Google-RTOC.
- James, William D.; Berger, Timothy; Elston, Dirk M. (2006). Andrews' diseases of the skin: clinical dermatology. Saunders Elsevier. p. 7. ISBN 978-0-7216-2921-6.
- Deakin, Barbara Young ... [et al.] ; drawings by Philip J. (2006). Wheater's functional histology : a text and colour atlas (5th ed. ed.). [Edinburgh?]: Churchill Livingstone/Elsevier. pp. 175–178, 413. ISBN 978-0-443-06850-8.
- Smith, K. R.; Thiboutot, D. M. (2007). "Thematic Review Series: Skin Lipids. Sebaceous Gland Lipids: Friend Or Foe?". Journal of Lipid Research 49 (2): 271–281. doi:10.1194/jlr.R700015-JLR200. PMID 17975220.
- Thiboutot, D (Jul 2004). "Regulation of human sebaceous glands.". The Journal of investigative dermatology 123 (1): 1–12. doi:10.1111/j.1523-1747.2004.t01-2-.x. PMID 15191536.
- Thody, A. J.; Shuster, S. (1989). "Control and Function of Sebaceous Glands". Physiological Reviews 69 (2): 383–416. PMID 2648418.
- "Exercise 15: Hair", VT.edu, 2008, webpage: Vetmed-lab15.
- Lampe, M.A.; A.L. Burlingame, J. Whitney, M.L. Williams, B.E. Brown, E. Roitman, and M. Elias (1983). "Human stratum corneum lipids: characterization and regional variations". J. Lipid Res. 24: 120–130.
- Cheng JB, Russell DW (September 2004). "Mammalian Wax Biosynthesis: II. EXPRESSION CLONING OF WAX SYNTHASE cDNAs ENCODING A MEMBER OF THE ACYLTRANSFERASE ENZYME FAMILY". The Journal of Biological Chemistry 279 (36): 37798–807. doi:10.1074/jbc.M406226200. PMC 2743083. PMID 15220349.
- Webster, Guy F.; Anthony V. Rawlings (2007). Acne and Its Therapy. Basic and clinical dermatology 40. CRC Press. p. 311. ISBN 0-8247-2971-4.
- Draelos, Zoe Diana (2005). Hair care : an illustrated dermatologic handbook. London ;New York: Taylor & Francis. p. 26. ISBN 978-1-84184-194-6.
- Britton, the editors Nicki R. Colledge, Brian R. Walker, Stuart H. Ralston ; illustrated by Robert (2010). Davidson's principles and practice of medicine. (21st ed. ed.). Edinburgh: Churchill Livingstone/Elsevier. pp. 1267–1268. ISBN 978-0-7020-3085-7.
- "Why do we have apocrine and sebaceous glands?". 10 May 2011. PMC 1281456.
- Youn, S. W. (2010). "The Role of Facial Sebum Secretion in Acne Pathogenesis: Facts and Controversies". Clinics in Dermatology 28 (1): 8–11. doi:10.1016/j.clindermatol.2009.03.011. PMID 20082943.
- Zouboulis, C. C. (2004). "Acne and Sebaceous Gland Function". Clinics in Dermatology 22 (5): 360–366. doi:10.1016/j.clindermatol.2004.03.004. PMID 15556719.
- Mackenna, R. M. B.; Wheatley, V. R.; Wormall, A. (1950). "The Composition of the Surface Skin Fat ('Sebum') from the Human Forearm". Journal of Investigative Dermatology 15 (1): 33–47. doi:10.1038/jid.1950.69. PMID 14774573.
- "Thematic Review Series: Skin Lipids. Antimicrobial lipids at the skin surface". 10 May 2011.
- Thiele, Jens J.; Weber, Stefan U.; Packer, Lester (1999). "Sebaceous Gland Secretion is a Major Physiologic Route of Vitamin E Delivery to Skin". Journal of Investigative Dermatology 113 (6): 1006–1010. doi:10.1046/j.1523-1747.1999.00794.x. PMID 10594744.
- Zouboulis, Christos C.; et al. , Jens Malte et al. (2008). "Frontiers in Sebaceous Gland Biology and Pathology". Experimental Dermatology 17 (6): 542–551. doi:10.1111/j.1600-0625.2008.00725.x. PMID 18474083.
- Doucet, Sébastien; Soussignan, Robert; Sagot, Paul; Schaal, Benoist (2009). "The Secretion of Areolar (Montgomery's) Glands from Lactating Women Elicits Selective, Unconditional Responses in Neonates". In Hausberger, Martine. PLoS ONE 4 (10): e7579. doi:10.1371/journal.pone.0007579. PMC 2761488. PMID 19851461.
- McCulley, JP; Shine, WE (Mar 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. PMID 15106913.
- Roeser, RJ; Ballachanda, BB (Dec 1997). "Physiology, pathophysiology, and anthropology/epidemiology of human earcanal secretions.". Journal of the American Academy of Audiology 8 (6): 391–400. PMID 9433685.
- 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. PMID 6451637.
- Goodfellow A, Alaghband-Zadeh J, Carter G et al. (August 1984). "Oral spironolactone improves acne vulgaris and reduces sebum excretion". The British Journal of Dermatology 111 (2): 209–14. doi:10.1111/j.1365-2133.1984.tb04045.x. PMID 6235834.
- Ya-e Zhao, et al. (2011). "Facial dermatosis associated with Demodex: a case-control study". Journal of Zhejiang University-SCIENCE B 12 (8): 1008–1015. doi:10.1631/jzus.B1100179. PMC 3232434. PMID 22135150.
- James, William D.; Berger, Timothy G.; et al. (2006). Andrews' Diseases of the Skin: clinical Dermatology. Saunders Elsevier. p. 662. ISBN 0-7216-2921-0.
- Dessinioti, C.; Katsambas, A. (2013). "Seborrheic dermatitis: etiology, risk factors, and treatments: facts and controversies.". Clin Dermatol 31 (4): 343–51. doi:10.1016/j.clindermatol.2013.01.001. PMID 23806151.
- Nelson BR, Hamlet KR, Gillard M, Railan D, Johnson TM (July 1995). "Sebaceous carcinoma". J. Am. Acad. Dermatol. 33 (1): 1–15; quiz 16–8. doi:10.1016/0190-9622(95)90001-2. PMID 7601925.
- Neville BW, Damm DD, Allen CA, Bouquot JE. (2002). Oral & maxillofacial pathology (2nd ed.). Philadelphia: W.B. Saunders. p. 31. ISBN 0-7216-9003-3.
- Harper, Douglas. "Sebaceous". Etymology Online. Retrieved 5 April 2014.
- Astruc, Jean (1746). A General and Compleat Treatise on All the Diseases Incident to Children. J. Nourse.
- Rosenthal, Stanley A; Furnari, Domenica. "Slide Agglutination as a Presumptive Test in the Laboratory Diagnosis of Candida Albicans1". The Journal of Investigative Dermatology 31 (5): 251–253. doi:10.1038/jid.1958.50.
- Mecklenburg, edited by Lars; Linek, Monika (2009). Hair loss disorders in domestic animals. Ames, Iowa: Wiley-Blackwell. pp. Chapter 4.8. Sebaceous adenitis. ISBN 9780813819341.
- Histology image: 08801loa - Histology Learning System at Boston University - "Integument: scalp"
- Sebaceous Glands at the US National Library of Medicine Medical Subject Headings (MeSH)