Sebaceous gland

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Sebaceous gland
HairFollicle.png
Schematic view of hair follicle & sebaceous gland.
Skin.png
Cross-section of all skin layers. A hair follicle with associated structures. (Sebaceous glands labeled at center left.)
Latinglandula sebacea
Gray'ssubject #234 1069
MeSHSebaceous+glands
 
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Sebaceous gland
HairFollicle.png
Schematic view of hair follicle & sebaceous gland.
Skin.png
Cross-section of all skin layers. A hair follicle with associated structures. (Sebaceous glands labeled at center left.)
Latinglandula sebacea
Gray'ssubject #234 1069
MeSHSebaceous+glands

The sebaceous glands are microscopic glands in the skin that secrete an oily/waxy matter, called sebum, to lubricate and waterproof the skin and hair of mammals.[1] In humans, they are found in greatest abundance on the face and scalp, though they are distributed throughout all skin sites except the palms and soles.[2] In the eyelids, meibomian sebaceous glands secrete a special type of sebum into tears. There are several related medical conditions, including acne, sebaceous cysts, hyperplasia, sebaceous adenoma and sebaceous gland carcinoma (see section below: Pathology).

Locations and morphology[edit]

A branched type of acinar gland, the sebaceous glands exist in humans throughout the skin except in the palms of the hands and soles of the feet. Sebaceous glands can usually be found in hair-covered areas, where they are connected to hair follicles (see image at top). 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 known as a pilosebaceous unit. Sebaceous glands are also found in non-haired areas (glabrous skin) of eyelids, nose, penis, labia minora and nipples. Here, the sebum traverses ducts that terminate in sweat pores on the surface of the skin.[citation needed] At the rim of the eyelids, meibomian glands are a specialized form of sebaceous gland. They secrete a form of sebum (called meibum) onto the eye, slowing the evaporation of tears.

Sebum[edit]

Sebaceous glands secrete the oily, waxy substance called sebum (Latin, meaning fat or tallow) that is made of triglyceride oils, wax, squalene, and metabolytes of fat-producing cells.[3][4] In the glands, sebum is produced within specialized cells and is released as these cells burst; sebaceous glands are thus classified as holocrine glands. Seborrhoea is the name for the condition of greasy skin caused by excess sebum.[5]

Sebum keeps hair and skin supple. Sebum is odorless, but its bacterial breakdown can produce odors. Sebum is the cause of some people's experiencing "oily" hair,[6] as in hot weather or if not washed for several days. Earwax is partly composed of sebum.


Function[edit]

All of the sebaceous glands in humans have been demonstrated to show similarity in structure and secrete sebum by a holocrine process. Sebum secreted by the sebaceous gland is primarily composed of tryglycerides, wax esters, and squalene.[7] Wax esters, like squalene, are unique to sebum and not produced anywhere else in the body.[8] Sebum also contains 45% water-insoluble fatty acids known to have broad antimicrobial activity.[9][10] Additionally, sebaceous gland secretion provides Vitamin E to the upper layers of facial skin.[11] Sebaceous lipids contribute to maintaining the integrity of the skin barrier, and express pro-inflammatory and anti-inflammatory properties.[12][13][14] Recent research suggests that sebum may represent a delivery system for antioxidants, antimicrobial lipids, pheromones, and hydration of stratum corneum.[9] During the last gestation trimester, it is known that sebaceous glands produce vernix caseosa which protects the embryonic skin from amniotic water.[15] 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.[14] Increased facial surface sebum secretion is also associated with the development of acne.[13]

Composition[edit]

The composition of sebum varies across species. In humans, the lipid content is as follows:[16]

Percent compositionSubstance
25%wax monoesters
41%triglycerides
16%free fatty acids
12%squalene

Sapienic acid is a sebum fatty acid that is unique to humans.

Control[edit]

The following treatments have been shown to reduce sebum secretion rates:

Changes during development[edit]

The sebaceous glands of a human fetus in utero secrete a substance called Vernix caseosa, a "waxy" or translucent white substance coating the skin of newborns.

The activity of the sebaceous glands increases during puberty because of heightened levels of androgens, producing smegma. In males, sebaceous glands begin to appear predominantly on the penis, on the shaft and around the rim of the penile head during and after puberty. This is however normal, not to be confused with an STD. In females, they appear predominantly in the labia minora.

Pathology[edit]

Sebaceous glands are involved in skin problems such as acne and keratosis pilaris. In the skin pores, sebum and keratin can create a hyperkeratotic plug called a "microcomedone". The prescription drug isotretinoin significantly reduces the amount of sebum produced by the sebaceous glands, and is used to treat acne.

The extreme use (up to 10 times doctor-prescribed amounts) of anabolic steroids by bodybuilders, for muscle gain can cause acne. The sebaceous gland is stimulated due to some steroids conversion into dihydrotestosterone. This may cause serious acne on the face, neck, chest, back and shoulders.

Epidermal cysts are sometimes incorrectly called "sebaceous cysts" but they do not actually involve the sebaceous gland. (Instead, they are collection of keratin and dead keratinocytes which develop within the epidermal skin layer.)

A condition involving enlarged sebaceous glands is known as sebaceous hyperplasia.

Sebaceous gland carcinoma is a rare and aggressive form of cancer involving the sebaceous glands; sebaceous adenoma is a more benign neoplasm of the sebaceous glands.

Sebum can also build up around body piercings.[20]

Importance to other animals[edit]

Demodex mite

Certain species of Demodex mites feed on sebum and are commonly found in the sebaceous glands of mammals, including those of humans.

The preputial glands of mice and rats are large modified sebaceous glands that produce pheromones.

Additional images[edit]

See also[edit]

References[edit]

  1. ^ Dellmann's textbook of veterinary histology (405 pages), Jo Ann Coers Eurell, Brian L. Frappier, 2006, p.29, weblink: Books-Google-RTOC.
  2. ^ 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. 
  3. ^ "Exercise 15: Hair", VT.edu, 2008, webpage: Vetmed-lab15.
  4. ^ 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. 
  5. ^ OED. 2nd edition (1989). Online version (November 2010). Oxford University Press Retrieved 2011-02-28. 
  6. ^ "Hair Care: An Illustrated Dermatologic Handbook", Zoe Diana Draelos, Zoe Kececioglu Draelos, 2005, p.26, web: Books-Google-5QC: oily hair & detergents.
  7. ^ Thody, A. J.; Shuster, S. (1989). "Control and Function of Sebaceous Glands". Physiological Reviews 69 (2): 383–416. PMID 2648418. 
  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. 
  9. ^ a b 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. 
  10. ^ "Thematic Review Series: Skin Lipids. Antimicrobial lipids at the skin surface". 10/5/2011. 
  11. ^ 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. 
  12. ^ "Why do we have apocrine and sebaceous glands?". 10/5/2011. PMC 1281456. 
  13. ^ a b 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. 
  14. ^ a b 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. 
  15. ^ 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. 
  16. ^ 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. 
  17. ^ 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. 
  18. ^ http://www.summitplc.com/uploads/RNSSeborrhoeatrialFINAL.pdf
  19. ^ 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. 
  20. ^ Playe, Stephen J (July 2002). "Infectious Complications of Body Art: Infection is reported in about 1% of tattoos and in up to 45% of piercings, depending on the technique employed, body location, and after care". Emergency Medicine News 24 (7): 10–3. doi:10.1097/01.EEM.0000334232.52899.06 (inactive 2010-06-14). 

External links[edit]