Sebaceous gland

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Sebaceous gland
HairFollicle.png
Schematic view of hair follicle and sebaceous gland.
Skin.png
Cross-section of all skin layers. A hair follicle with associated structures. (Sebaceous glands labeled at center left.)
Latin glandula sebacea
Gray's p.1069
MeSH Sebaceous+glands

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.[1] 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.[2] 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.

Structure[edit]

Location[edit]

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.[3]

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.[3]

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.[3] 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.[4]

Development[edit]

Sebaceous glands are first visible from the 13th to the 16th week of fetal development, as bulgings off hair follicles.[5] 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.[4]

The sebaceous glands of a human fetus secrete a substance called vernix caseosa, a waxy, translucent white substance coating the skin of newborns.[6] 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.[5]

Function[edit]

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.[7][8] 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.[3]

Sebum[edit]

Sebum secreted by the sebaceous gland is primarily composed of tryglycerides (~41%), wax esters (~26%), and squalene (~12%).,[6][9] although the composition of sebum varies across species.[9] Wax esters, like squalene, are unique to sebum and not produced anywhere else in the body.[4] Sapienic acid is a sebum fatty acid that is unique to humans, and is implicated in the development of acne.[10]

Sebum is odorless, but its breakdown by bacteria can produce odors.[11]

Some hormones, including androgens such as testosterone and estrogen, as well as progesterone, increase the rate of sebum secretion.[12]

Immune function and nutrition[edit]

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.[13][14][15] Sebum may act as a delivery system for antioxidants, antimicrobial lipids, pheromones, and hydration of the stratum corneum.[16] The insoluble fatty acids contained within sebum have broad antimicrobial activity.[16][17] Additionally, sebaceous gland secretion provides Vitamin E to the upper layers of facial skin.[18]

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.[15]

Unique sebaceous glands[edit]

During the last three months of pregnancy, it is known that sebaceous glands produce vernix caseosa which protects the embryonic skin from amniotic fluid.[19]

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.[20] Meibomian glands, which secrete a form of sebum called meibum onto the eye, that slows the evaporation of tears.[21] Earwax is partly composed of sebum.[22]

Clinical significance[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.

Acne[edit]

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.[12]

When treated, acne is generally treated with antibiotics or isotretinoin.[12] Isotretinoin reduces the amount of sebum produced by the sebaceous glands.[23] A better treatment is claimed for SMT D002.[24] Another treatment seen to be effective in females is Spironolactone[25]

Should the usual treatments fail, the presence of the Demodex mite could be looked for as the possible cause.[26]

Other[edit]

Other conditions that affect the sebaceous glands include:

History[edit]

The word sebaceous – consisting of sebum, was first termed in 1728 and comes from the Latin for tallow.[31]

Other animals[edit]

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

Sebaceous adenitis is an autoimmune disease affecting the sebaceous gland, known mainly to occur in dogs.

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. ^ a b c d 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. 
  4. ^ a b c d e 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. 
  5. ^ a b Thiboutot, D (2004 Jul). "Regulation of human sebaceous glands.". The Journal of investigative dermatology 123 (1): 1–12. PMID 15191536. 
  6. ^ a b Thody, A. J.; Shuster, S. (1989). "Control and Function of Sebaceous Glands". Physiological Reviews 69 (2): 383–416. PMID 2648418. 
  7. ^ "Exercise 15: Hair", VT.edu, 2008, webpage: Vetmed-lab15.
  8. ^ 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. 
  9. ^ a b 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. 
  10. ^ 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. 
  11. ^ a b Draelos, Zoe Diana (2005). Hair care : an illustrated dermatologic handbook. London ;New York: Taylor & Francis. p. 26. ISBN 978-1-84184-194-6. 
  12. ^ a b c Britton, the editors Nicki R. Colledge, Brian R. Walker, Stuart H. Ralston ; illustated 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. 
  13. ^ "Why do we have apocrine and sebaceous glands?". 10 May 2011. PMC 1281456. 
  14. ^ 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. 
  15. ^ 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. 
  16. ^ 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. 
  17. ^ "Thematic Review Series: Skin Lipids. Antimicrobial lipids at the skin surface". 10 May 2011. 
  18. ^ 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. 
  19. ^ 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. 
  20. ^ 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. 
  21. ^ McCulley, JP; Shine, WE (2004 Mar). "The lipid layer of tears: dependent on meibomian gland function.". Experimental eye research 78 (3): 361–5. PMID 15106913. 
  22. ^ Roeser, RJ; Ballachanda, BB (1997 Dec). "Physiology, pathophysiology, and anthropology/epidemiology of human earcanal secretions.". Journal of the American Academy of Audiology 8 (6): 391–400. PMID 9433685. 
  23. ^ 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. 
  24. ^ http://www.summitplc.com/uploads/RNSSeborrhoeatrialFINAL.pdf
  25. ^ 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. 
  26. ^ 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. 
  27. ^ 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. 
  28. ^ 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. 
  29. ^ 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. 
  30. ^ 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. 
  31. ^ Harper, Douglas. "Sebaceous". Etymology Online. Retrieved 5 April 2014. 

External links[edit]