Sebaceous gland
The sebaceous glands are microscopic (tiny) glands in the skin which produce an oily/waxy substance, called sebum, to lubricate (oil) the skin and hair.[1] In people, these glands are found in greatest amounts on the face and scalp, joined near the top inside hair follicles or sweat pores. However, they are in all skin areas except the palms and soles (undersides) of the feet.[2] There are different kinds of these glands and sebum. In the eyelids, meibomian sebaceous glands emit a special kind of sebum into tears. There are several related medical conditions, including: acne, sebaceous cysts, hyperplasia, sebaceous adenoma and sebaceous gland carcinoma (see section below: Diseases). Washing skin or hair with plain detergent can cut the amount of sebum in oily skin. Also, water temperature over 84 °F (29 °C) can keep sebum melted during a wash.
| Sebaceous gland | |
|---|---|
 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.) | |
| Details | |
| Identifiers | |
| Latin | glandula sebacea |
| MeSH | D012627 |
| TA | A16.0.00.030 A15.2.07.044 |
| FMA | 59160 |
| Anatomical terminology | |
Locations and morphology
changeAs a branched kind of acinar gland, the sebaceous glands are found in people all over the skin except in the palms of the hands and soles of the feet.
Sebaceous glands are often found in hair-covered areas, where they are connected near the top of hair follicles (see image at top). The glands deposit sebum on each hair, 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 the eyelids, nose, penis, labia minora, and nipples. Here, the sebum travels along ducts which end inside sweat pores near the surface of the skin.
At the rim of the eyelids, meibomian glands are a specialized form of sebaceous gland. They secrete a form of sebum (known as meibum) onto the eye, which slows the drying of tears.
Sebum
changeSebaceous glands produce the oily, waxy substance called sebum (Latin, meaning fat or tallow) that is made of fat (lipids), wax, and the leftover dead fat-producing cells.[3] In the glands, sebum is produced within specialized cells and is released as these cells burst; sebaceous glands are thus classified as holocrine glands.
Sebum has no odor, but its bacterial breakdown can produce a bad odor. Sebum is the cause of some people experiencing "oily" hair,[4] as in hot weather or if not washed for several days. Earwax is partly composed of sebum. Sebum can be washed using plain detergent, to dissolve the waxy material in the skin. Also, water for washing should be over 84 °F (29 °C) to keep the sebum molten.
Excess sebum has been linked to eating red meats, fried and oily foods and some other type of foods but different research shows multiple theories about it.
Function
changeIt is commonly believed that sebum acts to save skin from drying or to waterproof hair and skin. But some scientists have contended that "low levels of sebaceous gland activity are not correlated with dry skin", and it may serve little or no purpose in modern humans.[5]
Composition
changeThe substances in sebum vary between species. In people, the lipid content is as follows:[6]
| Percent composition | Substance |
| 25% | wax monoesters |
| 41% | triglycerides |
| 16% | free fatty acids |
| 12% | squalene |
Also, sapienic acid is a sebum fatty acid that is found only in people, not in animals.
Control
changeThe following medicines have been shown to reduce the flow of sebum from skin:
- Isotretinoin[7]
- SMT D002[8]
- Spironolactone[9] (suitable for females only)
Changes during development
changeThe sebaceous glands of a human fetus in utero secrete a substance called Vernix caseosa, a "waxy" or "cheesy" white substance covering the skin of newborn babies.
The activity of the sebaceous glands increases during puberty because of heightened levels of androgens. 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.
Diseases
changeSebaceous glands are involved in skin problems such as oily skin, acne, and keratosis pilaris. In the skin pores, sebum and keratin can create a hyperkeratotic plug called a "microcomedo" which can block a pore and cause pimples. The prescription medicine 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 also cause acne.[10] The sebaceous gland is stimulated due to some steroid conversion into dihydrotestosterone. This event may cause serious acne on the face, neck, chest, back and shoulders.
A blocked sebaceous gland can result in a sebaceous cyst.
A condition involving enlarged sebaceous glands is known as sebaceous hyperplasia.
Sebaceous gland carcinoma is a very uncommon 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.[11]
Importance to other animals
changeCertain 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
change-
Base of pilosebaceous unit -
Joining of sebaceous glands into hair shaft
-
Sagittal section through the upper eyelid. -
A hair follicle with the connected structures.
References
change- ↑ 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.
- ↑ "Exercise 15: Hair", VT.edu, 2008, webpage: Vetmed-lab15.
- ↑ "Hair Care: An Illustrated Dermatologic Handbook", Zoe Diana Draelos, Zoe Kececioglu Draelos, 2005, p.26, web: Books-Google-5QC: oily hair & detergents.
- ↑ Downing DT, Stewart ME, Wertz PW, Colton SW, Abraham W, Strauss JS (March 1987). "Skin lipids: an update". The Journal of Investigative Dermatology. 88 (3 Suppl): 2s–6s. doi:10.1111/1523-1747.ep12468850. PMID 2950180.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ↑ 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.
- ↑ 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.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ↑ "RNS Seborrhoea trial", summitplc.com, PDF file: RNS-PDF Archived 2011-07-16 at the Wayback Machine.
- ↑ 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. S2CID 24518527.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ↑ Official exposed skin care reviews Archived 2013-03-20 at the Wayback Machine website
- ↑ 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.
Other websites
change- Histology at Boston University: 08801loa - "Integument: scalp".
- MeSH name: Sebaceous+Glands.