Mammary gland
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2008-07-14T13:25:05Z
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{{Cleanup|date=November 2007}}
{{Infobox Anatomy
| Name = Mammary gland
| Latin = glandula mammaria
| GraySubject = 271
| GrayPage = 1267
| Image = illu_breast_anatomy.jpg
| Caption = Cross section of the breast of a human female.
| Image2 = Dissected lactating breast gray1172.png
| Caption2 = Dissection of a lactating breast.<br>1 - [[Fat]]<br>2 - [[Lactiferous duct]]/lobule<br>3 - [[Lobule]]<br>4 - [[Connective tissue]]<br>5 - [[Sinus (anatomy)|Sinus]] of lactiferous duct<br>6 - [[Lactiferous duct]]
| System =
| MeshName =
| MeshNumber =
| DorlandsPre = g_06
| DorlandsSuf = 12392474
}}
'''Mammary glands''' are the [[organ (anatomy)|organ]]s that, in [[mammal]]s, produce [[milk]] for the sustenance of the young. These [[exocrine gland]]s are enlarged and modified [[sweat gland]]s and give mammals their name. The mammary glands of domestic mammals containing more than two breasts are called '''dugs'''.
==Humans==
{{main|breast}}
===Structure===
The basic components of the mammary gland are the ''alveoli'' (hollow cavities, a few millimetres large) lined with milk-secreting [[cuboidal cells]] and surrounded by [[myoepithelial cell]]s. These alveoli join up to form groups known as ''lobules'', and each lobule has a ''lactiferous duct'' that drains into openings in the [[nipple]]. The myoepithelial cells can contract, similar to [[muscle]] cells, and thereby push the milk from the alveoli through the lactiferous ducts toward the nipple, where it collects in widenings (''sinuses'') of the ducts. As the infant begins to suck, the hormonally mediated "let down reflex" ensues and the mother's milk is secreted into – not sucked from the gland by – the baby's mouth.
All the milk-secreting tissue leading to a single lactiferous duct is called a "simple mammary gland"; a "complex mammary gland" is all the simple mammary glands serving one nipple. Humans normally have two complex mammary glands, one in each [[breast]], and each complex mammary gland consists of 10–20 simple glands. The presence of more than two nipples is known as [[polythelia]] and the presence of more than two complex mammary glands as [[Accessory breast|polymastia]].
===Development and hormonal control===
Mammary glands exist in both sexes, but are rudimentary until [[puberty]] when, in females, they begin to develop in response to [[ovarian]] hormones. [[Estrogen]] promotes formation, whereas [[testosterone]] inhibits it.
At the time of [[childbirth|birth]], the baby has lactiferous ducts but no alveoli. Little branching occurs before puberty when ovarian estrogens stimulate branching differentiation of the ducts into spherical masses of cells that will become alveoli. True secretory alveoli only develop in [[pregnancy]], where rising levels of estrogen and [[progesterone]] cause further branching and differentiation of the duct cells, together with an increase in [[adipose tissue]] and a richer [[blood flow]].
[[Colostrum]] is secreted in late pregnancy and for the first few days after giving birth. True milk secretion ([[lactation]]) begins a few days later due to a reduction in circulating [[progesterone]] and the presence of the hormone [[prolactin]]. The suckling of the baby causes the release of the hormone [[oxytocin]] which stimulates contraction of the myoepithelial cells.
===Breast cancer===
As described above, the cells of mammary glands can easily be induced to grow and multiply by hormones. If this growth runs out of control, [[cancer]] results. Almost all instances of [[breast cancer]] originate in the lobules or ducts of the mammary glands.
==Other mammals==
The number and positioning of complex and simple mammary glands varies widely in different mammals. The nipples and glands can occur anywhere along the two [[milk line]]s, two roughly-parallel lines along the [[ventral]] aspect of the body. In general most mammals develop mammary glands in pairs along these lines, with a number approximating the number of young typically birthed at a time. The number of nipples varies from 2 (in most primates) to 16 (in pigs). The [[Virginia Opossum]] has 13, one of the few mammals with an odd number<ref>[http://digitalcollections.fiu.edu/wild/transcripts/possums1.htm With the Wild Things - Transcripts<!-- Bot generated title -->]</ref><ref>[http://www.awrc.org/Baby%20Opossums.htm Raising Orphaned Baby Opossums<!-- Bot generated title -->]</ref>. The following table lists the number and position of glands normally found in a range of mammals:
<center>
{| class="wikitable"
!Species <ref>Merle Cunningham, ''Animal Science and Industry'' ISBN 9780130462565</ref>
!Anterior <br/>([[thoracic]])
!Intermediate <br/>([[abdominal]])
!Posterior <br/>([[Inguinal]])
!Total
|-
|[[Goat]], [[sheep]], [[horse]] <br/> [[guinea pig]]
|0
|0
|2
|2
|-
|[[Cattle]]
|0
|0
|4
|4
|-
|[[Cat]]
|2
|2
|2
|6
|-
|[[Dog]] <ref> Dog breeds vary in the number of mammary glands: larger breeds tend to have 5 pairs, smaller breeds have 4 pairs.</ref>
|4
|2
|2-4
|8-10
|-
|[[Mouse]]
|6
|0
|4
|10
|-
|[[Rat]]
|6
|2
|4
|12
|-
|[[Pig]]
|6
|6
|4
|16
|-
|[[Elephant]]s, [[primates]]
|2
|0
|0
|2
|-
|}
</center>
Male mammals typically have rudimentary mammary glands and nipples, with a few exceptions: male mice don't have nipples, and male horses lack nipples and mammary glands.{{Fact|date=May 2008}} The male Dyak fruit bat has lactating mammary glands. <ref>{{cite journal | last = Francis | first = Charles M. | authorlink = | coauthors = Anthony, Edythe L. P.; Brunton, Jennifer A.; Kunz, Thomas H. | title = Lactation in male fruit bats | journal = Nature | volume = 367 | issue = | pages = 691–692 | publisher = Nature Publishing Group | location = | date = [[1994-02-24]] | url = http://www.nature.com/nature/journal/v367/n6465/abs/367691a0.html | doi = 10.1038/367691a0 | accessdate = 2008-05-14}}</ref>
Mammary glands are true [[protein]] factories, and several companies have constructed [[Genetically modified organism|transgenic animal]]s, mainly [[goat]]s and [[cow]]s, in order to produce proteins for pharmaceutical use. Complex [[glycoprotein]]s such as [[monoclonal antibody|monoclonal antibodies]] or [[antithrombin]] cannot be produced by genetically engineered [[bacteria]], and the production in live mammals is much cheaper than the use of mammalian [[cell culture]]s.
==Evolution==
{{Expand-section|date=June 2008}}
It is believed that the mammary gland is a transformed sweat gland, more closely related to [[Apocrine sweat glands]]{{Fact|date=May 2007}}. There are many theories of how they evolved, but since they do not fossilize well, supporting such theories presents a major difficulty for the researcher. One theory proposes that mammary glands evolved from glands that were used to keep the eggs of early mammals moist.[http://nationalzoo.si.edu/ConservationAndScience/SpotlightOnScience/oftedalolav20030714.cfm]
== Gallery ==
<!--- It would be nice to have other unusual examples, such as whales, seals, etc.. -->
<center><gallery>
Image:Bezerro mamando REFON .jpg|[[Cow]]
Image:White Cat Nursing Four Kittens HQ.jpg|[[Cat]]
Image:Piglets1.jpg|[[Pig]]
Image:Baby sheep feeding.JPG|[[Sheep]]
Image:Kid feeding on mothers milk.jpg|[[Goat]]
Image:Elephant_breastfeading.jpg|[[Elephant]]
Image:Mousesuckling.jpg|[[Mouse]]
Image:Breastfeeding_infant.jpg|[[Human]]
</gallery></center>
==See also==
* [[Teat]]
* [[Breast]]
* [[Udder]]
* [[Milk line]]
* [[Breastfeeding]]
* [[Mammary tumor]] for mammary gland tumors in animals
* [[Witch's milk]]
* [[gynaecomastia]]
==External links==
* [http://web.archive.org/web/20051201011719/http://classes.aces.uiuc.edu/AnSci308/anatomycompar.html Comparative Mammary Gland Anatomy] by W. L. Hurley
* [http://jdc.jefferson.edu/cooper/61/ On the anatomy of the breast] by Sir Astley Paston Cooper (1840). Numerous drawings, in the public domain.
* {{eMedicineDictionary|mammary+gland}}
== References ==
{{reflist}}
{{Female reproductive system}}
[[Category:Breast]]
[[Category:Breastfeeding]]
[[Category:Exocrine system]]
[[Category:Female reproductive system]]
[[Category:Glands]]
[[Category:Secondary sexual characteristics]]
[[Category:Mammal anatomy]]
[[Category:Thorax]]
[[ar:غدد ثديية]]
[[bg:Млечна жлеза]]
[[cs:Mléčná žláza]]
[[da:Mælkekirtel]]
[[de:Milchdrüse]]
[[es:Glándula mamaria]]
[[eo:Laktoglando]]
[[fr:Glande mammaire]]
[[ia:Glandula mammari]]
[[is:Mjólkurkirtill]]
[[it:Ghiandola mammaria]]
[[he:בלוטת חלב]]
[[lt:Pieno liauka]]
[[nl:Melkklier]]
[[ja:乳腺]]
[[fi:Maitorauhanen]]
[[no:Melkekjertel]]
[[pl:Gruczoł mlekowy]]
[[ru:Молочная железа]]
[[simple:Mammary gland]]
[[zh:乳腺]]