Milk line
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2008-06-20T08:27:59Z
Closedmouth
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Reverted edits by [[Special:Contributions/70.165.247.176|70.165.247.176]] to last version by Dinferno (using [[WP:HG|Huggle]])
The '''milk lines''' are two parallel lines, formed by thickenings of the [[epidermis (skin)|epidermis]] along the [[ventral]] surface of [[mammal]]s of both sexes. They extend from the upper limbs (arms) to the lower limbs (legs) and are developed in the [[embryo]]. They give rise to the [[mammary gland]]s and [[nipple]]s but are otherwise usually not visible in the adult.
==Evolution of the milk line==
In [[evolution]]ary terms the [[mammary gland]] is a rather young [[Organ (anatomy)|organ]]. It is thought to be derived from [[apocrine]]-like skin [[gland]]s of [[synapsid]]s, which probably served as a source of moisture for the parchment-shelled eggs and as a source of nutrients for the hatchlings. The origin of the gland in close association with [[feather]]s and [[hair]] is still reflected on a molecular level. A number of [[gene]]s are expressed in hair, whiskers and feathers as well as in the mammary gland, and deletion of some genes affects development of all the epidermal appendages. Another common feature of all [[skin]] appendages is the dependence on reciprocal signals between the [[epithelium]] and the [[mesenchyme]].<ref name=Robinson> [http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=400673 Breast Cancer Res. 2004; 6(3): 105–108. Published online 2004 March 9. doi: 10.1186/bcr776.] article ''Identification of signaling pathways in early mammary gland development by mouse genetics'' by Gertraud W Robinson</ref>
==Morphology of early mammary development==
Mammary gland development is initiated during [[embryonic]] life. The first manifestation of the gland in many mammals is an elevated ridge or milk line, which then fragments into individual buds in specific regions lateral of the [[dorsal]] midline. These buds are either located in the [[thoracic]] region in [[primate]]s, in the [[inguinal]] area in [[ungulate]]s or along the entire length of the trunk in [[rodent]]s and [[pig]]s.<ref name=Robinson/>
==Milk lines in humans==
In [[human]]s, milk lines appear in the seventh week of embryonic development. After initial development of the milk lines they go into remission.
Nipples develop on the milk lines of mammals. Most of us have two nipples, but in some cases humans can develop more than two nipples. We have heard of 'third nipples', but there can be more than three nipples in very rare cases, all which would grow along the milk lines.
==See also==
* [[Supernumerary nipple]]
* [[Mammary gland]]
* [[Breast]]
* [[Nipple]]
* [[Teat]]
* [[Udder]]
* [[Lactation]]
* [[Milk]]
==References==
{{Reflist}}
[[Category:Integumentary system]]
[[Category:Breast]]
[[Category:Mammal anatomy]]
{{mammal-stub}}
{{developmental-biology-stub}}
[[De:Milchleiste]]