| Mammary crest | |
|---|---|
 Natural milk lines in the human body | |
| Details | |
| Identifiers | |
| Latin | crista mammaria |
| Anatomical terminology | |
Themammary ridge ormammary crest is aprimordium[1] specific for the development ofmammary glands.
The mammary ridge is primordial for the mammary glands on thechest in humans, and is associated withmammary gland andbreast development.
Inhuman embryogenesis, the mammary ridge usually appears as a narrow, microscopicectodermal thickening during the first seven weeks of pregnancy and growscaudally as a narrow, linear ridge.[1] In manymammals, these glands first appear as elevated ridges along the milk lines, which then separate into individual buds located in regions lateral to theventral midline. The location of these buds varies according to species: they are located in thethoracic region inprimates, in theinguinal area inungulates, and along the entire length of the trunk inrodents andpigs.[2]
A mammary ridge, or crest, usually stops growing at eight weeks and its length is regressed starting at the caudal end and extendingcranially,[1] so that what remains is a round,ectodermicplacode where theaxilla develops. When shortening of the mammary crest is complete, the structure remains prominent in the areas where the mammary glands eventually form. The mammary lines begin to shorten and ectodermal cells begin to divide and grow into themesenchymal cell layer.[3] Abasement membrane separating the expanding ectodermal crest structure and the underlyingmesoderm usually remains. The mammary ridge then becomes recognizable in thethoracic region in thehuman embryo. Nipples develop on the milk lines oftherian mammals.
Inhumans,milk lines form as thickenings of theepidermis of the mammary ridge, along the front surface of both sexes. Milk lines appear in the seventh week of embryonic development before humansexual differentiation, which explains why male humans have nipples. After initial development of the milk lines they go into remission. Most humans have two nipples, but in some casesmore than two will develop. These additional nipples usually grow along the milk line.
A recently published study demonstrates that focal fat pads on the front of human torsos are of mammary ridge origin. Eight pairs of fatty mounds were consistently found running along a curved line from the armpits to the groins in nearly all lean women and men, consistent with the location of the embryological mammary ridge line. This finding explains why fat on the front of the body is less responsive to diet and exercise than fat elsewhere in most people—because it is actually of breast origin and therefore sensitive more to hormonal influence than caloric intake or burn.[4]