Milk line

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The milk lines are two parallel lines, formed by thickenings of the epidermis along the ventral surface of mammals 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 glands and nipples but are otherwise usually not visible in the adult.

1 Evolution of the milk line
2 Morphology of early mammary development
3 Milk lines in humans
4 See also
5 References

[edit] Evolution of the milk line

In evolutionary terms the mammary gland is a rather young organ. It is thought to be derived from apocrine-like skin glands of synapsids, 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 feathers and hair is still reflected on a molecular level. A number of genes 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.^[1]

[edit] 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 primates, in the inguinal area in ungulates or along the entire length of the trunk in rodents and pigs.^[1]

[edit] Milk lines in humans

In humans, 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.