In female mammals, a gland responsible for the production and release of milk to feed their young. The number varies between 2 and 20. They are located on the surface of the chest or abdomen, and may be concentrated into an udder. They are probably derived from highly modified sweat glands.
Mammary glands are the organs that, in the female mammal, produce milk for the sustenance of the young. These exocrine glands are enlarged and modified sweat glands and are the characteristic of mammals which gave the class its name.
Humans
Structure
The basic components of the mammary gland are the areoli (hollow cavities, a few millimetres large) lined with milk-secreting epithelial cells and surrounded by myoepithelial cells.
One distinguishes between a simple mammary gland, which consists of all the milk-secreting tissue leading to a single lactiferous duct, and a complex mammary gland, which consists of 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 polymastia.)
Development and hormonal control
The development of mammary glands is controlled by hormones. The mammary glands exist in both sexes, but they are rudimentary until puberty when in response to ovarian hormones, they begin to develop in the female.
At the time of birth, the baby has lactiferous ducts but no areoli.
Breast cancer
As described above, the cells of mammary glands can easily be induced to grow and multiply by hormones. Almost all instances of breast cancer originate in the lobules or ducts of the mammary glands.
Other mammals
The number of complex and simple mammary glands varies widely in different mammals. The nipples and glands can occur anywhere along the two milk lines, two roughly-parallel lines along the front 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.
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.
Mammary glands are true protein factories, and several companies have constructed transgenic animals, mainly goats and cows, in order to produce proteins for pharmaceutical use.
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