Mammary glands are the that, in , produce for the sustenance of the young. These are enlarged and modified and give mammals their name. The mammary glands of domestic mammals containing more than two breasts are called dugs.
The basic components of the mammary gland are the alveoli (hollow cavities, a few millimetres large) lined with milk-secreting and surrounded by . These alveoli join up to form groups known as lobules, and each lobule has a lactiferous duct that drains into openings in the . The myoepithelial cells can contract under the stimulation of thereby excreting the milk from glands through the lactiferous ducts toward the nipple, where it collects in sinuses of the ducts. As the infant begins to suck, the hormonally (oxytocin) 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 , and each complex mammary gland consists of 10–20 simple glands. The presence of more than two nipples is known as and the presence of more than two complex mammary glands as .
Development and hormonal control
Mammary glands exist in both sexes, but are rudimentary until when, in females, they begin to develop in response to hormones. promotes formation, whereas inhibits it.
At the time of , 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 , where rising levels of estrogen and cause further branching and differentiation of the duct cells, together with an increase in and a richer .
is secreted in late pregnancy and for the first few days after giving birth. True milk secretion () begins a few days later due to a reduction in circulating and the presence of the hormone . The suckling of the baby causes the release of the hormone which stimulates contraction of the myoepithelial cells.
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, results. Almost all instances of originate in the lobules or ducts of the mammary glands.
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 , two roughly-parallel lines along the 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 has 13, one of the few mammals with an odd number. The following table lists the number and position of glands normally found in a range of mammals:
Species | Anterior
() | Intermediate
() | Posterior
() | Total
| 0 | 0 | 2 | 2
0 | 0 | 4 | 4
2 | 2 | 2 | 6
| 4 | 2 | 2 or 4 | 8 or 10
6 | 0 | 4 | 10
6 | 2 | 4 | 12
6 | 6 | 4 | 16
, | 2 | 0 | 0 | 2
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. The male Dyak fruit bat has lactating mammary glands; occurs infrequently in some species, including humans.
Mammary glands are true factories, and several companies have constructed , mainly and , in order to produce proteins for pharmaceutical use. Complex such as or cannot be produced by genetically engineered , and the production in live mammals is much cheaper than the use of mammalian .
It is believed that the mammary gland is a transformed sweat gland, more closely related to .