Mammary gland

Mammary gland
Cross section of the breast of a human female.
Dissection of a lactating breast.
1 - Fat
2 - Lactiferous duct/lobule
3 - Lobule
4 - Connective tissue
5 - Sinus of lactiferous duct
6 - Lactiferous duct
Latin glandula mammaria
Gray's subject #271 1267
Dorlands/Elsevier g_06/12392474




The basic components of the mammary gland are the alveoli (hollow cavities, a few millimetres large) lined with milk-secreting cuboidal cells and surrounded by myoepithelial cells. These alveoli join up to form groups known as lobules, and each lobule has a lactiferous duct that drains into openings in the nipple. The myoepithelial cells can contract, similar to muscle cells, and thereby push the milk from the alveoli through the lactiferous ducts toward the nipple, where it collects in widenings (sinuses) of the ducts. As the infant begins to suckle, the hormonally mediated "let down reflex" ensues and the mother's milk is secreted into - not sucked from the gland by - the baby's mouth.

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. Estrogen promotes formation, while testosterone inhibits it.



Breast cancer


Other mammals

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 milk lines, two roughly-parallel lines along the ventral 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, including humans, for example,) to 16 found in pigs. The Virginia Opossum has 13, one of the few mammals with an odd number[1][2]. The following table lists the number and position of glands normally found in a range of mammals:

Species [3] Anterior
Goat, sheep, horse
guinea pig
0 0 2 2
Cattle 0 0 4 4
Cat 2 2 0 4
Dog [4] 4 2 2-4 8-10
Mouse 6 0 4 10
Rat 6 2 4 12
Pig 6 6 4 16
elephants, primates 2 0 0 2



Evolution of Mammary gland

It is believed that the mammary gland is a transformed sweat gland, more closely related to Apocrine sweat glands[citation needed]. There are many theories of how they evolved, but since they do not fossilize well, supporting such theories presents a major difficulty for the researcher. One theory proposes that mammary glands evolved from glands that were used to keep the eggs of early mammals moist.[2]


See also

External links


  1. ^ http://digitalcollections.fiu.edu/wild/transcripts/possums1.htm
  2. ^ http://www.awrc.org/Baby%20Opossums.htm
  3. ^ Merle Cunningham, Animal Science and Industry ISBN 9780130462565
  4. ^ Dog breeds vary in number of mammary glands, larger breeds tend to have 5 pairs, smaller breeds have 4 pairs


The content of this article is licensed under the GNU Free Documentation License (local copy). It uses material from the Wikipedia article "Mammary gland" modified August 9, 2007 with previous authors listed in its history.