mammary gland

Any of the milk-producing glands in female mammals, consisting of lobes containing clusters of alveoli with a system of ducts to convey the milk to an external nipple or teat. These glands typically occur in pairs and begin secreting milk when young are born.

A unique anatomical structure of mammals that secretes milk for the nourishment of the newborn. The mammary gland contains thousands of milk-producing units called alveoli, each of which consists of a unicellular layer of epithelial cells arranged in a spheroid structure. The alveolar epithelial cells take up a variety of nutrients from the blood that perfuses the outer surface of the alveolar structures. Some of the nutrients are then secreted directly into the alveolar lumen; other nutrients are used to synthesize the unique constituents of milk which are then secreted. Each alveolus is connected to a duct through which milk flows. The ducts from many alveoli are connected via a converging ductal system which opens externally by way of the lactiferous pore.

Surrounding each alveolus and its associated small ducts are smooth muscle cells called myoepithelial cells. These cells contract in response to the posterior pituitary hormone oxytocin; milk is thus forced out of the alveoli, through the ductal system, and out the lactiferous pore for the nourishment of the newborn. The release of oxytocin is a neuroendocrine reflex triggered by the stimulation of sensory receptors by the suckling of the newborn.

Mammary glands are basically highly modified and specialized sebaceous glands which derive from ectoderm. In the embryo, mammary lines, formed on both sides of the midventral line, mark the location of future mammary glands. Along the mammary lines discrete ectodermal ingrowths, called mammary buds, produce a rudimentary branched system of ducts at birth. In all species (except the monotremes) a nipple or teat develops in concert with the mammary buds. In the most primitive mammal (the duckbill or platypus), which lacks nipples or teats, milk simply oozes out of the two mammary gland areas and is lapped up by the young.

From birth to sexual maturity the mammary gland consists of a nipple and a rudimentary ductal system in both males and females. At the onset of puberty in the female, the enhanced secretion of estrogen causes a further development of the mammary ductal system and an accumulation of lipids in fat cells. After puberty in women, the mammary gland consists of about 85% fat cells and a partially developed ductal system. See also Estrogen.

During pregnancy the mammary gland comes under the influence of estrogen and progesterone which are derived from both the ovary and placenta. These hormones cause a further branching of the ductal system and the development of milk-secreting structures, the alveoli. In humans, approximately 200 alveoli are surrounded by a connective tissue sheath forming a structure called a lobule. About 26 lobules are packaged via another connective tissue sheath into a larger structure called a lobe. Each of 15–20 lobes is exteriorized into the nipple via separate lactiferous pores. See also Progesterone.

A complement of hormones maximizes the development of the ductal andlobuloalveolar elements in the mammary gland. Optimal ductal growth is attainedwith estrogen, a glucocorticoid, prolactin, and insulin. Maximal lobuloalveolargrowth is obtained with estrogen, progesterone, growth hormone, prolactin, aglucocorticoid, and insulin. During pregnancy estrogen and progesteronestimulate mammary development but inhibit milk production.

During the final third of pregnancy, the alveolar epithelial cells beginsecreting a fluid called colostrum. This fluid fills the alveoli and causes agradual enlargement of the breast or udder. At parturition, the inhibitoryinfluence of estrogen and progesterone is removed, and the gland can secretemilk under the influence of a further complement of hormones includingprolactin, a glucocorticoid, insulin, and the thyroid hormones. See also Gland; Lactation; Mammalia; Milk; Pregnancy.

For more information on mammary gland, visit Britannica.com.

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

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 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.

At the time of birth, 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 pregnancy, where rising levels of estrogen and progesterone cause further branching and differentiation of the duct cells, together with an increase in adipose tissue and a richer blood flow.

Colostrum is secreted in late pregnancy and for the first few days after giving birth. True milk secretion (lactation) begins a few days later due to a reduction in circulating progesterone and the presence of the hormone prolactin. The suckling of the baby causes the release of the hormone oxytocin which stimulates contraction of the myoepithelial cells.

Breast cancer

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, cancer results. Almost all instances of breast cancer originate in the lobules or ducts of the mammary glands.

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 (thoracic)	Intermediate (abdominal)	Posterior (Inguinal)	Total
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

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. [1]

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. Complex glycoproteins such as monoclonal antibodies or antithrombin cannot be produced by genetically engineered bacteria, and the production in live mammals is much cheaper than the use of mammalian cell cultures.

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]

Examples

Cow	Pig	Sheep	Goat
Elephant	Mouse

See also

• Breast
• Nipple
• Teat
• Udder
• Milk line
• Lactation
• Milk
• Breastfeeding
• Mammary tumor for mammary gland tumors in animals
• Witch's milk

External links

• Comparative Mammary Gland Anatomy by W. L. Hurley
• On the anatomy of the breast by Sir Astley Paston Cooper (1840). Numerous drawings, in the public domain.
• mammary+gland at eMedicine Dictionary

References

Female reproductive system
Ovaries/Oogenesis	Germinal epithelium • Tunica albuginea • cortex (Cumulus oophorus, Stroma) • Medulla
Follicles/Folliculogenesis	corpus (hemorrhagicum, luteum, albicans) • Theca folliculi (externa, interna) • Follicular antrum (Follicular fluid) • Corona radiata • Zona pellucida • Membrana granulosa (Granulosa cells)
Ligaments	Proper • Suspensory • Round • Broad (Mesovarium, Mesosalpinx, Mesometrium) • Cardinal
Fallopian tubes	Isthmus • Ampulla • Infundibulum • Fimbria
Uterus	cervix/neck (External orifice, Canal of the cervix, Internal orifice) • corpus/body (Cavity of the body, Fundus) • layers (Endometrium, Myometrium, Perimetrium)
Vulva/mons pubis	Labium (Labial commissures, Frenulum labiorum pudendi, Labia majora, Cleft of venus, Labia minora, Vulval vestibule, interlabial sulci) Clitoris (Clitoral hood, Clitoral glans, Frenulum clitoridis, Clitoral crura, Corpus cavernosa, Vestibular bulbs) Vagina (Bartholin's glands, Bartholin's ducts, Skene's glands, Skene's ducts, Fossa of vestibule of vagina, Vaginal fornix, Hymen)
Breast	Mammary glands • Nipple • Areola • Lactiferous duct
Other	G-spot • Urethral sponge

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Dansk (Danish)
n. - kvindebryst, brystkasse
v. tr. - tage kampen op imod, trodse

idioms:

• breast milk    modermælk

Nederlands (Dutch)
borst, boezem, ribbenkast, confronteren, heuveltop bereiken

Français (French)
n. - sein, poitrine, poitrail, devant (d'une chemise), (Tech) ventre (de haut-fourneau), (Minér) front de taille ou d'abattage
v. tr. - affronter, faire front à, gravir (une colline)

idioms:

• breast milk    lait maternel

Deutsch (German)
n. - Brust
v. - trotzen

idioms:

• breast milk    Muttermilch

Ελληνική (Greek)
n. - θώρακας, στέρνο, στήθος, μαστός, βυζί
v. - αντιμετωπίζω άφοβα, αψηφώ, τα βγάζω πέρα, σκαρφαλώνω στην κορυφή (υψώματος)

idioms:

• breast milk    μητρικό γάλα

Italiano (Italian)
petto, seno

idioms:

• breast milk    latte materno
• make a clean breast of    confessare completamente

Português (Portuguese)
n. - mama (f)
v. - enfrentar, aplicar ou opor o peito

idioms:

• breast milk    leite (m) materno
• double breasted    trespassado
• make a clean breast of    contar toda a verdade

Русский (Russian)
грудь, противиться

idioms:

• breast milk    материнское молоко
• double breasted    двубортный
• make a clean breast of    чистосердечно сознаться

Español (Spanish)
n. - pecho, seno, mama
v. tr. - acometer de frente, dar pecho, amamantar

idioms:

• breast milk    leche materna

Svenska (Swedish)
n. - bröst, barm, bringa, gå rätt emot, möta, trotsa

中文（简体） (Chinese (Simplified))
乳房, 胸膛, 胸部, 胸脯, 以胸对着, 对付

idioms:

• breast milk    母奶

中文（繁體） (Chinese (Traditional))
n. - 乳房, 胸膛, 胸部, 胸脯
v. tr. - 以胸對著, 對付

idioms:

• breast milk    母奶

한국어 (Korean)
n. - 흉부, 유방, 마음
v. tr. - ~을 가슴으로 받다, ~에 과감히 맞서다

日本語 (Japanese)
n. - 乳房, 胸, 胸部, 胸中
v. - 立ち向かう, 胸に受ける

idioms:

• breast milk    母乳

العربيه (Arabic)
‏(الاسم) صدر, نهد, ثدي, واجه, جابه‏

עברית (Hebrew)
n. - ‮חזה, שדיים, בגד חזה‬
v. tr. - ‮התייצב מול, נאבק עם, דחף בחזה, נגע בחזהו ב-‬

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