Mammary gland

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"Mammary" redirects here. For the mountain in Alaska, see Mammary Peak.

Mammary gland
Lobules
  • Nipple
  • Areola
  • Milk duct
  • Fatty tissue
  • Skin
    • Details
    PrecursorMesoderm
     (blood vessels and connective tissue)
    Ectoderm[3]
     (cellular elements)
    ArteryInternal thoracic artery
    Lateral thoracic artery[1]
    VeinInternal thoracic vein
    Axillary vein[1]
    NerveSupraclavicular nerves
    Intercostal nerves[2]
     (lateral and medial branches)
    LymphPectoral axillary lymph nodes[1]
    Identifiers
    TA98A16.0.02.006
    TA27099
    FMA60088
    Anatomical terminology]

    A mammary gland is an

    sex steroids. In a few mammalian species, male lactation
    can occur. With humans, male lactation can occur only under specific circumstances.

    Mammals are divided into 3 groups: prototherians, metatherians, and eutherians. In the case of prototherians, both males and females have functional mammary glands, but their mammary glands are without nipples. These mammary glands are modified sebaceous glands. Concerning metatherians and eutherians, only females have functional mammary glands. Their mammary glands can be termed as breasts or udders. In the case of breasts, each mammary gland has its own nipple (e.g., human mammary glands). In the case of udders, pairs of mammary glands comprise a single mass, with more than one nipple (or teat) hanging from it. For instance, cows and buffalo udders have two pairs of mammary glands and four teats, whereas sheep and goat udders have one pair of mammary glands with two teats protruding from the udder. Each gland produces milk for a single teat. These mammary glands are modified sweat glands.

    Structure

    See also: Breast

    The basic components of a mature mammary gland are the

    lobules. Each lobule has a lactiferous duct that drains into openings in the nipple. The myoepithelial cells contract under the stimulation of oxytocin, excreting the milk secreted by alveolar units into the lobule lumen toward the nipple. As the infant begins to suck, the oxytocin-mediated "let down reflex" ensues, and the mother's milk is secreted — not sucked — from the gland into the baby's mouth.[4]

    All the milk-secreting tissue leading to a single lactiferous duct is collectively called a "simple mammary gland"; in a "complex mammary gland", all the simple mammary glands serve 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 opening of each simple gland on the surface of the nipple is called a "pore."

    polythelia and the presence of more than two complex mammary glands as polymastia
    .

    Maintaining the correct polarized morphology of the lactiferous duct tree requires another essential component – mammary epithelial cells extracellular matrix (ECM) which, together with adipocytes, fibroblast, inflammatory cells, and others, constitute mammary stroma.[6] Mammary epithelial ECM mainly contains myoepithelial basement membrane and the connective tissue. They not only help to support mammary basic structure, but also serve as a communicating bridge between mammary epithelia and their local and global environment throughout this organ's development.[7][8]

    Histology

    Normal histology of the breast.
    Light micrograph of a human proliferating mammary gland during estrous cycle. Sprouting gland tissue can be seen in the upper left field (haematoxylin eosin staining)

    A mammary gland is a specific type of apocrine gland specialized for manufacture of colostrum when giving birth. Mammary glands can be identified as apocrine because they exhibit striking "decapitation" secretion. Many sources assert that mammary glands are modified sweat glands.[9][10][11] Some authors dispute that and argue instead that they are sebaceous glands.[9]

    Development

    Further information: Breast development and Parathyroid hormone-related protein

    Mammary glands develop during different growth cycles. They exist in both sexes during the embryonic stage, forming only a rudimentary duct tree at birth. In this stage, mammary gland development depends on systemic (and maternal) hormones,

    BMP4. These mesenchymal cells can transform into a dense, mammary-specific mesenchyme, which later develop into connective tissue with fibrous threads, forming blood vessels and the lymph system.[13] A basement membrane, mainly containing laminin and collagen, formed afterward by differentiated myoepithelial cells, keeps the polarity of this primary duct tree. These components of the extracellular matrix are strong determinants of duct morphogenesis.[14]

    Biochemistry

    paracrine growth factors such as epidermal growth factor (EGF), transforming growth factor beta (TGF-β),[21] amphiregulin,[22] fibroblast growth factor (FGF), and hepatocyte growth factor (HGF)[23] are involved in breast development as mediators downstream to sex hormones and GH/IGF-1.[24][25][26]

    During embryonic development, IGF-1 levels are low, and gradually increase from birth to puberty.

    lobuloalveolar development in estrogen-primed mammary gland tissue, which occurs in preparation of lactation and nursing.[15][29]

    Androgens such as testosterone inhibit estrogen-mediated mammary gland development (e.g., by reducing local ER expression) through activation of androgen receptors expressed in mammary gland tissue,[29][30] and in conjunction with relatively low estrogen levels, are the cause of the lack of developed mammary glands in males.[31]

    Timeline

    Before birth

    Mammary gland development is characterized by the unique process by which the

    stroma. The development of the mammary gland occurs mainly after birth. During puberty, tubule formation is coupled with branching morphogenesis which establishes the basic arboreal network of ducts emanating from the nipple.[32]

    Developmentally, mammary gland epithelium is constantly produced and maintained by rare epithelial cells, dubbed as mammary progenitors which are ultimately thought to be derived from tissue-resident stem cells.[33]

    anlage (biology).[34]

    The primitive (stem) cells are detected in embryo and their numbers increase steadily during development[35]

    Growth

    Postnatally, the mammary ducts elongate into the mammary fat pad. Then, starting around four weeks of age, mammary ductal growth increases significantly with the ducts invading towards the lymph node
    . Terminal end buds, the highly proliferative structures found at the tips of the invading ducts, expand and increase greatly during this stage. This developmental period is characterized by the emergence of the terminal end buds and lasts until an age of about 7–8 weeks.

    By the pubertal stage, the mammary ducts have invaded to the end of the mammary fat pad. At this point, the terminal end buds become less proliferative and decrease in size. Side branches form from the primary ducts and begin to fill the mammary fat pad. Ductal development decreases with the arrival of sexual maturity and undergoes estrous cycles (proestrus, estrus, metestrus, and diestrus). As a result of estrous cycling, the mammary gland undergoes dynamic changes where cells proliferate and then regress in an ordered fashion.[36]

    Pregnancy

    During

    myoepithelial cells surrounding the alveoli will cause the milk to be ejected through the ducts and into the nipple for the nursing infant. Upon weaning of the infant, lactation stops and the mammary gland turns in on itself, a process called involution. This process involves the controlled collapse of mammary epithelial cells where cells begin apoptosis
    in a controlled manner, reverting the mammary gland back to a pubertal state.

    Postmenopausal

    During

    postmenopause
    , due to much lower levels of estrogen, and due to lower levels of GH and IGF-1, which decrease with age, mammary gland tissue atrophies and the mammary glands become smaller.

    Physiology

    Hormonal control

    Lactiferous duct development occurs in females in response to circulating

    myoepithelial and luminal (duct) epithelial cells, and the increased amount of activated MMPs can degrade surrounding ECM helping duct buds to reach further in the fat pads.[40][41] On the other hand, basement membrane along the mature mammary ducts is thicker, with strong adhesion to epithelial cells via binding to integrin and non-integrin receptors. When side branches develop, it is a much more "pushing-forward" working process including extending through myoepithelial cells, degrading basement membrane and then invading into a periductal layer of fibrous stromal tissue.[7] Degraded basement membrane fragments (laminin-5) roles to lead the way of mammary epithelial cells migration.[42] Whereas, laminin-1 interacts with non-integrin receptor dystroglycan negatively regulates this side branching process in case of cancer.[43]
    These complex "Yin-yang" balancing crosstalks between mammary ECM and epithelial cells "instruct" healthy mammary gland development until adult.

    There is preliminary evidence that soybean intake mildly stimulates the breast glands in pre- and postmenopausal women.[44]

    Pregnancy

    Secretory alveoli develop mainly in pregnancy, when rising levels of

    blood flow. In gestation, serum progesterone remains at a stably high concentration so signaling through its receptor is continuously activated. As one of the transcribed genes, Wnts secreted from mammary epithelial cells act paracrinely to induce more neighboring cells' branching.[45][46] When the lactiferous duct tree is almost ready, "leaves" alveoli are differentiated from luminal epithelial cells and added at the end of each branch. In late pregnancy and for the first few days after giving birth, colostrum is secreted. Milk secretion (lactation) begins a few days later due to reduction in circulating progesterone and the presence of another important hormone prolactin, which mediates further alveologenesis, milk protein production, and regulates osmotic balance and tight junction function. Laminin and collagen in myoepithelial basement membrane interacting with beta-1 integrin on epithelial surface again, is essential in this process.[47][48] Their binding ensures correct placement of prolactin receptors on the basal lateral side of alveoli cells and directional secretion of milk into lactiferous ducts.[47][48]
    Suckling of the baby causes release of the hormone oxytocin, which stimulates contraction of the myoepithelial cells. In this combined control from ECM and systemic hormones, milk secretion can be reciprocally amplified so as to provide enough nutrition for the baby.

    Weaning

    During weaning, decreased prolactin, missing mechanical stimulation (baby suckling), and changes in osmotic balance caused by milk stasis and leaking of tight junctions cause cessation of milk production. It is the (passive) process of a child or animal ceasing to be dependent on the mother for nourishment. In some species there is complete or partial

    proteinase is under the control of somatostatin and other growth inhibiting hormones and local factors.[52]
    This major structural change leads loose fat tissue to fill the empty space afterward. But a functional lactiferous duct tree can be formed again when a female is pregnant again.

    Clinical significance

    Tumorigenesis in mammary glands can be induced biochemically by abnormal expression level of circulating hormones or local ECM components,[53] or from a mechanical change in the tension of mammary stroma.[54] Under either of the two circumstances, mammary epithelial cells would grow out of control and eventually result in cancer. Almost all instances of breast cancer
    originate in the lobules or ducts of the mammary glands.

    Other mammals

    General

    The breasts of female humans vary from most other mammals that tend to have less conspicuous mammary glands. The number and positioning of mammary glands varies widely in different mammals. The protruding teats and accompanying glands can be located anywhere along the two

    milk lines. 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 teats varies from 2 (in most primates) to 18 (in pigs). The Virginia opossum has 13, one of the few mammals with an odd number.[55][56]
    The following table lists the number and position of teats and glands found in a range of mammals:

    Species[57] Anterior
    (
    thoracic
    )     		Intermediate
    (
    abdominal
    )     		Posterior
    (inguinal)     		Total
    Goat, sheep, horse
    guinea pig     		0 0 2 2
    Cattle 0 0 4 4
    Cat 2 2 4 8
    Dog[58] 4 2 2 or 4 8 or 10
    Mouse 6 0 4 10
    Rat 6 2 4 12
    Pig 6 6 6 18
    primates
    		 2 0 0 2
    Virginia opossum[55][56] 0 0 13 13
    Southern red-sided opossum[59] 0 0 25 to 27 25 to 27

    Male mammals typically have rudimentary mammary glands and nipples, with a few exceptions: male mice do not have nipples,[60] male marsupials do not have mammary glands,[61] and male horses lack nipples.[62] The male dayak fruit bat has lactating mammary glands.[63] Male lactation occurs infrequently in some species.[64]

    Mammary glands are true

    cows, to produce proteins for pharmaceutical use.[66] 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

    There are many theories on how mammary glands evolved. For example, it is thought that the mammary gland is a transformed sweat gland, more closely related to

    apocrine sweat glands.[67] Because mammary glands do not fossilize well, supporting such theories with fossil evidence is difficult. Many of the current theories are based on comparisons between lines of living mammals—monotremes, marsupials, and eutherians. One theory proposes that mammary glands evolved from glands that were used to keep the eggs of early mammals moist[68][69] and free from infection[70][71] (monotremes still lay eggs). Other theories suggest that early secretions were used directly by hatched young,[72] or that the secretions were used by young to help them orient to their mothers.[73]

    Lactation is thought to have developed long before the evolution of the mammary gland and mammals; see evolution of lactation.

    Additional images

    See also

    This article uses anatomical terminology.

    List of distinct cell types in the adult human body

    References

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    Bibliography

    External links

    Wikisource has the text of the 1911 Encyclopædia Britannica article "Mammary Gland".
    Retrieved from "https://en.wikipedia.org/w/index.php?title=Mammary_gland&oldid=1215317178"