Instruct: Given a question, retrieve passages that answer the question.
Query: How many mammary glands do human women have?
All the milk-secreting tissue leading to a single lactiferous duct is 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 presence of more than two nipples is known as polythelia and the presence of more than two complex mammary glands as polymastia.
Females have enlarged breasts due to functional mammary glands, which develop from puberty onward due to the effects of estrogen. Mammary glands do not contain muscle tissue. The shape of female breasts is affected by age, genetic factors, and body weight.
Male mammals typically have rudimentary mammary glands and nipples, with a few exceptions: male mice do not have nipples,[56] and male horses lack nipples and mammary glands. The male Dayak fruit bat has lactating mammary glands.[57] Male lactation occurs infrequently in some species, including humans.
hPL has been found to bind to the prolactin receptor with equal affinity to that of prolactin in rabbit milk fat gobule membrane, and hPL and prolactin have been found to possess very similar lactogenic activity "in vitro" in mouse and rat mammary gland explants. In addition, hPL has been found to stimulate DNA synthesis in human mammary fibroadenoma cells transplanted into mice, which suggests that hPL promotes the growth of the human mammary gland similarly to prolactin. As hPL circulates at concentrations that are 100-fold higher than those of prolactin during pregnancy, these findings suggest that hPL may play an important role in human mammogenesis during this time. However, the relative affinities of hPL and prolactin for the human prolactin receptor have yet to be published and the effects of hPL on normal human mammary epithelial tissue have not yet been investigated, and so a definitive role of hPL in human mammary gland development during pregnancy has not been established at present.
The human body contains about 15–20mg of iodine, mostly concentrated in thyroid tissue (70–80%). Extra-thyroidal iodine exists in several other organs, including the mammary glands, eyes, gastric mucosa, cervix, ovary and salivary glands. In the cells of these tissues the iodide ion (I−) enters directly by the sodium-iodide symporter (NIS). Different tissue responses for iodine and iodide occur in the mammary glands and the thyroid gland of rats.[4] The role of iodine in mammary tissue is related to fetal and neonatal development, but its role in the other tissues is unknown.[5] It has been shown to act as an antioxidant[5] and antiproliferant[6] in various tissues that can uptake iodine. Molecular iodine (I2) has been shown to have a suppressive effect on benign and cancerous neoplasias.[6]
Mammary stem cells provide the source of cells for growth of the mammary gland during puberty and gestation and play an important role in carcinogenesis of the breast.[4] Mammary stem cells have been isolated from human and mouse tissue as well as from cell lines derived from the mammary gland. Single such cells can give rise to both the luminal and myoepithelial cell types of the gland, and have been shown to have the ability to regenerate the entire organ in mice.[4]
The mammary glands of mammals are specialized to produce milk, a liquid used by newborns as their primary source of nutrition. The monotremes branched early from other mammals and do not have the nipples seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly.
The mammary ridge is primordial for the mammary gland on the breast in humans, and is associated with mammary gland and breast development.
Cyclooxygenase-2 (COX-2) overexpression in mammary gland tissue produces mammary gland hyperplasia as well as precocious mammary gland development in female mice, indicating a strong stimulatory effect of this enzyme on the growth of the mammary glands. These effects appear to be downstream actions of increased activation of the prostaglandin EP, EP, and EP receptors, but not the EP receptor, in mammary gland tissue, which in turn results in the potent induction of amphiregulin expression, a critical growth factor involved in normal mammary gland development. In addition, agonists of the epidermal growth factor receptor (EGFR), the molecular target of amphiregulin, induce COX-2 expression in mammary gland tissue, potentially resulting in a self-perpetuating cycle of growth amplification by COX-2. This mechanism is closely related to formation, growth, and spreading of cancers with poor prognosis, and is in accordance with the fact that long-term administration of aspirin, a COX inhibitor, as well as of other COX-inhibiting nonsteroidal anti-inflammatory drugs (NSAIDs), have been found to slightly reduce the risk of breast cancer in women (it is notable here that breast growth/size and breast cancer risk are positively associated). Taken together, these findings indicate that COX-2 inhibitors, such as aspirin, ibuprofen, naproxen, and celecoxib, may decrease breast cell proliferation.
The mammary glands of mammals are specialized to produce milk, the primary source of nutrition for newborns. The monotremes branched early from other mammals and do not have the nipples seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly.
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,[27][28] and in conjunction with relatively low estrogen levels, are the cause of the lack of developed mammary glands in males.[29]
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.[33]
A mammary tumor is a neoplasm originating in the mammary gland. It is a common finding in older female dogs and cats that are not spayed, but they are found in other animals as well. The mammary glands in dogs and cats are associated with their nipples and extend from the underside of the chest to the groin on both sides of the midline. There are many differences between mammary tumors in animals and breast cancer in humans, including tumor type, malignancy, and treatment options. The prevalence in dogs is about three times that of women. In dogs, mammary tumors are the second most common tumor (after skin tumors) over all and the most common tumor in female dogs with a reported incidence of 3.4%. Multiple studies have documented that spaying female dogs when young greatly decreases their risk of developing mammary neoplasia when aged. Compared with female dogs left intact, those spayed before puberty have 0.5% of the risk, those spayed after one estrous cycle have 8.0% of the risk, and dogs spayed after two estrous cycles have 26.0% of the risk of developing mammary neoplasia later in life. Overall, unspayed female dogs have a seven times greater risk of developing mammary neoplasia than do those that are spayed. While the benefit of spaying decreases with each estrous cycle, some benefit has been demonstrated in female dogs even up to 9 years of age. There is a much lower risk (about 1 percent) in male dogs and a risk in cats about half that of dogs.
Lactation is thought to have developed long before the evolution of the mammary gland and mammals; see evolution of lactation.
In the 1960s, CMA was introduced as a component of oral contraceptives. However, around 1970, such formulations were withdrawn from many markets such as the United States and United Kingdom due to the finding that CMA induced alarming mammary gland tumors in Beagle dogs. The doses administered that caused the nodules were 10 or 25 times the recommended human dosage for an extended period of time (2–4 years), while no tumors were found in dogs treated with 1–2 times the human dosage. In addition to CMA, mammary tumors were found in dogs with various other 17α-hydroxyprogesterone derivatives, including medroxyprogesterone acetate, megestrol acetate, and anagestone acetate, and they were also discontinued for the indication of hormonal contraception (although medroxyprogesterone acetate has since been reintroduced). Tumors were also observed with progesterone, as well as with ethynerone and chloroethynylnorgestrel, but notably not with the non-halogenated 19-nortestosterone derivatives norgestrel, norethisterone, noretynodrel, or etynodiol diacetate, which remained on the market. In any case, according to Hughes et al., "It is still doubtful how much relevance these findings have for humans as the dog mammary gland seems to be the only one which can be directly maintained by progestogens." Subsequent research revealed species differences between dogs and humans and established that there is no similar risk in humans.
In 1969, along with a variety of other progestogens including progesterone, chlormadinone acetate, megestrol acetate, medroxyprogesterone acetate, ethynerone, and chloroethynyl norgestrel, anagestone acetate was found to induce the development of mammary gland tumors in Beagle dogs after extensive treatment (2–7 years) with very high doses (10–25 times the recommended human dose), though notably not with 1–2 times the human dosage. In contrast, the non-halogenated 19-nortestosterone derivatives norgestrel, norethisterone, noretynodrel, and etynodiol diacetate were not found to produce such nodules. Because of these findings, anagestone acetate was voluntarily withdrawn from the market by the manufacturer in 1969. The findings also led to the virtual disappearance of most 17α-hydroxyprogesterone derivatives as hormonal contraceptives from the market (though medroxyprogesterone acetate, cyproterone acetate, and chlormadinone acetate have continued to be used). According to Hughes et al., "It is still doubtful how much relevance these findings have for humans as the dog mammary gland seems to be the only one which can be directly maintained by progestogens." Subsequent research revealed species differences between dogs and humans and established that there is no similar risk in humans.
This is the most common cause of anovulation and is thought to account for about 70% of all cases. About half the women with hormonal imbalances do not produce enough follicles to ensure the development of an ovule, possibly due to poor hormonal secretions from the pituitary gland or the hypothalamus. The pituitary gland controls most other hormonal glands in the human body. Therefore, any pituitary malfunctioning affects other glands under its influence, including the ovaries. This occurs in around 10% of cases. The mammary glands are also controlled by the pituitary gland, so lactation can also be affected. The pituitary gland is controlled by the hypothalamus. In 10% of cases, alterations in the chemical signals from the hypothalamus can easily seriously affect the ovaries.
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.[7][8][9] Some authors dispute that and argue instead that they are sebaceous glands.[7]
A possible mechanism of the negative regulatory effects of the VDR on breast development may be indicated by a study of vitamin D supplementation in women which found that vitamin D suppresses cyclooxygenase-2 (COX-2) expression in the breast, and by doing so, reduces and increases, respectively, the levels of prostaglandin E (PGE) and transforming growth factor β2 (TGF-β2), a known inhibitory factor in breast development. Moreover, suppression of PGE in breast tissue is relevant because, via activation of prostaglandin EP receptors, PGE potently induces amphiregulin expression in breast tissue, and activation of the EGFR by amphiregulin increases COX-2 expression in breast tissue, in turn resulting in more PGE, and thus, a self-perpetuating, synergistic cycle of growth amplification due to COX-2 appears to potentially be present in normal breast tissue. Accordingly, overexpression of COX-2 in mammary gland tissue produces mammary gland hyperplasia as well as precocious mammary gland development in female mice, mirroring the phenotype of VDR knockout mice, and demonstrating a strong stimulatory effect of COX-2, which is downregulated by VDR activation, on the growth of the mammary glands. Also in accordance, COX-2 activity in the breasts has been found to be positively associated with breast volume in women.
Estetrol is weakly estrogenic in breast/mammary gland, but shows very low potency in this tissue and, when administered in combination with estradiol, antagonizes the effects of estradiol. Relative to estradiol, estetrol shows 100-fold lower potency in stimulation of the proliferation of human breast epithelial cells "in vitro" and of mouse mammary gland cells "in vivo". In animal models, estetrol shows antiestrogenic effects in mammary gland tissue comparable to those of the SERM tamoxifen and of ovariectomy, antagonizing the stimulatory effects of estradiol and preventing tumor development in a 7,12-dimethylbenz(a)anthracene (DMBA) mammary tumor model. As such, it is anticipated that estetrol may cause minimal proliferation of breast tissue and that it may be useful in the treatment of breast cancer.
The breasts of the adult human female 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.[52][53] The following table lists the number and position of teats and glands found in a range of mammals:
Breastfeeding Mammary tumor Mammaglobin Gynecomastia Hypothalamic–pituitary–prolactin axis Udder Witch's milk Milk line List of glands of the human body#Skin
Tumorigenesis in mammary glands can be induced biochemically by abnormal expression level of circulating hormones or local ECM components,[50] or from a mechanical change in the tension of mammary stroma.[51] 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.
A mammary ridge, or crest, usually stops growing at eight weeks and its length is regressed starting at the caudal end and extending cranially, so that what remains is a round, ectodermic placode where the axilla develops. When shortening of the mammary crest is complete, the structure remains prominent in the areas where the mammary glands eventually form. The mammary lines begin to shorten and ectodermal cells begin to divide and grow into the mesenchymal cell layer. A basement membrane separating the expanding ectodermal crest structure and the underlying mesoderm usually remains. The mammary ridge then becomes recognizable in the thoracic region in the human embryo. Nipples develop on the milk lines of mammals.
In humans, milk lines form as thickenings of the epidermis of the mammary ridge, along the front surface of both sexes of mammals. Milk lines appear in the seventh week of embryonic development before human sexual differentiation, which explains why male humans have nipples. After initial development of the milk lines they go into remission. Most humans have two nipples, but in some cases more than two will develop. These additional nipples usually grow along the milk line.
A mammary gland is an exocrine gland in mammals that produces milk to feed young offspring. Mammals get their name from the Latin word mamma, "breast". The mammary glands are arranged in organs such as the breasts in primates (for example, humans and chimpanzees), the udder in ruminants (for example, cows, goats, and deer), and the dugs of other animals (for example, dogs and cats). Lactorrhea, the occasional production of milk by the glands, can occur in any mammal, but in most mammals, lactation, the production of enough milk for nursing, occurs only in phenotypic females who have gestated in recent months or years. It is directed by hormonal guidance from sex steroids. In a few mammalian species, male lactation can occur.