Instruct: Given a question, retrieve passages that can help answer the question.
Query: Do men have mammary glands?
In zoology, male lactation is the production of milk from a male mammal's mammary glands. Such production is known as lactation. It is well-documented in the Dayak fruit bat. The term male lactation is not used in human medicine. It has been used in popular literature, such as Louise Erdrich's The Antelope Wife, to describe the phenomenon of male galactorrhea, which is a well-documented condition in humans, unrelated to childbirth or nursing. Newborn babies of both sexes can occasionally produce milk; this is called neonatal milk (also known as "Witch's milk") and not considered male lactation.
A distinguishing characteristic of the class "Mammalia" is the presence of mammary glands. The mammary glands are modified sweat glands that produce milk, which is used to feed the young for some time after birth. Only mammals produce milk. Mammary glands are most obvious in humans, as the female human body stores large amounts of fatty tissue near the nipples, resulting in prominent breasts. Mammary glands are present in all mammals, although they are seldom used by the males of the species.
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]
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.
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:
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]
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.
During pregnancy, the ductal systems undergo rapid proliferation and form alveolar structures within the branches to be used for milk production. After delivery, lactation occurs within the mammary gland; lactation involves the secretion of milk by the luminal cells in the alveoli. Contraction of the 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.
A mammary alveolus (plural: alveoli, from Latin "alveolus", "little cavity") is a small cavity or sac found in the mammary gland. Mammary alveoli are the site of milk production and storage in the mammary gland. Mammary alveoli cluster into groups called lobules, and each breast may contain 15–20 of these lobules. The lobules drain milk through the lactiferous ducts out of the nipples.
The mammary ridge is primordial for the mammary gland on the breast in humans, and is associated with mammary gland and breast development.
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 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.
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.
The concept of synapomorphy is relative to a given clade in the tree of life. What counts as a synapomorphy for one clade may well be a primitive character or plesiomorphy at a less inclusive or nested clade. For example, the presence of mammary glands is a synapomorphy for mammals in relation to tetrapods but is a symplesiomorphy for mammals in relation to one another—rodents and primates, for example. So the concept can be understood as well in terms of "a character newer than" (autapomorphy) and "a character older than" (plesiomorphy) the apomorphy: mammary glands are evolutionarily newer than vertebral column, so mammary glands are an autapomorphy if vertebral column is an apomorphy, but if mammary glands are the apomorphy being considered then vertebral column is a plesiomorphy.
The evolution of the mammary gland is difficult to explain; this is because mammary glands are typically required by mammals to feed their young. 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.[59] 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[60][61] and free from infection[62][63] (monotremes still lay eggs). Other theories suggest that early secretions were used directly by hatched young,[64] or that the secretions were used by young to help them orient to their mothers.[65]
The first agent-based model is a multiscale model of mammary gland development starting with a rudimentary mammary ductal tree at the onset of puberty (during active proliferation) all the way to a full mammary gland at adulthood (when there is little proliferation). The model consists of millions of agents, with each agent representing a mammary stem cell, a progenitor cell, or a differentiated cell in the breast. Simulations were first run on the Lawrence Berkeley National Laboratory Lawrencium supercomputer to parameterize and benchmark the model against a variety of "in vivo" mammary gland measurements. The model was then used to test the three different mechanisms to determine which one led to simulation results that matched "in vivo" experiments the best. Surprisingly, radiation-induced cell inactivation by death did not contribute to increased stem cell frequency independently of the dose delivered in the model. Instead the model revealed that the combination of increased self-renewal and cell proliferation during puberty led to stem cell enrichment. In contrast epithelial-mesenchymal transition in the model was shown to increase stem cell frequency not only in pubertal mammary glands but also in adult glands. This latter prediction, however, contradicted the "in vivo" data; irradiation of adult mammary glands did not lead to increased stem cell frequency. These simulations therefore suggested self-renewal as the primary mechanism behind pubertal stem cell increase.
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]
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.
This type of tissue can be observed in sweat glands, mammary glands, circumanal glands, and salivary glands. They protect areas such as the ducts of sweat glands, mammary glands, and salivary glands.
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.
Estrogen and growth hormone (GH) are essential for the ductal component of mammary gland development, and act synergistically to mediate it.[13][14][15][16][17] Neither estrogen nor GH are capable of inducing ductal development without the other.[14][15][16][17] The role of GH in ductal development has been found to be mostly mediated by its induction of the secretion of insulin-like growth factor 1 (IGF-1), which occurs both systemically (mainly originating from the liver) and locally in the mammary fat pad through activation of the growth hormone receptor (GHR).[14][15][16][17][18] However, GH itself also acts independently of IGF-1 to stimulate ductal development by upregulating estrogen receptor (ER) expression in mammary gland tissue, which is a downstream effect of mammary gland GHR activation.[17] In any case, unlike IGF-1, GH itself is not essential for mammary gland development, and IGF-1 in conjunction with estrogen can induce normal mammary gland development without the presence of GH.[17] In addition to IGF-1, other paracrine growth factors such as epidermal growth factor (EGF), transforming growth factor beta (TGF-β),[19] amphiregulin,[20] fibroblast growth factor (FGF), and hepatocyte growth factor (HGF)[21] are involved in breast development as mediators downstream to sex hormones and GH/IGF-1.[22][23][24]
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.
Lactation is thought to have developed long before the evolution of the mammary gland and mammals; see evolution of lactation.
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]
Specialized sweat glands, including the ceruminous glands, mammary glands, ciliary glands of the eyelids, and sweat glands of the nasal vestibulum, are modified apocrine glands. Ceruminous glands are near the ear canals, and produce cerumen (earwax) that mixes with the oil secreted from sebaceous glands. Mammary glands use apocrine secretion to produce milk.