Mammary gland
Encyclopedia
A mammary gland is an organ
in mammal
s that produces milk
to feed young offspring
. Mammals get their name from the word "mammary". In ruminants such as cows, goats, and deer, the mammary glands are contained in their udders. The mammary glands of other mammals that have more than two breasts, such as dogs and cats, are sometimes called dugs.
gland specialized for manufacture of colostrum at the time of parturition. Mammary glands can be identified as apocrine because they exhibit striking "decapitation" secretion. Whether mammary glands are modified sweat glands or sebaceous glands still remains controversial.
. The myoepithelial cells can contract under the stimulation of oxytocin
thereby excreting milk secreted from alveolar units into the lobule lumen toward the nipple, where it collects in sinuses of the ducts. As the infant begins to suck, the hormonally (oxytocin) mediated "let down reflex" ensues and the mother's milk is secreted – not sucked from the gland – into the baby's mouth.
All the milk-secreting tissue leading to a single lactiferous duct is called a "simple mammary gland"; a "complex mammary gland" is 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
.
To keep the correct polarized morphology of the lactiferous duct tree requires another essential component - mammary epithelial cells extracellular matrix
(ECM), which together with adipocyte
s, fibroblast
, inflammatory cells etc. constitute mammary stroma. 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 epithelials and their local and global environment throughout this organ's development.
and collagen
, formed thereafter by differentiated myoepithelial cells keeps the polarity of this primary duct tree.
Lactiferous duct development occurs in females in response to circulating hormones, a first development is frequently seen during pre- and postnatal stages and later during puberty
. Estrogen
promotes branching differentiation, whereas in males testosterone
inhibits it. A mature duct tree reaching the limit of the fat pad of the mammary gland comes into being by bifurcation of duct terminal end buds (TEB), secondary branches sprouting from primary ducts and proper duct lumen formation. These processes are tightly modulated by components of mammary epithelial ECM interacting with systemic hormones and local secreting factors. However, for each mechanism the epithelial cells' "niche
" can be delicately unique with different membrane receptor profiles and basement membrane
thickness from specific branching area to area, so as to regulate cell growth or differentiation sub-locally. Important players include beta-1 integrin
, epidermal growth factor receptor
(EGFR), laminin-1/5
, collagen-IV
, matrix metalloproteinase
(MMPs), heparan sulfate proteoglycans etc. Elevated circulating level of growth hormone and estrogen get to multipotent cap cells on tip of TEB through a leaky thin layer of basement membrance and promote specific gene expression. Hence cap cells can differentiate into 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. 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. Degraded basement membrane
fragments (laminin-5) roles to lead the way of mammary epithelial cells migration. Whereas, laminin
-1 interacts with non-integrin receptor dystroglycan
negatively regulates this side branching process in case of cancer
. These complex "Yin-yang" balancing crosstalks between mammary ECM and epithelial cells "instruct" healthy mammary gland development until adult
.
Secretory alveoli develop mainly in pregnancy
, when rising levels of prolactin
, estrogen and progesterone cause further branching, together with an increase in adipose tissue
and a richer 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, Wnt
s secreted from mammary epithelial cells act paracrinely to induce more neighboring cells branching. 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. Their binding ensures correct placement of prolactin receptors on basal lateral side of alveoli cells and directional secretion of milk into lactiferous ducts. Suckling of the baby causes release of hormone oxytocin
which stimulates contraction of the myoepithelial cells. In this way of combined control from ECM and systemic hormones, milk secretion can be reciprocally amplified so as to provide enough nutrition for the baby.
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. In some species there is complete or partial involution
of alveolar structures after weaning, in humans there is only partial involution and the level of involution in humans appears to be highly individual. In some other species (such as cows) all alveoli and secretory duct structure collapse by programmed cell death (apoptosis
) and autophagy
for lack of growth promoting factors either from the ECM or circulating hormones. At the same time, apoptosis of blood capillary endothelial cells speeds up the regression of lactation ductal beds. Shrinkage of the mammary duct tree and ECM remodeling by various proteinase is under the control of somatostatin
and other growth inhibiting hormones and local factors. This big structure change leads loose fat tissue to fill up the empty space thereafter. But a functional lactiferous duct tree can be formed again when a female is pregnant again.
. Almost all instances of breast cancer
originate in the lobules or ducts of the mammary glands.
s, 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) to 18 (in pigs). The Virginia Opossum
has 13, one of the few mammals with an odd number. The following table lists the number and position of glands normally found in a range of mammals:
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 Dayak fruit bat
has lactating mammary glands; male lactation
occurs infrequently in some species, including humans.
Mammary glands are true protein
factories, and several companies have constructed transgenic animal
s, mainly goat
s and cows, in order to produce proteins for pharmaceutical use. Complex glycoprotein
s 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 culture
s.
. There are many theories of how they evolved, but since they do not fossilize well, supporting such theories 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 and free from infection (monotremes still lay eggs). Other theories propose that early secretions were used directly by hatched young, or that the secretions were used by young to help them orient to their mothers.
Lactation developed long before the evolution of the mammary gland and mammals, see evolution of lactation.
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...
in mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s that produces milk
Milk
Milk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many...
to feed young offspring
Offspring
In biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...
. Mammals get their name from the word "mammary". In ruminants such as cows, goats, and deer, the mammary glands are contained in their udders. The mammary glands of other mammals that have more than two breasts, such as dogs and cats, are sometimes called dugs.
Histology
A mammary gland is a specific type of apocrineApocrine
Apocrine is a term used to classify exocrine glands in the study of histology. Cells which are classified as apocrine bud their secretions off through the plasma membrane producing membrane-bound vesicles in the lumen. This method is also called decapitation secretion...
gland specialized for manufacture of colostrum at the time of parturition. Mammary glands can be identified as apocrine because they exhibit striking "decapitation" secretion. Whether mammary glands are modified sweat glands or sebaceous glands still remains controversial.
Structure
The basic components of a mature 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 nippleNipple
In its most general form, a nipple is a structure from which a fluid emanates. More specifically, it is the projection on the breasts or udder of a mammal by which breast milk is delivered to a mother's young. In this sense, it is often called a teat, especially when referring to non-humans, and...
. The myoepithelial cells can contract under the stimulation of oxytocin
Oxytocin
Oxytocin is a mammalian hormone that acts primarily as a neuromodulator in the brain.Oxytocin is best known for its roles in sexual reproduction, in particular during and after childbirth...
thereby excreting milk secreted from alveolar units into the lobule lumen toward the nipple, where it collects in sinuses of the ducts. As the infant begins to suck, the hormonally (oxytocin) mediated "let down reflex" ensues and the mother's milk is secreted – not sucked from the gland – into the baby's mouth.
All the milk-secreting tissue leading to a single lactiferous duct is called a "simple mammary gland"; a "complex mammary gland" is all the simple mammary glands serving one nipple. Humans normally have two complex mammary glands, one in each breast
Breast
The breast is the upper ventral region of the torso of a primate, in left and right sides, which in a female contains the mammary gland that secretes milk used to feed infants.Both men and women develop breasts from the same embryological tissues...
, 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
Accessory breast
Accessory breasts, also known as polymastia, supernumerary breasts, or mammae erraticae, is the condition of having an additional breast...
.
To keep the correct polarized morphology of the lactiferous duct tree requires another essential component - mammary epithelial cells extracellular matrix
Extracellular matrix
In biology, the extracellular matrix is the extracellular part of animal tissue that usually provides structural support to the animal cells in addition to performing various other important functions. The extracellular matrix is the defining feature of connective tissue in animals.Extracellular...
(ECM), which together with adipocyte
Adipocyte
However, in some reports and textbooks, the number of fat cell increased in childhood and adolescence. The total number is constant in both obese and lean adult...
s, fibroblast
Fibroblast
A fibroblast is a type of cell that synthesizes the extracellular matrix and collagen, the structural framework for animal tissues, and plays a critical role in wound healing...
, inflammatory cells etc. constitute mammary stroma. Mammary epithelial ECM mainly contains myoepithelial basement membrane
Basement membrane
The basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs including skin, or the endothelium, which lines the interior surface of blood vessels.- Composition :...
and the connective tissue. They not only help to support mammary basic structure, but also serve as a communicating bridge between mammary epithelials and their local and global environment throughout this organ's development.
Development and hormonal control
Mammary glands develop during different growth cycles. They exist in both sexes during embryonic stage, forming only a rudimentary duct tree at birth. In this stage, mammary gland development depends on systemic (and maternal) hormones, but is also under the (local) regulation of paracrine communication between neighboring epithelial and mesenchymal cells by parathyroid hormone-related protein(PTHrP). This locally secreted factor gives rise to a series of outside-in and inside-out positive feedback between these two types of cells, so that mammary bud epithelial cells can get to proliferate and sprout down into the mesenchymal layer until they reach the fat pad to begin the first round of branching. At the same time, the embryonic mesenchymal cells around the epithelial bud get secrecting factors activated by PTHrP, such as BMP4, can transform into a dense, mammary-specific mesenchyme, which later develop into connective tissue with fibrous threads, forming blood vessels and the lymph system. Basement membrane, mainly containing lamininLaminin
Laminins are major proteins in the basal lamina , a protein network foundation for most cells and organs...
and collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
, formed thereafter by differentiated myoepithelial cells keeps the polarity of this primary duct tree.
Lactiferous duct development occurs in females in response to circulating hormones, a first development is frequently seen during pre- and postnatal stages and later during puberty
Puberty
Puberty is the process of physical changes by which a child's body matures into an adult body capable of reproduction, as initiated by hormonal signals from the brain to the gonads; the ovaries in a girl, the testes in a boy...
. Estrogen
Estrogen
Estrogens , oestrogens , or œstrogens, are a group of compounds named for their importance in the estrous cycle of humans and other animals. They are the primary female sex hormones. Natural estrogens are steroid hormones, while some synthetic ones are non-steroidal...
promotes branching differentiation, whereas in males testosterone
Testosterone
Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands...
inhibits it. A mature duct tree reaching the limit of the fat pad of the mammary gland comes into being by bifurcation of duct terminal end buds (TEB), secondary branches sprouting from primary ducts and proper duct lumen formation. These processes are tightly modulated by components of mammary epithelial ECM interacting with systemic hormones and local secreting factors. However, for each mechanism the epithelial cells' "niche
Niche cell
Niché cells are specific anatomic locations that regulate how stem-cell populations participate in tissue generation, maintenance and repair. The niché performs several functions:...
" can be delicately unique with different membrane receptor profiles and basement membrane
Basement membrane
The basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs including skin, or the endothelium, which lines the interior surface of blood vessels.- Composition :...
thickness from specific branching area to area, so as to regulate cell growth or differentiation sub-locally. Important players include beta-1 integrin
Integrin
Integrins are receptors that mediate attachment between a cell and the tissues surrounding it, which may be other cells or the ECM. They also play a role in cell signaling and thereby regulate cellular shape, motility, and the cell cycle....
, epidermal growth factor receptor
Epidermal growth factor receptor
The epidermal growth factor receptor is the cell-surface receptor for members of the epidermal growth factor family of extracellular protein ligands...
(EGFR), laminin-1/5
Laminin
Laminins are major proteins in the basal lamina , a protein network foundation for most cells and organs...
, collagen-IV
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
, matrix metalloproteinase
Matrix metalloproteinase
Matrix metalloproteinases are zinc-dependent endopeptidases; other family members are adamalysins, serralysins, and astacins. The MMPs belong to a larger family of proteases known as the metzincin superfamily....
(MMPs), heparan sulfate proteoglycans etc. Elevated circulating level of growth hormone and estrogen get to multipotent cap cells on tip of TEB through a leaky thin layer of basement membrance and promote specific gene expression. Hence cap cells can differentiate into 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. On the other hand, basement membrane
Basement membrane
The basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs including skin, or the endothelium, which lines the interior surface of blood vessels.- Composition :...
along the mature mammary ducts is thicker with strong adhesion to epithelial cells via binding to integrin
Integrin
Integrins are receptors that mediate attachment between a cell and the tissues surrounding it, which may be other cells or the ECM. They also play a role in cell signaling and thereby regulate cellular shape, motility, and the cell cycle....
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. Degraded basement membrane
Basement membrane
The basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs including skin, or the endothelium, which lines the interior surface of blood vessels.- Composition :...
fragments (laminin-5) roles to lead the way of mammary epithelial cells migration. Whereas, laminin
Laminin
Laminins are major proteins in the basal lamina , a protein network foundation for most cells and organs...
-1 interacts with non-integrin receptor dystroglycan
Dystroglycan
Dystroglycan is a protein that in humans is encoded by the DAG1 gene.Dystroglycan is one of the dystrophin-associated glycoproteins, which is encoded by a 5.5 kb transcript in Homo sapiens by chromosome 3. There are two exons that are separated by a large intron...
negatively regulates this side branching process in case of cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
. These complex "Yin-yang" balancing crosstalks between mammary ECM and epithelial cells "instruct" healthy mammary gland development until adult
Adult
An adult is a human being or living organism that is of relatively mature age, typically associated with sexual maturity and the attainment of reproductive age....
.
Secretory alveoli develop mainly in pregnancy
Pregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
, when rising levels of prolactin
Prolactin
Prolactin also known as luteotropic hormone is a protein that in humans is encoded by the PRL gene.Prolactin is a peptide hormone discovered by Henry Friesen...
, estrogen and progesterone cause further branching, together with an increase in adipose tissue
Adipose tissue
In histology, adipose tissue or body fat or fat depot or just fat is loose connective tissue composed of adipocytes. It is technically composed of roughly only 80% fat; fat in its solitary state exists in the liver and muscles. Adipose tissue is derived from lipoblasts...
and a richer blood flow
Blood flow
Blood flow is the continuous running of blood in the cardiovascular system.The human body is made up of several processes all carrying out various functions. We have the gastrointestinal system which aids the digestion and the absorption of food...
. In gestation
Gestation
Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time ....
, serum progesterone remains at a stably high concentration so signaling through its receptor is continuously activated. As one of the transcribed genes, Wnt
WNT
WNT is a three-letter abbreviation with multiple meanings, as described below:*Windows NT*WNT *Wnt signaling pathway, a complex protein network...
s secreted from mammary epithelial cells act paracrinely to induce more neighboring cells branching. 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
Colostrum
Colostrum is a form of milk produced by the mammary glands of mammals in late pregnancy. Most species will generate colostrum just prior to giving birth...
is secreted. Milk secretion (lactation
Lactation
Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process occurs in all female mammals, however it predates mammals. In humans the process of feeding milk is called breastfeeding or nursing...
) begins a few days later due to reduction in circulating progesterone
Progesterone
Progesterone also known as P4 is a C-21 steroid hormone involved in the female menstrual cycle, pregnancy and embryogenesis of humans and other species...
and the presence of another important hormone prolactin, which mediates further alveologenesis, milk protein production, and regulates osmotic balance and tight junction
Tight junction
Tight junctions, or zonula occludens, are the closely associated areas of two cells whose membranes join together forming a virtually impermeable barrier to fluid. It is a type of junctional complex present only in vertebrates...
function. Laminin and collagen in myoepithelial basement membrane interacting with beta-1 integrin
Integrin
Integrins are receptors that mediate attachment between a cell and the tissues surrounding it, which may be other cells or the ECM. They also play a role in cell signaling and thereby regulate cellular shape, motility, and the cell cycle....
on epithelial surface again, is essential in this process. Their binding ensures correct placement of prolactin receptors on basal lateral side of alveoli cells and directional secretion of milk into lactiferous ducts. Suckling of the baby causes release of hormone oxytocin
Oxytocin
Oxytocin is a mammalian hormone that acts primarily as a neuromodulator in the brain.Oxytocin is best known for its roles in sexual reproduction, in particular during and after childbirth...
which stimulates contraction of the myoepithelial cells. In this way of combined control from ECM and systemic hormones, milk secretion can be reciprocally amplified so as to provide enough nutrition for the baby.
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. In some species there is complete or partial involution
Involution (medicine)
-Thymus:The thymus continues to grow between birth and puberty and then begins to atrophy, a process directed by the high levels of circulating sex hormones. Proportional to thymic size, thymic activity is most active before puberty. Upon atrophy, the size and activity are dramatically reduced,...
of alveolar structures after weaning, in humans there is only partial involution and the level of involution in humans appears to be highly individual. In some other species (such as cows) all alveoli and secretory duct structure collapse by programmed cell death (apoptosis
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
) and autophagy
Autophagy
In cell biology, autophagy, or autophagocytosis, is a catabolic process involving the degradation of a cell's own components through the lysosomal machinery. It is a tightly regulated process that plays a normal part in cell growth, development, and homeostasis, helping to maintain a balance...
for lack of growth promoting factors either from the ECM or circulating hormones. At the same time, apoptosis of blood capillary endothelial cells speeds up the regression of lactation ductal beds. Shrinkage of the mammary duct tree and ECM remodeling by various proteinase is under the control of somatostatin
Somatostatin
Somatostatin is a peptide hormone that regulates the endocrine system and affects neurotransmission and cell proliferation via interaction with G-protein-coupled somatostatin receptors and inhibition of the release of numerous secondary hormones.Somatostatin...
and other growth inhibiting hormones and local factors. This big structure change leads loose fat tissue to fill up the empty space thereafter. But a functional lactiferous duct tree can be formed again when a female is pregnant again.
Breast cancer
Tumorigenesis in mammary glands can be induced biochemically by abnormal expression level of circulating hormones or local ECM components, or from a mechanical change in the tension of mammary stroma. Under either of the two circumstances, mammary epithelial cells would grow out of control and eventually result in cancerCancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
. Almost all instances of breast cancer
Breast cancer
Breast cancer is cancer originating from breast tissue, most commonly from the inner lining of milk ducts or the lobules that supply the ducts with milk. Cancers originating from ducts are known as ductal carcinomas; those originating from lobules are known as lobular carcinomas...
originate in the lobules or ducts of the mammary glands.
Other mammals
The constantly protruding breasts of the adult human female, unusually large relative to body size, are a unique evolutionary development whose purpose is not yet fully known (see breasts); other mammals tend to have less conspicuous mammary glands that protrude only while actually filling with milk. 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 lineMilk line
The milk lines are two parallel lines, formed by thickenings of the epidermis along the ventral surface of mammals of both sexes. They extend from the upper limbs to the lower limbs and are developed in the embryo...
s, 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) to 18 (in pigs). The Virginia Opossum
Virginia Opossum
The Virginia opossum , commonly known as the North American opossum or tlacuache in Mexico, is the only marsupial found in North America north of Mexico. A solitary and nocturnal animal about the size of a domestic cat, and thus the largest opossum, it is a successful opportunist...
has 13, one of the few mammals with an odd number. The following table lists the number and position of glands normally found in a range of mammals:
| Species | Anterior (thoracic) |
Intermediate (abdominal) |
Posterior (inguinal Inguinal In human anatomy, the inguinal region refers to either the groin or the lower lateral regions of the abdomen. It may also refer to:* Conjoint tendon, previously known as the inguinal aponeurotic falx, a structure formed from the transversus abdominis insertion into the pecten pubis* Granuloma... ) |
Total |
|---|---|---|---|---|
| Goat Goat The domestic goat is a subspecies of goat domesticated from the wild goat of southwest Asia and Eastern Europe. The goat is a member of the Bovidae family and is closely related to the sheep as both are in the goat-antelope subfamily Caprinae. There are over three hundred distinct breeds of... , sheep, horse Horse The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today... guinea pig Guinea pig The guinea pig , also called the cavy, is a species of rodent belonging to the family Caviidae and the genus Cavia. Despite their common name, these animals are not in the pig family, nor are they from Guinea... |
0 | 0 | 2 | 2 |
| Cattle Cattle Cattle are the most common type of large domesticated ungulates. They are a prominent modern member of the subfamily Bovinae, are the most widespread species of the genus Bos, and are most commonly classified collectively as Bos primigenius... |
0 | 0 | 4 | 4 |
| Cat Cat The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests... |
2 | 2 | 4 | 8 |
| Dog Dog The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in... |
4 | 2 | 2 or 4 | 8 or 10 |
| Mouse Mouse A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles... |
6 | 0 | 4 | 10 |
| Rat Rat Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, Rattus norvegicus... |
6 | 2 | 4 | 12 |
| Pig Pig A pig is any of the animals in the genus Sus, within the Suidae family of even-toed ungulates. Pigs include the domestic pig, its ancestor the wild boar, and several other wild relatives... |
6 | 6 | 6 | 18 |
| proboscideans, 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 Dayak fruit bat
Dayak Fruit Bat
The Dayak fruit bat or Dyak fruit bat is a relatively rare frugivorous megabat species found only on the Sunda Shelf of southeast Asia, specifically the Malay Peninsula south of the Isthmus of Kra, and the islands of Borneo and Sumatra. There are three species in the genus Dyacopterus: D....
has lactating mammary glands; male lactation
Male lactation
Male lactation in zoology means production of milk from mammary glands in the presence of physiological stimuli connected with nursing infants. It is well documented in the Dayak fruit bat.The term male lactation is not used in human medicine...
occurs infrequently in some species, including humans.
Mammary glands are true protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
factories, and several companies have constructed transgenic animal
Genetically modified organism
A genetically modified organism or genetically engineered organism is an organism whose genetic material has been altered using genetic engineering techniques. These techniques, generally known as recombinant DNA technology, use DNA molecules from different sources, which are combined into one...
s, mainly goat
Goat
The domestic goat is a subspecies of goat domesticated from the wild goat of southwest Asia and Eastern Europe. The goat is a member of the Bovidae family and is closely related to the sheep as both are in the goat-antelope subfamily Caprinae. There are over three hundred distinct breeds of...
s and cows, in order to produce proteins for pharmaceutical use. Complex glycoprotein
Glycoprotein
Glycoproteins are proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...
s such as monoclonal antibodies or antithrombin
Antithrombin
Antithrombin is a small protein molecule that inactivates several enzymes of the coagulation system. Antithrombin is a glycoprotein produced by the liver and consists of 432 amino acids. It contains three disulfide bonds and a total of four possible glycosylation sites...
cannot be produced by genetically engineered bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, and the production in live mammals is much cheaper than the use of mammalian cell culture
Cell culture
Cell culture is the complex process by which cells are grown under controlled conditions. In practice, the term "cell culture" has come to refer to the culturing of cells derived from singlecellular eukaryotes, especially animal cells. However, there are also cultures of plants, fungi and microbes,...
s.
Evolution
It is believed that the mammary gland is a transformed sweat gland, more closely related to Apocrine sweat glandsApocrine sweat glands
Apocrine sweat glands are sweat glands composed of a coiled secretory portion located at the junction of the dermis and subcutaneous fat, from which a straight portion inserts and secretes into the infundibular portion of the hair follicle...
. There are many theories of how they evolved, but since they do not fossilize well, supporting such theories 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 and free from infection (monotremes still lay eggs). Other theories propose that early secretions were used directly by hatched young, or that the secretions were used by young to help them orient to their mothers.
Lactation developed long before the evolution of the mammary gland and mammals, see evolution of lactation.
See also
- BreastfeedingBreastfeedingBreastfeeding is the feeding of an infant or young child with breast milk directly from female human breasts rather than from a baby bottle or other container. Babies have a sucking reflex that enables them to suck and swallow milk. It is recommended that mothers breastfeed for six months or...
- Mammary tumorMammary tumorA mammary tumor is a tumor 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...
- GynecomastiaGynecomastiaGynecomastia or Gynaecomastia, , is the abnormal development of large mammary glands in males resulting in breast enlargement. The term comes from the Greek γυνή gyné meaning "woman" and μαστός mastós meaning "breast"...
- Teat
- UdderUdderAn udder is an organ formed of the mammary glands of female quadruped mammals, especially ruminants such as cattle, goats, sheep and deer. The udder is a single mass hanging beneath the animal, consisting of pairs of mammary glands...
- 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.