Lactation
Overview
Lactation describes the secretion of 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...

 from the mammary gland
Mammary gland
A mammary gland is an organ in mammals 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...

s and the period of time that a mother
Mother
A mother, mum, mom, momma, or mama is a woman who has raised a child, given birth to a child, and/or supplied the ovum that grew into a child. Because of the complexity and differences of a mother's social, cultural, and religious definitions and roles, it is challenging to specify a universally...

 lactates to feed her young. The process occurs in all female
Female
Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...

 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, however it predates mammals. In humans the process of feeding milk is called breastfeeding
Breastfeeding
Breastfeeding 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...

 or nursing. In most species milk comes out of the mother's nipples; however, the platypus
Platypus
The platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...

 (a non-placental mammal) releases milk through ducts in its abdomen. In only one species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of mammal, the 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....

, is milk production a normal male function.
Discussions
Encyclopedia
Lactation describes the secretion of 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...

 from the mammary gland
Mammary gland
A mammary gland is an organ in mammals 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...

s and the period of time that a mother
Mother
A mother, mum, mom, momma, or mama is a woman who has raised a child, given birth to a child, and/or supplied the ovum that grew into a child. Because of the complexity and differences of a mother's social, cultural, and religious definitions and roles, it is challenging to specify a universally...

 lactates to feed her young. The process occurs in all female
Female
Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...

 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, however it predates mammals. In humans the process of feeding milk is called breastfeeding
Breastfeeding
Breastfeeding 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...

 or nursing. In most species milk comes out of the mother's nipples; however, the platypus
Platypus
The platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...

 (a non-placental mammal) releases milk through ducts in its abdomen. In only one species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of mammal, the 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....

, is milk production a normal male function. In some other mammals, the male may produce milk as the result of a hormone
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...

 imbalance. This phenomenon may also be observed in newborn infants as well (for instance witch's milk).

Galactopoiesis is the maintenance of 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...

 production. This stage requires 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...

 (PRL) and 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...

.

Purpose

The chief function of lactation is to provide nutrition
Nutrition
Nutrition is the provision, to cells and organisms, of the materials necessary to support life. Many common health problems can be prevented or alleviated with a healthy diet....

 and immune protection to the young after birth. In almost all mammals, lactation induces a period of infertility
Infertility
Infertility primarily refers to the biological inability of a person to contribute to conception. Infertility may also refer to the state of a woman who is unable to carry a pregnancy to full term...

, which serves to provide the optimal birth spacing for survival of the offspring.

Human lactation

Hormonal influences

From the twenty-fourth week of 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...

 (the second and third trimesters), a woman's body produces hormone
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...

s that stimulate the growth of the milk duct system in the 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...

s:
  • 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...

    —influences the growth in size of alveoli
    Alveolar gland
    In contrast to tubular glands, in the second main variety of gland, the secretory portion is enlarged and the lumen variously increased in size. These are termed alveolar glands ....

     and lobes, high levels of progesterone inhibit lactation before birth. Progesterone levels drop after birth, this triggers the onset of copious milk production.
  • Oestrogen—stimulates the milk duct system to grow and differentiate. Like progesterone high levels of oestrogen also inhibit lactation. Oestrogen levels also drop at delivery and remain low for the first several months of breastfeeding. It is recommended that breastfeeding mothers avoid oestrogen-based birth control methods, as a spike in estrogen levels may reduce a mother's milk supply.
  • 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...

    —contributes to the increased growth and differentiation of the alveoli, also influences differentiation of ductal structures. High levels of prolactin during pregnancy and breastfeeding also increase insulin resistance, increase growth factor levels (IGF-1) and modify lipid metabolism in preparation for breastfeeding. During lactation prolactin is the main factor maintaining 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...

    s of the ductal epithelium and regulating milk production through osmotic balance.
  • Growth hormone
    Growth hormone
    Growth hormone is a peptide hormone that stimulates growth, cell reproduction and regeneration in humans and other animals. Growth hormone is a 191-amino acid, single-chain polypeptide that is synthesized, stored, and secreted by the somatotroph cells within the lateral wings of the anterior...

     is structurally very similar to prolactin and contributes to its galactopoietic function.
  • ACTH and glucocorticoids have an important lactation inducing function in several animal species. ACTH is thought to contribute as it is structurally similar to prolactin. Glucocorticoids play a complex regulating role in the maintenance of tight junctions.
  • TSH
    TSH
    TSH may refer to:* Tanzanian shilling, the currency of Tanzania* Thyroid-stimulating hormone, a hormone synthesized and secreted by thyrotrope cells* Tshiuetin Rail Transportation, a Canadian railway between Labrador and Quebec...

     is a very important galactopoietic hormone, its levels are naturally increased during pregnancy.
  • 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...

    —contracts the smooth muscle
    Smooth muscle
    Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...

     of the uterus
    Uterus
    The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...

     during and after birth, and during orgasm(s). After birth, oxytocin contracts the smooth muscle layer of band-like cells surrounding the alveoli to squeeze the newly-produced milk into the duct system. Oxytocin is necessary for the milk ejection reflex, or let-down to occur.
  • Human placental lactogen
    Human placental lactogen
    Human placental lactogen , also called human chorionic somatomammotropin, is a polypeptide placental hormone. Its structure and function is similar to that of human growth hormone. It modifies the metabolic state of the mother during pregnancy to facilitate the energy supply of the fetus. HPL has...

     (HPL)—From the second month of pregnancy, the placenta
    Placenta
    The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...

     releases large amounts of HPL. This hormone appears to be instrumental in breast, nipple, and areola growth before birth.
  • Follicle stimulating hormone (FSH)
  • Luteinizing hormone
    Luteinizing hormone
    Luteinizing hormone is a hormone produced by the anterior pituitary gland. In females, an acute rise of LH called the LH surge triggers ovulation and development of the corpus luteum. In males, where LH had also been called interstitial cell-stimulating hormone , it stimulates Leydig cell...

     (LH)


By the fifth or sixth month of pregnancy, the breasts are ready to produce milk. It is also possible to induce lactation without pregnancy.

Lactogenesis I

During the latter part of pregnancy, the woman's breasts enter into the Lactogenesis I stage. This is when the breasts make 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...

 (see below), a thick, sometimes yellowish fluid. At this stage, high levels of progesterone inhibit most milk production. It is not a medical concern if a pregnant woman leaks any colostrum before her baby's birth, nor is it an indication of future milk production.

Lactogenesis II

At birth
Childbirth
Childbirth is the culmination of a human pregnancy or gestation period with the birth of one or more newborn infants from a woman's uterus...

, prolactin levels remain high, while the delivery of the placenta results in a sudden drop in progesterone, estrogen, and HPL levels. This abrupt withdrawal of progesterone in the presence of high prolactin levels stimulates the copious milk production of Lactogenesis II.

When the breast is stimulated, prolactin levels in the blood rise, peak in about 45 minutes, and return to the pre-breastfeeding state about three hours later. The release of prolactin triggers the cells in the alveoli to make milk. Prolactin also transfers to the breast milk. Some research indicates that prolactin in milk is greater at times of higher milk production, and lower when breasts are fuller, and that the highest levels tend to occur between 2 a.m. and 6 a.m.

Other hormones—notably insulin, thyroxine, and cortisol—are also involved, but their roles are not yet well understood. Although biochemical markers indicate that Lactogenesis II begins about 30–40 hours after birth, mothers do not typically begin feeling increased breast fullness (the sensation of milk "coming in the breast") until 50–73 hours (2–3 days) after 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 the first milk a breastfed baby receives. It contains higher amounts of white blood cells and antibodies
Antibody
An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...

 than mature milk, and is especially high in immunoglobulin A
Immunoglobulin A
Immunoglobulin A is an antibody that plays a critical role in mucosal immunity. More IgA is produced in mucosal linings than all other types of antibody combined; between three and five grams are secreted into the intestinal lumen each day....

 (IgA), which coats the lining of the baby's immature intestines, and helps to prevent pathogens from invading the baby's system. Secretory IgA also helps prevent food allergies. Over the first two weeks after the birth, colostrum production slowly gives way to mature breast milk.

Lactogenesis III

The hormonal endocrine control system drives milk production during pregnancy and the first few days after the birth. When the milk supply is more firmly established, autocrine (or local) control system begins. This stage is called Lactogenesis III

During this stage, the more that milk is removed from the breasts, the more the breast will produce milk. Research also suggests that draining the breasts more fully also increases the rate of milk production. Thus the milk supply is strongly influenced by how often the baby feeds and how well it is able to transfer milk from the breast. Low supply can often be traced to:
  • not feeding or pumping
    Breast pump
    A breast pump is a mechanical device that extracts milk from the breasts of a lactating woman. Breast pumps may be manual devices powered by hand or foot movements or electrical devices powered by mains electricity or batteries.- History :...

     often enough
  • inability of the infant to transfer milk effectively caused by, among other things:
    • jaw or mouth structure deficits
    • poor latching technique
  • rare maternal endocrine disorders
  • hypoplastic breast tissue
  • a metabolic or digestive inability in the infant, making it unable to digest the milk it receives
  • inadequate calorie intake or malnutrition of the mother

Milk ejection reflex

Milk ejection is initiated in the mother's breast by the act of suckling by the baby. The let-down reflex is not always consistent, especially at first. Once a women is conditioned to nursing, let-down can be triggered by a variety of stimuli, including the sound of any baby or even thinking about breastfeeding can stimulate this reflex, causing unwanted leakage, or both breasts may give out milk when an infant is feeding from one breast. However, this and other problems often settle after two weeks of feeding. Stress
Stress (medicine)
Stress is a term in psychology and biology, borrowed from physics and engineering and first used in the biological context in the 1930s, which has in more recent decades become commonly used in popular parlance...

 or anxiety can cause difficulties with breastfeeding. The release of the 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...

 leads to the milk ejection or let-down reflex. Oxytocin stimulates the muscles surrounding the breast to squeeze out the milk. Breastfeeding mothers describe the sensation differently. Some feel a slight tingling, others feel immense amounts of pressure or slight pain/discomfort, and still others do not feel anything different.

A poor milk ejection reflex can be due to sore or cracked nipples, separation from the infant, a history of breast surgery
Surgery
Surgery is an ancient medical specialty that uses operative manual and instrumental techniques on a patient to investigate and/or treat a pathological condition such as disease or injury, or to help improve bodily function or appearance.An act of performing surgery may be called a surgical...

, or tissue damage from prior breast trauma. If a mother has trouble breastfeeding, different methods of assisting the milk ejection reflex may help. These include feeding in a familiar and comfortable location, massage of the breast or back, or warming the breast with a cloth or shower.

Afterpains

A surge of oxytocin also causes the uterus to contract. During breastfeeding, mothers may feel these contractions as afterpains. These may range from period-like cramps to strong labour-like contractions and can be more severe with second and subsequent babies. Some women's breasts also become dry and chapped and even crack open and bleed while breast feeding. Rubbing lanolin on the nipples and areola can help with those problems.

Lactation without pregnancy, induced lactation, relactation

In humans induced lactation and relactation has been observed frequently in primitive cultures and demonstrated with varying success in adoptive mothers. It appears plausible that the possibility of induction of lactation in women (or females of other species) who are not biological mothers does confer an evolutionary advantage especially in groups with high maternal mortality and tight social bonds. The phenomenon has been also observed in most primates, some lemurs and dwarf mongooses.

Lactation can be induced in humans by a combination of physical and psychological stimulation, by drugs, or by a combination of those methods. Some couples may stimulate lactation outside of pregnancy for sexual purposes
Erotic lactation
Erotic lactation refers to a person achieving sexual arousal by breastfeeding or sucking on a female's breast. Depending on the context, the practice can also be referred to as adult suckling, adult nursing, and adult breastfeeding. Practitioners sometimes refer to themselves as being in an adult...

.

Rare accounts of 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...

 (as distinct from galactorrhea
Galactorrhea
Galactorrhea or galactorrhoea is the spontaneous flow of milk from the breast, unassociated with childbirth or nursing.Contemporary Maternal-Newborn Nursing Care defines galactorrhea as "nipple discharge." -Causes:...

) exist in historical medical and anthropological literature, although the phenomenon has not been confirmed by more recent literature.

Evolution

Darwin correctly recognised that mammary glands developed from cutaneous glands and postulated the hypothesis that they evolved from glands in brood pouches of fish where they provided nourishment for eggs. The later aspect of his hypothesis has not been confirmed, but recently the same mechanism has been postulated for early synapsid
Synapsid
Synapsids are a group of animals that includes mammals and everything more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having an opening low in the skull roof behind each eye, leaving a bony arch beneath each, accounting for their name...

s. Instead the discus fish (Symphysodon aequifasciata) became known for (biparentally) feeding their offspring by epidermal mucus secretion. A closer look reveals that similar to most mammals the secretion of the nourishing fluid may be controlled by prolactin.

Later therapsids
Therapsida
Therapsida is a group of the most advanced synapsids, and include the ancestors of mammals. Many of the traits today seen as unique to mammals had their origin within early therapsids, including hair, lactation, and an erect posture. The earliest fossil attributed to Therapsida is believed to be...

 such as cynodonts appear to have secreted complex, nutrient-rich milk. This brought them evolutionary advantage by allowing a decline in egg size.

During early evolution of lactation the secretion was through pilosebaceous glands, and mammary hairs transported the nourishing fluids to the eggs or youngs. Later the development of the mammary patch rendered mammary hairs obsolete.

Other well known example of nourishing young with secretions of glands is the crop milk
Crop milk
Crop milk is a secretion from the lining of the crop of parent birds that is regurgitated to young birds. They are found among all pigeons and doves where they are referred to as pigeon milk...

 of pigeons. Like in mammals and disc fish this also appears closely related to prolactin. Other birds such as flamingos and penguins are utilizing similar feeding techniques.

See also

  • Lactation room
    Lactation room
    Lactation room is an American term for a private room where a breastfeeding woman can use a breast pump in private one or more times a day.-Purpose:...

  • Galactogogue
  • Milk line
    Milk 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...

  • Udder
    Udder
    An 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...

  • Breastfeeding
    Breastfeeding
    Breastfeeding 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...


External links




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