Lactation (zoology)

Lactation is the process by which female mammals produce milk to feed their young. The ability to produce milk is one of the defining characteristics of the class Mammalia: all mammals, but no other animals, possess the highly specialized glands necessary for lactation. In evolutionary terms, the appearance of lactation coincides with the tendency of mammals to produce only a few offspring at a time; providing milk for these offspring helps to ensure their survival while removing competition between the adults and the young for food.

The Mammary Glands

The mammary glands are the milk-producing organs. The number varies among species from two to about twenty, with a rough correlation between the number of young born and the number of glands present. The glands are located on the ventral surface of the body, either in the thoracic (in humans, for example) or abdominal region (in horses and cows) or in two lines extending almost the length of the body (in dogs and rodents). Both male and female mammals have mammary glands because, in early mammalian development, the basic body plan of male and female embryos is identical. The mammary glands of males are nonfunctional, however, since they lack the hormonal stimulation necessary for lactation.

Internally, the mammary glands of all mammals follow the same basic plan, consisting of alveoli that produce milk and ducts that carry the milk to openings on the skin's surface. The alveoli are surrounded by myoepithelial cells that contract to squeeze the milk into the ducts during suckling by the young.

Externally, considerable variation exists among mammals in the appearance of the mammary glands and their associated openings. In the spiny anteater and platypus, the many lobes of the mammary glands each open directly to the surface of the abdominal skin through individual ducts, and the young suck the hair-covered skin to obtain the milk. In other mammals, the mammary glands are more obvious as swellings beneath the skin, with a raised area, the nipple or teat, that contains the duct openings. In some four-legged animals (cows, horses, and goats), the mammary glands are located in a baglike structure called the udder, from which are suspended the elongated teats. In humans, the nipple, which contains the openings of fifteen to twenty-five ducts, is surrounded by pigmented skin, the areola. The areola contains glands (tubercles of Montgomery) that secrete a lubricating fluid.

Milk Production

Lactogenesis (milk production) does not begin until a female has produced young. During pregnancy, a complex of hormones prepares the mammary glands for milk production by promoting their growth and internal development. These hormones include prolactin from the mother’s anterior pituitary gland, placental lactogen from the placenta within the uterus, and estrogen and progesterone, which are produced in the corpus luteum of the mother’s ovary and in the placenta. Other hormones, including cortisol from the adrenal gland, thyroxine from the thyroid gland, and insulin from the pancreatic islets, may also be involved. Progesterone appears to participate in the induction of mammary development, but paradoxically, it also prevents milk secretion during pregnancy.

Although true milk is not produced during pregnancy, a precursor to milk, colostrum, can be produced in small amounts by the mammary glands of most species. Colostrum is a sticky, yellowish, transparent liquid. Colostrum secretion continues in the first few days after the birth of the young; there is then a gradual transition to the production of true milk. Colostrum is especially beneficial for the nutrition of the newborn.

Milk contains water, proteins, fats, vitamins, minerals, and a unique sugar, lactose. The exact concentration of the various components varies greatly between species according to the nutritional demands of the young. The milk of seals is high in fat and other solids that contribute to rapid weight gain in the pups, a strategy that is essential for their survival.

Noteworthy among the constituents of milk are antibodies produced by the mother. These antibodies help protect the newborn from disease in the period when the newborn’s own immune system is immature and incapable of providing significant defense. The antibody concentration of colostrum is higher than that of true milk. For this reason, the first few days of nursing are considered the most important for the immunological protection of the newborn. During the COVID-19 pandemic, this became important when new mothers passed along antibodies in their breast milk that offered protection from the virus to their newborns.

The transition in production from colostrum to true milk is brought about by a change in the hormonal status of the mother. At the time of birth, the placenta is expelled from the mother’s body, thus removing the source of progesterone, estrogen, placental lactogen, and other hormones. The decrease in progesterone levels is thought to be essential for the onset of lactogenesis. In addition, at the time of birth, there are changes in prolactin secretion that may play a role in initiating milk secretion.

Suckling

Once lactogenesis is established, a set of hormonal reflexes act to match milk production and delivery to the needs of the newborn. Suckling of the nipple involves motions similar to chewing as the infant takes the nipple between the tongue and the palate. This suckling motion stimulates nerve endings in the mother’s nipple that relay signals about the stimulation back to the mother’s brain. Within thirty to sixty seconds, these signals result in the release of prolactin from the mother’s anterior pituitary gland and oxytocin from her posterior pituitary gland. Prolactin causes the continued production of milk by the alveolar cells of the mammary glands. Oxytocin acts immediately on the myoepithelial cells of the mammary gland, causing them to contract and push milk from the alveoli into the ducts and through the nipple into the infant’s mouth. Thus, the infant does not actually remove milk from the mammary gland by suction but instead is responsible for promoting a hormonal reflex that results in active milk ejection, or letdown, from the mammary gland.

Because of the operation of the prolactin and oxytocin reflexes during suckling, lactation is a biological example of the principle of supply meeting demand. All that is necessary to increase milk production is to increase the suckling stimulus by nursing the young more often. Once established, lactation in some species can be sustained in this manner for years, assuming the nutritional needs of the mother are met. On the other hand, if the mother fails to nurse her offspring, the absence of the suckling stimulus will cause the mammary glands gradually to cease milk production.

The exact composition of the milk is altered as lactation continues to meet the changing nutritional needs of the growing offspring. This is seen in all species. The most extreme example of the ability of the mammary gland to change the composition of milk is seen in the kangaroo. In this animal, the newborn attaches to a teat in the mother’s pouch shortly after birth and remains there for a month or more. A mother kangaroo may nurse offspring of different ages from separate teats, and each teat supplies a milk with the appropriate nutritional composition for that young.

Species vary in time spent suckling the young. The rabbit nurses her litter for only about five minutes once a day, while the rat nurses for about half an hour at a time, at intervals throughout the day. Lactation lasts about ten days in rodents, but it may persist for months in large species such as horses and cows. Some human cultures practice extended breastfeeding, where a child nurses into the second and third years. Continued lactation has a suppressive effect on ovulation that is thought to be attributable to interference by prolactin with the normal hormonal mechanisms that cause ovulation.

The Study of Lactation

Although it has always been clear that milk is expelled from the mammary glands, the realization that the glands themselves actually produce the milk is a relatively recent one. Early anatomists erroneously assumed that milk must be a product of the uterus since the uterus is involved in the support and nourishment of the fetus. Thus, much of the early anatomical work attempted to show some sort of connection between the uterus and the mammary glands. It was not until the late 1800s that the light microscope clearly demonstrated that milk is formed within the mammary glands. In the twentieth century, electron microscopy showed that during pregnancy the intracellular organization of the alveoli becomes increasingly more complex as the cells become capable of milk secretion.

Various techniques for labeling compounds with radioactive or fluorescent markers have been used in conjunction with electron microscopy to examine how milk is synthesized in the alveoli. The alveoli cells extract necessary precursors from blood flowing through the mammary gland, assemble the precursors into milk components, and then secrete the constituents of milk into the mammary gland ducts. Specific routes of secretion have been identified for the major components of milk.

More recently, researchers have used cell-free systems to study the biochemical pathways involved in milk synthesis. These systems use isolated fragments of deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and perhaps some cell organelles to examine the intermediate chemical steps in synthesizing milk components. Using these techniques, researchers have been able to “watch” as complex milk proteins and constituents are assembled step by step. The knowledge of how the components of milk are assembled is leading to a fuller understanding of how the amounts of these substances in milk are hormonally regulated.

Knowledge of the hormones involved in inducing and maintaining lactation has come about through a systematic assembly of information from several lines of research. Test animals can be treated with a specific hormone to determine if that hormone causes or suppresses lactation. The test animals may be males or immature females, with the goal being the duplication of the specific mix of hormones that cause lactation in the adult female. The opposite approach may also be taken: an endocrine gland can be removed from a lactating female to determine if the hormonal products of that gland are necessary for lactation. Another approach is to carefully measure the levels of hormones circulating in the blood as the lactational state changes; any hormone that shows a correlated change may be a good candidate for further investigation by treatment or removal from test animals. These methods have led to an understanding of the importance of prolactin, placental lactogen, oxytocin, estrogen, and progesterone in promoting lactation, but researchers still do not understand how the system is fine-tuned. For example, considerable variation exists in the volume and quality of milk produced by different individuals—or by the same individual at different times—but these differences cannot currently be explained by any known change in hormone levels. Research is focusing not only on describing changes in circulating levels of hormones but also on elucidating the exact effects of these hormones on the biosynthetic pathways within the mammary gland.

Another important area of study in terms of lactation is the rearing of dairy cattle in order to produce greater quantities and better quality milk. Though milk consumption in the United States is down from twenty years agoHolstein-Friesian breed is the most common in the U.S. for milk production, and the average dairy cow produces more than seven gallons of milk per day. Typically, they are milked two to three times per day, either manually by humans or by robotic machines. In some larger dairy farms, cows can choose when they want to be milked by walking into a robotic milking stale at will. This choice is rewarded with feed to promote future milkings. However, some farmers argue that milking only once a day makes for a healthier, more fertile cow in the long run, despite somewhat smaller yields, as there is less stress on the cow's body. The International Committee for Animal Recording (ICAR) has placed guidelines and standards on yield per animal and how to track genetic information. However, as technology continues to advance, so too will farmers' and researchers' understanding of how to most efficiently produce the best milk possible, both for human consumption and for the health of the cow.

Principal Terms

Alveoli: the milk-producing areas within the mammary glands

Colostrum: the precursor to milk that is formed in the mammary gland during pregnancy and immediately after the birth of the young

Ducts: the tubular structures that carry milk from the alveoli to the outside through the nipple or teat

Lactation: the process of producing and delivering milk to the young; also, the time period during which milk is produced

Mammary Glands: the milk-producing glands found in all mammals; for example, the cow’s udder contains the mammary glands

Milk Ejection: also known as milk letdown, this is the reflex response of the mammary gland to the suckling of the nipple; the hormone oxytocin mediates this reflex

Myoepithelial Cells: the specialized cells within the mammary gland that surround the alveoli and contract to force milk into the ducts during milk ejection

Nipple: the raised area on the surface of the skin over the mammary gland that contains the duct openings

Teat: an elongated form of nipple that contains one duct opening

Bibliography

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Cole, John B., et al. “The Effects of Breeding and Selection on Lactation in Dairy Cattle.” Animal Frontiers, vol. 13, no. 3, 2023, pp. 55-63. Oxford Academic, doi.org/10.1093/af/vfad044. Accessed 14 Sept. 2024.

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