The Mammary Gland

The Mammary Gland

Key Points

Anatomical aspects of the mammary gland

1. The milk-secreting cells of the mammary gland develop through the proliferation of epithelium into hollow structures called alveoli.

2. Most of the milk that accumulates before suckling or milking is stored in the alveoli, even though animals have enlarged milk-storage areas called cisterns.

3. A suspensory system involving the udder of the cow allows the animal to carry a large amount of milk.

Control of mammogenesis

1. Initial development of the mammary gland is programmed by embryonic mesenchyme.

2. Proliferation of the mammary duct system begins at puberty, with ducts under the control of estrogens, growth hormone, and adrenal steroids, and alveoli under the control of progesterone and prolactin.


1. Prepartum milk secretion (without removal) results in the formation of colostrum.

2. The ingestion of colostrum is important because of the passive immunity it confers through the presence of high concentrations of immunoglobulins.

3. The time immunoglobulins can be absorbed through the neonatal gut is limited to the first 24 to 36 hours of life.

4. Lipids (particularly vitamin A) and proteins (caseins and albumins) are high in concentration in colostrum; carbohydrates (lactose) are low.


1. Prolactin, inhibited by dopamine and stimulated by vasoactive intestinal peptide, is the most important hormone involved in the process of milk synthesis, or lactogenesis; growth hormone is also important for lactogenesis.

2. The release of fat into milk from the alveolar cell involves constriction of the plasma membrane around the fat droplet; fats are dispersed in milk in droplet form.

3. Milk proteins and lactose are released from alveolar cells by the process of exocytosis.

Milk removal

1. Efficient milk removal requires the release of oxytocin, which causes contraction of muscle cells that surround the alveoli (myoepithelial cells), and movement of milk into the ducts and cisterns.

First nursing

1. Carbohydrate stores are good in neonates born as singles or twins, whereas carbohydrate stores are low in neonates born in litters; consequently the former can stand a longer interval to first suckling than can the latter.

Composition of milk

1. Fats are the most important energy source in milk.

2. Lactose, composed of glucose and galactose, is the main carbohydrate of mammalian milk.

3. The main proteins in milk are called caseins and are found in curd.

The lactation cycle

1. Milk production peaks at 1 month postpartum in dairy cattle, followed by a slow decline in production; milking usually stops at 305 days of lactation so that the animal can prepare the mammary gland for the next lactation.

2. Lactation can be induced by hormone administration (estrogen and progesterone) and enhanced by growth hormone and increased photoperiod exposure.

Diseases associated with the mammary gland

1. The main diseases that affect the mammary gland directly are mastitis (prevalent in dairy cattle and dogs) and neoplasia (prevalent in intact dogs and cats).

2. The main conditions that involve the mammary gland indirectly are passive transfer of red blood cell agglutinating antibodies by the ingestion of colostrum (mare, queen) and hypocalcemia caused by the transient drain of calcium that occurs with initiation of lactation (dairy cattle) or during the perinatal period (dog).

Animals that belong to the class Mammalia are characterized as having bodies that are basically covered with hair, delivering live young instead of eggs (the monotremes are an exception), and, pertinent to this chapter, nurturing their young through the use of structures called mammary glands. The ability of mammals to nurture their young through milk secretion by mammary glands during the early part of post-fetal life has given these animals survival advantages. Because the reproductive strategy of mammals involves the production of far fewer young, compared with reptiles, amphibians, and birds, mammary glands have allowed mammals to be much more efficient in the nurture of their young. Egg-laying classes of animals, such as fish, reptiles, and amphibians, depend on favorable environmental factors for the nurture of their young; the offspring are often vulnerable to the vagaries of nature. Mammalian young do not require teeth for the suckling process and thus can be delivered with immature maxillae and mandibles, which facilitates the delivery of the head. The development of teeth coincides with the need to consume food other than milk.

Anatomical Aspects of the Mammary Gland

The Milk-Secreting Cells of the Mammary Gland Develop Through the Proliferation of Epithelium into Hollow Structures Called Alveoli

Embryonic ectoderm is the source of the mammary glands. The mammary ectoderm is first represented by parallel linear thickenings on the ventral belly wall. The continuity of the ridge that is formed is broken into the appropriate number of mammary buds, from which the functional part of the mammary gland will be derived.

The parenchyma, or milk-secreting cells, of the mammary gland develops through the proliferation of epithelial cells that arise from the primary mammary cord. The epithelial cells eventually form hollow, circular structures called alveoli, which are the fundamental milk-secreting units of the mammary gland (Figure 39-1). In concert with this development, an enlarged area of epithelium, the nipple, which is the external connection to the internal milk-secreting system, develops on the surface. In males, although nipples often develop, the underlying primary mammary cord does not develop into substantial glandular tissue.

Most of the Milk That Accumulates Before Suckling or Milking Is Stored in the Alveoli, Even Though Animals Have Enlarged Milk-Storage Areas Called Cisterns

Duct systems connect alveoli with the nipple, or teat, enabling milk to pass from the area of formation to the area of delivery (nipple). The ducts may come together so that there is only one final duct per gland, which has one opening through the nipple, or teat, such as occurs in cattle, goats, and sheep. Two main ducts and associated openings occur in the mare and sow, whereas the cat and dog can have 10 or more openings in the nipple, with each opening representing separate glands (Figure 39-2). Both the cow and the doe (goat) have specialized areas for holding milk, called cisterns, which are located in the ventral part of the gland and into which all main ducts empty (Figure 39-3). This has enabled the cow, for example, to synthesize and store larger amounts of milk than would otherwise be possible. Despite this adaptation, it is important to realize that a majority of the milk present at the time of milking is stored in the duct system of the mammary glands.

Mammary glands develop typically as paired structures. The number of pairs in domestic animals varies from one in goats, horses, and sheep; two in cattle; to seven to nine in the sow and seven to ten in the bitch and queen. The position of mammary glands varies in animals, being thoracic in primates; extending the length of the thorax and abdomen in cats, dogs, and pigs; and being inguinal in cattle, goats, and horses. In domestic species, such as cattle, goats, horses, and sheep, pairs of mammary glands are closely apposed to each other; the resulting structure is called an udder. In the cow, for example, two pairs of glands (four quarters) compose the udder.

A Suspensory System Involving the Udder of the Cow Allows the Animal to Carry a Large Amount of Milk

One of the important anatomical adaptations of the udder that allows dairy cows to carry large amounts of milk is the development of a suspension system for the udder. This system is formed by the median suspensory ligament (formed between pairs of mammary glands) composed of elastic connective tissue that originates from the abdominal tunic. The lateral (nonelastic) suspensory ligament, which originates from prepubic and subpubic ligaments, enters the glands laterally at various levels to become part of the interstitial connective tissue framework of the udder. It is not unusual for heavy-producing dairy cows to have 25 kg (55 lb) of milk in their udder immediately before milking. If the suspensory support system were not in place, the mammary gland system would soon break down from the weight of the milk.

Control of Mammogenesis

Initial Development of the Mammary Gland Is Programmed by Embryonic Mesenchyme

The fetal development of the mammary gland is under both genetic and endocrine control. The initial development of the mammary bud is under control of embryonic mesenchyme (connective tissue). If mammary mesenchyme is transplanted to another area, mammary bud formation will occur at the site of transplantation. Although little is known about fetal mammary development, it is not thought to be driven by hormones. However, actively secreting mammary glands may be present at birth as a result of exogenous administration of certain hormones to the mother.

Proliferation of the Mammary Duct System Begins at Puberty, with Ducts Under the Control of Estrogens, Growth Hormone, and Adrenal Steroids, and Alveoli Under the Control of Progesterone and Prolactin

Development of the mammary gland in post-fetal life usually starts in concert with puberty. Cyclical ovarian activity results in the production of estrogen and progesterone. Estrogen, with growth hormone and adrenal steroids, is responsible for proliferation of the duct system. The development of alveoli from the terminal ends of the ducts requires the addition of progesterone and prolactin (Figure 39-4).

Although the development of the mammary gland begins with the onset of puberty, the gland remains relatively undeveloped until the occurrence of pregnancy. In most domestic animals, udder development usually becomes evident by the middle of gestation; the secretion of milk often begins during the latter part of gestation (mainly from increasing prolactin secretion) and results in the formation of colostrum, as discussed later. By the end of pregnancy, the mammary gland has been transformed from a structure involving mostly stromal (connective tissue) elements to a structure that is filled with alveolar cells that are actively synthesizing and secreting milk. Groups of adjacent alveoli form lobules that further combine into larger structures called lobes. Connective tissue bands delineate the lobules and the lobes (Figure 39-5).


Prepartum Milk Secretion (Without Removal) Results in the Formation of Colostrum

The milk formed before parturition is called colostrum. Its formation represents a secretory process in which lactogenesis occurs in the absence of milk removal. Lactation cannot fully blossom until pregnancy is terminated, however, because of the inhibitory effects of progesterone and estrogen on milk secretion, inhibitory factors that are removed at or just before delivery.

The Ingestion of Colostrum Is Important Because of the Passive Immunity It Confers Through the Presence of High Concentrations of Immunoglobulins

When colostrum is formed before parturition, certain substances are concentrated in the process. Ingestion of colostrum is important for the well-being of the neonate. In addition to nutrition, colostrum has an important function in temporary, or passive, protection against infectious agents. Immunoglobulins (e.g., immunoglobulin A, or IgA) are produced in the mammary gland by plasma cells (derived from B lymphocytes originating in the gut) as a result of exposure of the mother to certain microorganisms. The immunoglobulins gain access to the milk system through the migration of the plasma cells from adjacent tissue sites. The immunoglobulins are highly concentrated in colostrum, and through the consumption of colostrum, the neonate can receive passive immunity against pathogens experienced by the mother. This allows the young to receive immediate protection from environmental organisms. The neonates of all domestic animals acquire antibodies through the ingestion of colostrum. The absorption of antibodies through milk in domestic animals contrasts with the situation in other species, including humans, rabbits, and guinea pigs, in which a more substantial amount of antibody is passed to the fetus through the placenta.

Lipids (Particularly Vitamin A) and Proteins (Caseins and Albumins) Are High in Concentration in Colostrum; Carbohydrates (Lactose) Are Low

Colostrum is a rich source of nutrients, especially vitamin A, in addition to immunoglobulins. Placental transfer of vitamin A is limited in domestic animals, with calves and piglets being particularly low in vitamin A at birth. This deficiency is corrected by the ingestion of colostrum. Lipids and proteins, including caseins and albumins, are also present in relatively high concentration in colostrum. One exception is lactose; its synthesis is significantly inhibited by progesterone until about the time of delivery. Nevertheless, at the moment of delivery, the newborn’s milk supply is nutritive (high protein, fat, and vitamin A content) and protective (immunoglobulins) (Table 39-1).

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on The Mammary Gland

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