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.

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).

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.

The Time Immunoglobulins Can Be Absorbed Through the Neonatal Gut Is Limited to the First 24 to 36 Hours of Life

Neonates usually have a limited time (24 to 36 hours) in which immunoglobulins (proteins) can be absorbed through the gut. Thus the feeding of colostrum within this period is important to ensure the presence of immunoglobulins in the newborn. Other antimicrobial factors found in milk that are important for protection against the development of pathogenic enteric bacterial flora include lysozymes, lactoferrin, and the lactoperoxidase system.

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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