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.

Colostrum

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.

Lactogenesis

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) and hypocalcemia caused by the transient drain of calcium that occurs with initiation of lactation (dairy cattle) or during the perinatal period (dog).