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Cows walking to the barn.


Recall from The Beginning Independent Study Module that lactation is the combined processes of milk secretion and milk removal. This current Module on Lactation will focus on the regulation of milk secretion and milk removal. Other lessons also are relevent to this section. For example, the Module on Mammary Structure: Macro & Micro includes sections on the structure and cell biology of the alveolar epithelial cells, the cells that synthesize and secrete milk. Many of the underlying principles of regulation of lactogenesis, dicsussed in the Mother & Neonate Module, hold true for the maintenance of lactation once it is established.

Galactopoeisis is the maintenance of lactation once lactation has been established. Two key interrelated components contribute to the maintenance of lactation, galactopoietic hormones and removal of accumulated milk. Because of the importance of galactopoietic hormones in milk production, sometimes the word galactopoiesis also is used to indicate enhancement of lactation, especially in dairy animals. Inhibition of secretion of key galactopoietic hormones will depress milk production to varying degrees depending on the species, stage of lactation, and the particular hormone being suppressed. Much of the fundamental knowledge that we have on galactopoietic hormones comes from classic studies demonstrating that inhibition of hormone secretion will inhibit milk production. Conversely, administration of additional amounts of galactopoietic hormones during lactation can enhance milk protuction, again depending on the species, stage of lactation, and particular hormone. The most widely known example of this has led to the common practice of administration of bovine somatotropin (bST, or bovine growth hormone) to lactating dairy cattle resulting in relatively dependable increases in milk yield. Any substance that is administered to a lactating animal and that results in increased milk production would be considered a galactagogue.

The role of galactopoietic hormones such as prolactin in maintenance of lactation is well established (reviewed by Tucker 1994), although specific cellular mechanisms of action continue to be investigated. Prolactin is considered the major galactopoietic hormone in nonruminants. Prolactin is released at the time of milk removal in ruminants and nonruminants, and it remains a key systemic modulator of milk secretion during lactation. Conversely, growth hormone is generally considered to be the predominant galactopoietic hormone in ruminants (Bauman 1992; Tucker 1994). Inhibition of prolactin secretion or administration of prolactin to lactating cows has little effect on milk yields (Karg and Schams 1974; Plaut et al. 1987). However, these apparently clear-cut roles of prolactin vs. growth hormone in maintenance of lactation in nonruminants vs. ruminants are probably an oversimplification (Wilde and Hurley 1996). For example, in lactating sheep both prolactin and growth hormone seem to be important for galactopoiesis (Hooley et al. 1978; Tucker 1994). Even in the rat, recent studies have demonstrated an important role for growth hormone, independent of the role of prolactin (Flint et al. 1992; Flint and Gardner 1994).

Regardless of the hormones involved, all attempts to evaluate milk secretion must account for continued removal of milk. This is a reminder of the critical role of local mammary factors in maintenance of milk secretion. One such factor that plays a major role in regulating milk secretion in many species is a feedback inhibitor of lactation (FIL) found in milk (Wilde et al. 1995). FIL is thought to be produced by the mammary cells as they synthesize and secrete milk. Accumulation of FIL in the milk-producing alveoli results in feedback inhibition of milk synthesis and secretion. Frequent removal of milk from the gland minimizes local inhibitory effects of FIL and increases milk secretion (Wilde et al. 1987; Wilde and Knight 1989; Wilde and Peaker 1990).

Milk removal involves several mechanisms that impact milk production, including removal of local inhibitory components, regulation of local blood flow, and even physical factors in the alveolus. The effects of frequency of milk removal are tied closely with the local regulation of milk secretion. The mechanism by which the alveoli physically express milk from the lumen during milk removal is called milk ejection. Stimulation of the mammary gland, particularly the teats or nipples, results in secretion of the hormone oxytocin from the posterior pituitary. Oxytocin travels via the blood to the mammary gland and causes contraction of the myoepithelial cells surrounding the alveolus (for a reminder of myoepithelial cells, see the lesson on Mammary Structure: Macro & Micro). This results in expulsion of the luminal milk from the alveolus into the ducts and out of the gland, resulting in the physical removal of milk from the alveoli.

The role of milk removal complicates interpretation of the hormonal requirements for milk synthesis and secretion. Without frequent emptying of the mammary gland (milk removal), milk synthesis will not persist in spite of adequate hormonal status. Conversely, maintenance of intense suckling or milking stimulus will not maintain lactation indefinitely. Nevertheless, suckling or actual removal of milk from the gland is required to maintain lactation.

This module will begin with a discussion of the major hormones involved in galactopoiesis, followed by discussion of milk removal and its implications, and finally by the mechanisms of milk ejection.

Lactation Resources