Galactopoiesis 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. The role of galactopoietic hormones such as prolactin in maintenance of lactation is well established. 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. Inhibition of prolactin secretion or administration of prolactin to lactating cows has little effect on milk yields.
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. 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. 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.