Mammary involution in dairy animals.

Lifetime milk production is maximized when dairy cows are pregnant during approximately 70% of each lactation. Between lactations, a nonlactating period is necessary for optimal milk production in the succeeding lactation. With cessation of milking, alveolar structure is largely maintained and little or no loss of cells occurs. However, increased apoptosis and cell proliferation, relative to that in lactating glands during the same stage of gestation, suggest that a nonlactating period serves to promote cell turnover prior to the next lactation. Even in the absence of pregnancy, mammary involution in dairy animals occurs at a slower rate than in rodents; alveolar structure is maintained for several weeks and lactation can be reinitiated after four weeks or more of involution. Although apoptosis appears to be initiated within a similar time frame to that in rodents, the maximum proportion of apoptotic epithelial cells appears to be lower than in rodents, and apoptosis may be accompanied by an initial increase in cell proliferation. The ability to manipulate apoptosis and cell proliferation during the nonlactating period and during lactation is expected to provide enormous benefits to the dairy industry.