Rates of cell proliferation and death, along with changes in secretory activity of mammary epithelial cells, account for the shape of the lactation curve.[5,6] In litter-bearing species such as rodents and pigs, considerable mammary growth occurs during early stages of lactation (pre-peak lactation), and may even equal the extent of mammary growth that occurred during gestation. In ruminants, the magnitude of mammary growth during lactation is more limited and appears to be restricted to the first weeks of lactation. In goats, mammary growth during lactation may equal approximately 20% of the mammary growth that occurred during gestation. In dairy cows, nearly all mammary growth occurs prior to parturition, but increased milking stimuli during the first three weeks of lactation may promote postparturient proliferation of mammary epithelium. Increased milk production during early lactation may be due to increased mammary growth and increasing secretory activity per cell until peak lactation. In dairy cows, increased synthetic activity per cell is typically the predominant factor. After peak milk production there is a steady and gradual decline in milk production with advancing lactation. This is due to loss of secretory cells by programmed cell death, with a smaller contribution due to declining secretory activity per cell, particularly during late lactation when concomitant pregnancy may play an important role in the decline of milk yield. Although absolute rates of cell proliferation and death during lactation appear to be very low, the balance of these processes promotes gradual cell loss, but considerable cell turnover during a long lactation.
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