The importance of cell division in udder development and lactation

The mammary secretory cell population increases in an exponential fashion during pregnancy in all species studied, as a consequence of very high rates of cell division. After parturition the mitotic index drops dramatically, but a limited amount of proliferation does continue, at least until the time of maximum milk yield. This is particularly true of rodents but also occurs in dairy species. During declining lactation apoptosis exceeds cell division, so the size of the cell population falls and it is this decrease which is responsible for the reduction in milk yield. Many factors influence cell division. In addition to well known hormones such as ovarian steroids, prolactin and growth hormone, and growth factors such as epidermal growth factor and insulin-like growth factor-1, there are also effects of milking frequency and nutrition. Some of these same factors are now also known to regulate apoptosis. The challenge for the future is to understand more about the relationships between apoptosis and cell division in the mammary gland; for instance, are the two mutually exclusive and independent or is apoptosis important in preparing the gland for renewed cell division? To this end, we have developed a lactation rescue model which will allow us to study interactions between apoptosis and cell division in lactating mouse and cow mammary glands. (C) 2000 Elsevier Science B.V. All rights reserved.