The importance of cell division in udder development and lactation

The mammary secretory cell population increases in an exponential fashion d uring 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 tim e of maximum milk yield. This is particularly true of rodents but also occu rs in dairy species. During declining lactation apoptosis exceeds cell divi sion, so the size of the cell population falls and it is this decrease whic h is responsible for the reduction in milk yield. Many factors influence ce ll division. In addition to well known hormones such as ovarian steroids, p rolactin and growth hormone, and growth factors such as epidermal growth fa ctor and insulin-like growth factor-1, there are also effects of milking fr equency and nutrition. Some of these same factors are now also known to reg ulate apoptosis. The challenge for the future is to understand more about t he relationships between apoptosis and cell division in the mammary gland; for instance, are the two mutually exclusive and independent or is apoptosi s 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 int eractions between apoptosis and cell division in lactating mouse and cow ma mmary glands. (C) 2000 Elsevier Science B.V. All rights reserved.