Mammary Cell Number, Proliferation, and Apoptosis During a Bovine Lactation: Relation to Milk Production and Effect of bST

Abstract

This investigation evaluated mammary cell loss and replacement during lactation and the impact of admin- istration of bST on these processes. During lactation, a gradual decrease in number of mammary epithelial cells within the mammary glands occurs and largely accounts for the decline in milk production with advanc- ing lactation. This decrease is not appreciably impacted by the loss of viable epithelial cells in milk. Rather, the net decline in cell number (∼50% during the entire lactation) results from continual death by apoptosis. Accompanying the decline in mammary cell number by apoptosis is a degree of cell renewal. Approximately 0.3% of mammary cells in lactating, nonpregnant cows were labeled by a 24-h in vivo treatment with the thymi- dine analog, bromodeoxyuridine. During the entire lac- tation, the number of new cells amounts to approxi- mately 50% of the number of cells initially present. By the end of lactation, most cells present in the mammary gland were formed after calving. Increasing cell replace- ment or decreasing apoptosis during lactation may pro- vide a means to increase persistency of lactation. In- deed, administration of bST to Holstein cows during midlactation increased the proportion of mammary epi- thelial cells expressing the nuclear proliferation anti- gen, Ki-67, from 0.5 to 1.6%. Bovine somatotropin ap- pears to increase the rate of cell renewal in the lactating mammary gland. Knowledge of molecular regulation of apoptosis and cell proliferation should provide a means to modulate cell turnover in the mammary gland. A change in the ratio of epithelial proliferation to cell death during lactation will affect the persistency of lac- tation.