Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 25, 2001
Publication Date: October 1, 2001
Citation: Capuco, A.V., Wood, D.L., Baldwin, R.L., Mcleod, K.R., Paape, M.J. 2001. Mammary cell number, proliferation and apoptosis during the bovine lactation cycle: relation to milk production and effect of bst. Journal of Dairy Science. 84(10):2177-2187. Interpretive Summary: The biological reasons for the decrease in milk yield with advancing lactation have been unclear. This experiment evaluated loss and replacement of mammary secretory cells during lactation and the impact of administration of bovine somatotropin on these processes. A gradual decrease in number of mammary epithelial cells accounts for the decline in milk production with advancing lactation in dairy cows. Approximately 50% of mammary cells are lost during the entire lactation via programmed cell death (apoptosis). Accompanying the decline in mammary cell number by apoptosis is some cell renewal. During the entire lactation, the number of new cells formed amounts to approximately 50% of the cells present at the beginning of lactation. Increasing cell replacement or decreasing cell death during lactation will prolong lactation. Indeed, administration of bovine growth hormone to Holstein cows increased the percentage of proliferating cells approximately three-fold and seemingly increased the rate of cell renewal in the lactating mammary gland. Knowledge of molecular regulation of apoptosis and cell proliferation should enable scientists to modulate cell turnover in the mammary gland and thus prolong lactation. Prolonging lactation will increase profitability for dairy farmers by reducing the number of lactations per cow; thus reducing exposure to increased health risks that accompany calving and early lactation, reducing necessity for breeding during early lactation, and reducing the proportion of time that a cow is not lactating.
Technical Abstract: This investigation evaluated mammary cell loss and replacement during lactation and the impact of administration 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 advancing lactation. This decrease is not appreciably impacted by loss of viable epithelial cells in milk. Rather, the net decline in cell number (~50% during the entire lactation) appears to result from a continuous process of cell 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 thymidine analog, bromodeoxyuridine. Thus, during the entire lactation, the number of new cells amounts to approximately 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 replacement or decreasing apoptosis during lactation may provide a means to increase persistency of lactation. Indeed, administration of bST to Holstein cows during midlactation increased the proportion of mammary epithelial cells that expressed the nuclear proliferation antigen, Ki-67, from 0.5 to 1.6%. Bovine somatotropin appears to increase the rate of cell renewal in the lactating mammary gland. Knowledge of molecular regulation of apoptosis and cell proliferation should provide 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 lactation.