Thymidine incorporation by lactating mammary epithelium during compensatory mammary growth in beef cattle

Ten Hereford cows, 100 d into first lactation, were assigned to treatment or control groups to study compensatory growth of mammary glands. The right udder half of treatment cows was covered to prevent suckling by the calf, whereas control cows were suckled on all quarters. Milk production was estimated the day treatment began and 4 d later by machine milking following removal of calves for 12 h and i.v. injection of oxytocin. Five to 7 d after beginning treatment, cows were killed and mammary tissue was obtained from three regions within left and right glands for in vitro incubation with [3H]thymidine. Deoxyribonucleic acid of lactating udder halves did not increase in response to treatment although RNA: DNA ratio and milk production tended to increase. Incorporation of [3H]thymidine was greater in lactating quarters of treated cows than control cows (35,000 vs. 19,000 cpm/mg of DNA) with greatest incorporation in the basal regions of each gland. Furthermore, greatest incorporation of [3H]thymidine occurred in non-suckled glands. Autoradiographic analyses confirmed incorporation data and indicated that 81% of proliferating cells were epithelial. Data suggest that proliferation of mammary epithelial cells, within both the lactating and nonlactating glands, occurred in response to milk stasis.