Diurnal changes in mouse mammary gland DNA synthesis

DNA synthesis was studied in mammary tissue from 13- and 15-day pregnant Balb/c mice at different times during the day. Tissue was labeled with 3H-thymidine (3H-TdR) using both in vivo and in vitro techniques. The labeling index of mammary epithelial cells was determined by autoradiography. Incorporation of 3H-TdR into TCA-precipitable material after in vitro labeling was also measured. The labeling index of mammary alveolar cells is 2.5 times higher in tissue from animals studied at midnight when compared to tissue from animals studied at noon; ductal epithelial cells do not show any variation in labeling index at these time periods. Precursor incorporation into TCA-precipitable material is also increased at midnight when compared to values at noon. Tissue from animals at 6 a.m. and 6 p.m. show intermediate levels of 3H-TdR incorporation when measured by both of these methods. Diurnal variations in DNA synthesis in mammary tissue may reflect daily changes in either serum levels of those hormones directly stimulating this process or fluctuations in tissue sensitivity to these hormones, and might provide a means of analyzing the regulation of mammary epithelial cell proliferation during pregnancy.