Dynamics of Leukocytes in Rat Mammary Epithelium During Pregnancy and Lactation1

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Abstract
Female rats in which the time of mating was known, received 1 μCi of [3H]-thymidine per gram body weight for 4 days prior to each experimental time period and were then sacrificed 1 h after the final injection. Experimental time periods were 8, 14 and 20 days gestation and 2 and 6 days postpartum. Autoradiographs were prepared by the dipping technique with an exposure time of 6 weeks, and double-blind cell counts were performed on a total of 4000 cells from each animal. Counts included alveolar epithelial cells and intraepithelial leukocytes. The rate of cell proliferation was quantitated by determining the mean cell counts and percent labeled cells. Labeling studies demonstrated that a high percentage (50%) of mammary epithelial cells were labeled early in pregnancy, but decreased later in pregnancy and during lactation [20 days gestation (31%); 2 and 6 days postpartum (10%)]. During mammary alveolar development, the intraepithelial leukocyte population was constant (3.5%); however, with the onset of lactation, the percentage of leukocytes (8.7%) was significantly increased. Although the total percentage of intraepithelial leukocytes was increased, the number of labeled cells was unchanged between 20 days gestation and 2 days postpartum and actually showed a significant decrease between 2 and 6 days postpartum. This decrease in the incidence of labeled cells during the period of increased alveolar infiltration of leukocytes may indicate that many of these cells are a population which had been formed prior to the first thymidine injection, possibly at another site, and are redistributed to the mammary gland in response to the onset of lactation.

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