Survival of Mammary Epithelial Cells from Virgin Female BALB/c Mice Following in Vivo Gamma Irradiation

To examine the cytotoxic effects of ionizing radiation on mouse mammary epithelial cells, a transplantation assay was used to generate a single-dose survival curve. In these experiments, virgin female BALB/c mice were irradiated with 0, 100, 300, 600, 900, 1200, or 1500 rad whole-body137Cs radiation. Mammary glands of control and irradiated mice were removed and the mammary epithelial cells were enzymatically dissociated, either immediately or 24 hr following irradiation, serially diluted, and injected into gland-free mammary fat pads of 3 1/2-week-old virgin female BALB/c host mice. The number of cells required to give ductal growth at 10 weeks in half the fat pads (the mammary cell dose 50%,${\rm MCD}_{50}$) was determined by a maximum likelihood procedure and the ratio of${\rm MCD}_{50}$for nonirradiated control cells to${\rm MCD}_{50}$for irradiated cells provided an estimate of cell survival. Fitting the data using a single-hit multitarget model resulted in an estimate for D0of 250 rad with an n of 2.4. No significant difference in the results was obtained whether the cells were dissociated and transplanted immediately after irradiation or the cells were allowed to remain in situ for 24 hr following irradiation. Thus no "in situ repair" was detected for mouse mammary epithelial cells.