Response to prolactin and ovarian steroids of normal mammary epithelial cell cultures

Mouse mammary epithelial cells in confluent primary monolayer cultures retain responsiveness to the specific hormones that induce mammary growth in vivo. Simultaneous stimulation by prolactin, progesterone and estrogen, in the presence of 5 percent fetal calf serum, is required to induce an increase in both thymidine uptake into DNA and in cell replication (as judged by mitotic indexes) over the hormone-free control. This increase in mitogenic response could not be elicited in either mouse fibroblasts or in mouse mammary tumor cells, the latter known to be hormone insensitive.