Cultured mouse mammary epithelial cells: normal phenotype after implantation.

Mammary epithelial cells from normal virgin BALB/c mice were cultivated in vitro by the feeder cell technique developed and reported previously. These cells were cultured up to the 10th passage, equivalent to 60 cell divisions in culture. They were then tested for normality by several criteria, namely, the ability to regrow into normal mammary glands after implantation into cleared mammary fat pads of both syngeneic and nude mice, chromosome numbers, and response to mammogenic hormones. The cultured cells did form normal mammary ducts after implantation. The fraction of fat pads with ductal outgrowths as well as the size of the outgrowths was proportional to the number of cells implanted. When 10(6) cells were implanted into BALB/c mice, 83% of the fat pads contained outgrowths, filling, on the average, approximately 87% of the fat pad. More ductal outgrowths occurred from implanted cells taken from lower tissue culture passages than from high ones, and the number of outgrowths was greater in BALB/c mice than in nude mice. A small fraction of the cells in culture reacted with antibodies to casein, but there was no evidence of alpha-lactalbumin in the cells. However, ductal outgrowths from implanted cells responded to hormone stimulation of an estrogen deoxycorticosteroid pellet by forming well-developed lobulo-alveolar structures characteristic of pregnancy. Of the cells that were studied in passages 3 and 7, 85% were diploid. An abnormally growing culture in passage 10 was composed of cells in the tetraploid range. These tetraploid cells formed normal mammary ducts when implanted into animals.