Distribution of developmental markers in rat mammary tumors induced by N-nitrosomethylurea.

We have examined the distribution of immunological markers in 55 intraductal carcinomas induced in rats by N-nitrosomethylurea using several different regimens of carcinogen treatment. The goal was to determine the possible relationship of the marker distribution to those existing at various stages of development of the mammary gland. We have also examined two passage lines in syngeneic rats derived from two of the tumors. The distribution of markers was not affected by the regimen of administration. The primary tumors were found to maintain the general topography of mammary ducts but with infoldings of the basal layer without accompanying stroma. We attribute this to an abnormal expression of the tendency of basal cells to migrate towards the lumen to generate luminal cells. The distribution of markers in the tumors was evaluated by identifying 13 special features of distribution that are common in tumors, using all-or-none criteria. The distribution of these features bears out the high heterogeneity of tumors in which the various features vary in a seemingly independent way. There is also heterogeneity within tumors, adjacent nodes often having different marker distributions. The distribution of the markers is related to that found in the early stages of mammary development. Because of this characteristic and of the fact that the tumors contain both basal and luminal cells, they must originate from multipotent cells, probably the stem cells present in end buds and ducts as already proposed by other work. As they develop, the tumors can both differentiate towards the adult type and dedifferentiate towards a fetal type. Of the two transplanted lines one retained the same general features of primary tumors in several passages, whereas the other evolved into a fusiform cell type with a marker distribution not seen at any stage of mammary development. Foci of similar cells were already present in the primary tumor, suggesting that the tendency to progress was already determined at an early stage. The fusiform cells are similar and probably equivalent to the fusiform cells that arise in vitro in cultures of rat mammary cancers.