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Biological and morphological characteristics of mammary tumors in GR mice.

Mammary tumor incidence was studied in virgins, in females bearing 1, 4, or 8 pituitary isografts, in ovariectomized females treated with estrone, and in force-bred mice of the GRI A strain. The frequency of mammary tumors was high in all groups of mice. The tumors appeared with significant acceleration in mice bearing pituitary grafts and in mice treated with estrone. Four pituitary grafts had more effect than one graft. Eight pituitary grafts had no further effect. The average age at development of tumor in force-bred females was much lower than that in mice of any other group. Over 80% of the force-bred mice had mammary tumors at the end of the first pregnancy. Of the tumors in force breeders, 98% regressed after breeding was stopped. Nearly all tumors in the mice of the other groups grew progressively. After transplantation, all 13 tumors derived from force breeders retained their hormone dependency, whereas only 3 of the 25 tumors from mice of the other groups retained their hormone dependency. Nearly all tumors in the force breeders had distinct morphological features and were classified as a separate group, mammary tumor type P. Type P was very rare in the other groups, in which adenocarcinoma types A and B predominated. Another type of tumor was frequently observed; it was classified as pale cell carcinoma, which has not been described extensively before. The relation of the hormone dependency and the morphology of the tumors is discussed.-J Nat Cancer Inst 46: 885-897, 1971.