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Effect of Hormones and of Parity on the Occurrence of Hyperplastic Alveolar Nodules and Tumors in the Mammary Glands of Female A/Crgl Mice^*

( Department of Zoology and Its Cancer Research Genetics Laboratory, University of California, Berkeley; Department of Anatomy, University of California School of Medicine, San Francisco; and the Hormone Research Laboratory, University of California, Berkeley, Calif.)

Within the A/Crgl strain of mice, virgin females almost never developed mammary tumors, whereas breeding females had a high incidence of tumor development. Tumor incidence was proportional to the number of litters borne. Appreciable tumor incidence could be induced in intact virgin females by administration of estradiol or a combination of estrone and progesterone. Induction of prelactational lobuloalveolar development by exogenous hormones resulted in 24 per cent tumor incidence. This incidence was not altered appreciably by one to four hormonal inductions of milk secretion. Exogenous hormone administration which led to tumor development also increased nodule incidence.

^* This study was aided by cancer research funds of the University of California. We are indebted to Virgil Finster and Donald R. Gillies for important assistance, to Dr. G. K. Hawkins, of Schering Corporation, for the estradiol and the progesterone, and to Dr. J. R. Beem, of Merck, Sharp and Dohme, for the cortisol acetate.

Received 7/15/60.