Mammary Tumors and Mammary-Gland Development in Virgin Mice of Strains C3H, RIII, and Their F1 Hybrids2

Abstract

The incidence of mammary tumors was high in virgin females of strain C3H and in reciprocal F₁ hybrids between strains C3H and RIII, but comparatively low in virgin females of strain RIII. The incidence of tumors differed in the reciprocal hybrids, both, however, resembling strain C3H more than strain RIII. The mean tumor age in the (C3H × RIII)F₁ virgin hybrids was less than in either parental strain but more closely resembled that of C3H. The mean tumor age in the (RIII × C3H)F₁ virgin hybrids was greater than that in strain C3H and not significantly different from that in strain RIII mice. The differences in incidence of mammary tumors and in mean tumor age between the two hybrid groups suggest that the agent in the milk of strains C3H and RIII differs in quality or quantity, that of C3H being the more active. The mammary glands of strain RIII virgin females usually were not well developed, whereas the glands of strain C3H usually had many lateral buds or clusters of acini along the main ducts. The mammary glands of virgin hybrids between strains C3H and RIII resembled those of strain C3H virgin females. Frequently there was better development of acini in glands of hybrids than in those of strain C3H. No differences were observed in the extent of lateral budding and acinar development in the reciprocal hybrids.