Morphologic characteristics of pregnancy-dependent mammary tumors in GRS/A mice

Groups of pregnancy-dependent mammary tumors from GRS/A female mice, composed of early, maximally grown and regressed tumors, were investigated morphologically in comparison with functionally active normal mammary gland of the same host and pregnancy-independent mammary tumors of the same strain. In the dependent tumors, earlier ones consisted of proliferating, non-secretory ductular cells in loose stroma, which grotesquely resembled normal mammary gland architecture, and the maximally grown feature was the localized overgrowth of the ductal lobular unit, in which papillomatous proliferations of ductal structures were observed. Histochemically, few myoepithelial cells in the tumours, low secretory activity and poor expression of murine mammary tumor virus antigens in these tumor cells were detected, as contrasted with neighboring mammary glands of pregnant and lactating mice. Particularly severe necrosis occurred in the papillomatous foci of ductal elements within the dependent tumors soon after parturition, and these finally changed to a group of microcysts. Small foci of adenocarcinoma type B were rarely seen in the dependent tumors during later pregnancies, and some cancer cells showed an intracytoplasmic focal staining for the viral antigens. Such an immunohistochemical finding was never detected in the dependent tumors.