Pregnancy Dependence of Mammary Tumors in Strain DDD Mice 2

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Abstract
Of 2411 strain DDD retired breeder mice, under ≈ 12 months of age, from our production colony, mammary tumors developed in 22 (0.91%). These tumors were all transplanted into virgins. Half were force-bred so that the tumors could be examined for pregnancy dependence: 4 were classified as completely pregnancydependent (CPD), 2 as moderately pregnancy-dependent (MPD), 3 as slightly pregnancy-dependent (SPD), and 13 as pregnancy-independent (PID). These CPD tumors did not grow in virgins but grew rapidly in pregnant breeders and regressed after parturition, reaching ascending peaks in successive pregnancies. Morphologically, 4 CPD tumors were type A composed mainly of glandular spaces of various sizes surrounded by a single layer of small, cuboidal epithelial cells and filled with a milky secretion. One MPD tumor was type A similar to the above-mentioned and the other, a pale-cell carcinoma. Two type A + B tumors and 1 type P were SPD. PID tumors consisted of 3 type A + B without secretory activity, 5 type P, 3 pale-cell carcinomas, and 2 mixed tumors. These findings suggest that the mammary tumor virus of strain DDD may belong to plaque-inducing MTV.

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