A Highly Inbred Line of Wistar Rats Yielding Spontaneous Mammo-somatotropic Pituitary and Other Tumors2

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Abstract
Spontaneous neoplasms of female rats of a highly inbred Wistar line are described. All neoplasms tested were found to be transplantable within the strain; many were hormone-dependent. Pituitary tumors were the most common neoplasm occurring in over 27 percent of rats surviving beyond 17 months of age. These “chromophobe adenomas,” like those induced by estrogens, were functional, having marked mammosomatotropic effects, and were variously responsive to estrogens. The secondary changes were more marked in rats with grafted than with spontaneous tumors. The almost equally common mammary fibroadenomas proved transplantable to all rats conditioned with grafted mammosomatotropic tumors but to only a few rats without them. Thus, these tumors appear to be autonomous but responsive to the mammotropic hormone(s) of the anterior pituitary. Their poor transplantability was due not to heterozygocity of the Wistar strain but to hormone dependence. For example, whereas transplantation succeeded in only 2 of 11 normal females (3 strains) after about a year of latency, all 3 mammary-tumor grafts in mammotropic-tumor-bearing animals took in all 13 rats after a mean latent period of 70 to 216 days. Secondary changes in the host suggested that the spontaneous adrenocortical tumor observed secreted corticoids. Three spontaneous leukemias and one malignant lymphoma tested proved transplantable to all adult animals of the strain. The usefulness of this strain in studies of the genesis and control of mammary tumors, leukemias, and some endocrine neoplasia is discussed.