Spontaneous Testicular Tumors in Mice*

Testicular interstitial cell tumors have appeared in estrogen-treated mice of at least three different inbred strains (1, 2, 11, 12, 13, 21), rarely in those of other strains treated in a comparable manner (2, 11, 21). Spontaneous testicular tumors were not described in those inbred strains that acquired such tumors subsequent to prolonged treatment with estrogens. In fact such tumors occur rarely in mice. Among 19,000 mice autopsied, of which approximately one-half were males, only 28 primary tumors of the testes were found (27). All except one of the 28 mice bearing a testicular tumor were from one family or its hybrids, an observation indicating a tendency to hereditary predisposition. Only one of the 28 tumors consisted of the interstitial cells of Leydig; the others were spindle cell sarcomas or composed "of cells closely resembling the epithelium of the seminiferous tubules arranged in alveolar structure." Mice of the Strong A strain, when adequately treated with estrogens, show hypertrophy of the Leydig cells (1, 7, 8, 13) and finally testicular tumors composed largely of such cells. Estrogen-treated mice of several other inbred strains failed to show a hypertrophy of the Leydig cells, and tes'ticular tumors rarely or never appeared. The tendency for such tumors to occur in estrogen-treated animals was thus strainlimited. The question arises whether or not estrogen merely results in the attainment of potentialities for testicular neoplasia that are usually not realized. The appearance of spontaneous tumors among mice that would acquire a high incidence of tumors if estrogens were injected is reported here.