Background: Dogs develop germ cell tumors of the testis at a relatively high rate. It is not known to what degree these tumors resemble various human testicular neoplasms.
Experimental design: The epidemiology and morphology of a series of spontaneous canine testicular tumors, collected between 1985 and 1991, was analyzed, and compared with human testicular germ cell tumors. DNA content analysis of representative samples was performed using flow cytometry and image cytometry. Eight human spermatocytic seminomas were studied in parallel.
Results: All canine tumors had the histopathologic features reported as typical for dog testis seminomas. These tumors could show both an intratubular and an invasive component. Most of them were pure (78%), while they could be combined with a Leydig cell tumor, a Sertoli cell tumor, or both. No somatic, placental or yolk sac cells were identified, and there was no carcinoma in situ (CIS). A bimodal age distribution, with a peak around 1 year of age and between 4 and 16 years of age, was found for all pure and mixed testicular tumors, except for those composed of a Leydig cell and a seminoma component. These tumors were all present in dogs older than 7 years, being significantly more older (p < 0.01) than dogs with a pure tumor of either type. All Sertoli cell and Leydig cell tumors were diploid. No consistent peritriploid DNA content, characteristic of human testicular germ cell tumors, was found for canine seminomas, which most often had a diploid DNA content. Human spermatocytic seminomas always contained diploid tumor cells, and showed a relatively low number of high ploidy cells, comparable to canine seminomas of the testis.
Conclusions: The so-called seminomas of the testis are tumors of old age. Histologically, these tumors are composed of a single cell type with some variation without evidence of differentiation. It is proposed that canine seminoma correspond to human spermatocytic seminomas. It is thought that the Leydig elements in these tumors represent a reactive change rather than biphasic differentiation of a single stem cell capable of germinal and sex-cord cell development.