Testicular Teratomas in Fetal Mice2

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Abstract
Fifty-nine teratomatous foci from 27 testes of fetal 129/Sv-Sl mice 15 to 19 days of gestation are described. The incidence of tumors in fetuses is the same as in older mice, which demonstrates origin during a prenatal period and not later. Judged by the appearance of these fetal tumors, teratocar-cinogenesis may be initiated shortly before 15 days' gestation, and the period of susceptibility to teratocarcinogenesis may extend to as late as (but not later than) 17 days' gestation. All 24 tumorous foci in testes of 15- and 16-day fetuses were entirely enclosed within intact seminiferous tubules, which demonstrated that they were derived from a component of the seminiferous tubules, probably the primordial germ cell. Seven foci in 16- and 17-day fetuses were completely intratubular, while 14 were larger and had invaded the walls of the tubules in which they originated. Tumor cells migrated into the interstitial area. All four 19-day tumors had intratubular areas continuous with extratubular areas. There was no indication that any extratubular element contributed to the formation of teratomas. Usually tumorous testes had a small number of foci located near each other within the same tubule. These foci appeared to represent clones derived from a primordial germ cell in which neoplastic transformation occurred prior to its establishment in its definitive peripheral position in the tubule. Many foci contained primitive epithelium similar in appearance to embryonic ectoderm. All the varied components of teratomas in older animals, including germ layers and immature and adult tissues, are derived from intratubular tumors in fetuses.