Testicular development and fertility of mice treated prenatally with N-nitroso-N-ethylurea at various gestational stages.

N-nitroso-N-ethylurea (ENU) was injected intraperitoneally (i.p.) into ICR female mice at 50 mg/kg on day 8, 10, 12, or 16 of gestation (plug day = day 0). Male newborns treated prenatally with ENU were obtained. Body and testes weights of males were measured on postnatal days 4, 12, and 21 and at 12 weeks of age, as well as histopathological observation of their testes. The treated males were mated at 10 weeks to untreated females of the same strain. Subsequently, the plasma testosterone concentration in each group was determined by enzyme immunoassay. Body weight on postnatal day 4 in the group treated on day 16 of gestation was significantly lower compared to that of the control group. On postnatal day 21 and at 12 weeks of age, body weight was significantly lower in all groups treated with ENU compared to that of the control group. Among the embryonic stages tested, embryonic day 10 is the most susceptible to ENU insult with respect to the postnatal development of testes and epididymides, when judged by the relative weight at 12 weeks of age. The fertility of the male offspring was drastically impaired by the prenatal ENU treatment on embryonic day 10, followed by day 12, while the fertility of male offspring treated on embryonic days 8 and 16 was not affected. Histopathological sections of testes of male offspring treated with ENU on embryonic day 10 resulted in the most severe changes in the seminiferous tubules. The plasma testosterone concentration was drastically lower in male offspring treated on embryonic day 10 compared to the control level. These results demonstrate that the impaired fertility of the ENU-affected mice was the result of paucity of germ cells and that the critical period in the male mouse fetus with respect to the disturbance of postnatal testicular development and fertility was around embryonic day 10, which is the period of primordial germ cell migration.