Exposure to Exogenous Estrogen During Testis Development Permanently Alters Meiosis in the Adult Mouse.

Exposures to estrogenic chemicals during pre- and perinatal development are postulated to cause spermatogenic impairment and associated male reproductive disorders. Consistent with this, male rodents exposed during perinatal development exhibit reductions in testis size and sperm count as adults. Because meiotic defects result in the elimination of spermatocytes, we tested the hypothesis that estrogenic exposures during testis development cause the elimination of spematocytes in the adult testis by increasing the frequency of meiotic defects. Male CD-1 and C57BL/6 mice were exposed postnatally (1-12 days postpartum) with oral doses of 20 or 500 (ng/g/day) bisphenol A, or 0.25 (ng/g/day) ethinyl estradiol, or a placebo. Meiotic analyses of juvenile (20 days postpartum) and adult (12 weeks-old) testes were performed (n≥6 per group). Specifically, surface spread preparations of spermatocytes were immunostained with SYCP3 and MLH1 antibodies. These antibodies provide markers of the synaptonemal complex and recombination sites, respectively, allowing us to analyze synapsis and recombination between homologous chromosomes during meiotic prophase. Unexpectedly, in both mouse strains and in both control and treated animals, recombination levels were significantly lower in juveniles than in adult littermates. In addition, CD-1 males exposed to bisphenol A or ethinyl estradiol exhibited a significant reduction in recombination in both the juvenile and adult testis. The effect was strongest in ethinyl estradiol exposed animals, where one-third of spermatocytes had only a single recombination site per homologous pair, the lowest level of recombination compatible with germ cell survival. In contrast, no effects were observed in C57BL/6 males for any exposure. Although genetic background differences frequently account for differences among mouse strains, in this instance we hypothesize that differences in uterine environment account for the lack of estrogen sensitivity in the C57BL/6 male. Specifically, we are currently testing the hypothesis that C57BL/6 males are estrogenized in utero, and that this renders them relatively insensitive to exogenous estrogens. Taken together, our results indicate that 1) the meiotic process is significantly different during the first wave of spermatogenesis in the juvenile testis, with significantly lower levels of recombination; 2) exposure to low levels of exogenous estrogens during testis development can permanently alter male meiosis in the adult testis; and 3) the response to exogenous estrogens is variable among mouse strains. Finally, because recombination is vital for the correct segregation of chromosomes during meiosis and the normal progression of cells through spermatogenesis, our findings support the hypothesis that meiotic defects contribute to the testicular impairment reported in previous studies. (poster)