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Abnormal Fertilization Is Responsible for Reduced Fecundity Following Thiram-Induced Ovulatory Delay in the Rat¹

Gamete and Early Embryo Biology Branch, Reproductive Toxicology Division, National Health and Environmental Effects Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711

Brief exposure to some pesticides, applied during a sensitive window for the neural regulation of ovulation, will block the preovulatory surge of LH and, thus, delay ovulation. Previously, we have shown that a single i.p. injection of 50 mg/kg of thiram, a dithiocarbamate fungicide that decreases norepinephrine synthesis, on proestrus (1300 h) suppresses the LH surge and delays ovulation for 24 h without altering the number of oocytes released. However, when bred, the treated dams had a decreased litter size and increased postimplantation loss. We hypothesized that the reduced litter size in thiram-delayed rats was a consequence of altered oocyte function arising from intrafollicular oocyte aging. To test this hypothesis, we examined delayed oocytes, zygotes, and 2-cell embryos for evidence of fertilization and polyspermy. In addition, we used confocal laser-scanning microscopy to evaluate and characterize cortical granule localization in oocytes and release in zygotes, because the cortical granule response is a major factor in the normal block to polyspermy. Our results demonstrate that a thiram-induced, 24-h delay in ovulation alters the fertilizability of the released oocyte. Although no apparent morphological differences were observed in the unfertilized mature oocytes released following the thiram-induced delay, the changes observed following breeding include a significant decrease in the percentage of fertilized oocytes, a significant increase in polyspermic zygotes (21%), and a 10-fold increase in the number of supernumerary sperm in the perivitelline space. Importantly, all the polyspermic zygotes exhibited an abnormal pattern of cortical granule exudate, suggestive of a relationship between abnormal cortical reaction and the polyspermy in the delayed zygotes. Because polyspermy is associated with polyploidy, abnormal development, and early embryonic death, the observed polyspermy could explain the abnormal development and decreased litter size that we observed previously following thiram-delayed ovulation.

¹ The research described in this article has been reviewed by the National Health and Environmental Effects Research Laboratory, U.S. Environmental Protection Agency, and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use.

² Correspondence: Tammy E. Stoker, MD-72, Gamete and Early Embryo Biology Branch, Reproductive Toxicology Division, National Health and Environmental Effects Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711. FAX: 919 541 5138; stoker.tammy{at}epa.gov

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				T. E. Stoker, S. D. Perreault, K. Bremser, R. S. Marshall, A. Murr, and R. L. Cooper Acute Exposure to Molinate Alters Neuroendocrine Control of Ovulation in the Rat Toxicol. Sci., March 1, 2005; 84(1): 38 - 48. [Abstract] [Full Text] [PDF]