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ABNORMAL FERTILIZATION IS RESPONSIBLE FOR REDUCED FECUNDITY FOLLOWING THIRAM-INDUCED OVULATORY DELAY IN THE RAT
Citation:
Stoker, T E., S C. Jeffay, R M. Zucker, R L. Cooper, AND S D. PERREAULT. ABNORMAL FERTILIZATION IS RESPONSIBLE FOR REDUCED FECUNDITY FOLLOWING THIRAM-INDUCED OVULATORY DELAY IN THE RAT. BIOLOGY OF REPRODUCTION 68(6):2142-2149, (2003).
Description:
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 which 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 is a consequence of altered oocyte function arising from intra follicular oocyte aging. To test this hypothesis, we examined delayed oocytes, zygotes and two-cell embryos for evidence of fertilization and polyspermy. In addition, we used confocal laser scanning microscopy (CLSM) to evaluate and characterize cortical granule localization in oocytes and release in zygotes, since the cortical granule response is a major factor in the normal block to polyspermy. Our results demonstrate that a thiram-induced, 24 hour 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 of 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. Since polyspermy is associated with polyploidy, abnormal development, and early embronic death, the observed polyspermy could explain the abnormal development and decreased litter size we observed previously following thiram-delayed ovulation.