Failure of fertilization as a cause of preimplantation loss induced by methyl chloride in Fischer 344 rats

Abstract

Methyl chloride (MeCl) is a testicular and epididymal toxicant in the Fischer 344 rat that induces significant decreases in sperm quality in males and significant increases in preimplantation embryonic loss in females mated to exposed males. This study examined the possibility that these losses are due to failure of fertilization and not to preimplantation embryonic death. Groups of males either were exposed to 1000 or 3000 ppm MeCl 6 hr/day for 5 days or received a single ip injection of 0.2 mg/kg triethylenemelamine (TEM) on Day 5 and were bred to two females weekly for up to 8 weeks. Females were killed 12 hr postmating; embryos and ova were isolated and scored as fertilized or unfertilized. Nearly 90% (602/683) of the normal ova recovered from females bred to control males were fertilized, and fertilization rates in the 1000-ppm MeCl and TEM groups were not significantly depressed from that level. The percentage of fertilized ova in the 3000-ppm MeCl group was significantly decreased, ranging from 3 to 72% over the 8 weeks. The frequency of unfertilized ova in this group always equaled or exceeded the frequency of preimplantation loss recorded in an earlier dominant lethal assay. In contrast, only a fraction of the preimplantation losses caused by the genotoxicant TEM could be accounted for by failure of fertilization. After culture of fertilized ova for 24 hr, only the TEM group exhibited a significant decrease in the number of fertilized ova which cleaved (85 vs 96% in the control). We conclude that the preimplantation loss caused by MeCl exposure is due to failure of fertilization and not to a genotoxic effect of MeCl. The methods used here permit a more accurate assessment of the reproductive toxicity of a chemical by discriminating between its cytotoxic and genotoxic effects.