Ovarian follicle resilience in mice orally dosed with methoxychlor: Are reproductive impacts possible in mammals as ecological receptors?

Mammals are routinely evaluated with chemical exposure modeling in ecological risk assessments for contaminated terrestrial properties, such as sites managed under the U.S. EPA's Superfund program. Despite modeling outcomes frequently indicating that site mammals are experiencing severe reproductive impacts, there are virtually no known instances of site mammal populations being either unhealthful or reduced in size. This observation has triggered the development of the one-time patented and recently ASTM International-certified Rodent Sperm Analysis method that directly assesses male reproductive health, and thereby the overall health of mammals that occur at sites. Just as experimentation has led to an understanding of the degree of sperm count reduction, sperm motility reduction, and increase in morphologically altered sperm that need occur for reproduction to be impacted, the degree of ovarian follicle count reduction to similarly impact reproductive success should be able to be determined. Thus, adult female CD-1 mice were orally exposed to vehicle control or methoxychlor at 32 and 64 mg/kg/day for 30 days. Pursuant to resting periods of 30 and 60 days post-dosing, half of the treated animals were euthanized and their ovaries were collected and subjected to histological evaluations of follicle numbers and health. The other half of the animals were mated to proven breeder males to enable evaluation of breeding success. At 30 days post-dosing, primary and pre-antral follicle numbers were increased at both methoxychlor doses compared to controls. Further, the number of atretic follicles nearly doubled at both methoxychlor doses compared to controls. In contrast, at 60 days post-dosing, follicle numbers did not differ between methoxychlor treatment and control groups. These results were corroborated by 60-day mated and non-mated mice displaying an increase in serum anti-Müllerian hormone level, a recognized marker for the quantitative aspect of ovarian reserve. At both 30 day and 60 days post-dosing, pup weights and litter sizes in methoxychlor exposed animals were the same as those of controls. Collectively, these data indicate that oral exposure to methoxychlor did not increase follicle atresia or reduce follicle numbers enough to compromise female fertility. These outcomes may be indicators of ovarian follicle resiliency to chemical exposure, a capability that could benefit mammals in the wild. Ovarian follicle resiliency could, in part, explain the recurrent discovery at contaminated terrestrial sites, of rodent populations that are identical (in terms of size, sex ratio, and age distribution) to those of nearby (non-contaminated) reference locations.