Differential follicle counts as a screen for chemically induced ovarian toxicity in mice: results from continuous breeding bioassays.

Ovaries from National Toxicology Program Reproductive Assessment by Continuous Breeding (RACB) bioassays were used to directly compare differential ovarian follicle counts and reproductive performance for 15 chemicals. Ovaries of 10 animals per group from 16 studies in CD-1 mice and 1 study each in C3H and C57BL/6 mice were sectioned serially at 6 microm. Counts of small, growing, and antral follicles were obtained in every 10th section. For all follicle types, younger mice had more follicles than older mice, and CD-1 mice had more follicles than age-matched animals from either inbred strain. The in-life portion of the RACB protocols demonstrated that 9 of 15 chemicals altered reproductive outcome in one or both sexes of mice, with six agents affecting females (R. E. Morrissey et al., 1989, Fundam. Appl. Toxicol. 13, 747-777). Three of six female toxicants [2,2-bis(bromoethyl)-1,3-propanediol, BPD; ethylene glycol monomethyl ether, EGME; methoxyacetic acid, MAA] significantly decreased counts of small and/or growing follicles by 33 to 92% in CD-1 mice; EGME also reduced follicle counts in the other strains. Follicle counts were decreased in progeny of animals treated with EGME or its active metabolite, MAA. For BPD, reductions in follicle numbers were proportional to dose. In CD-1 mice, female toxicants di-N-hexyl phthalate, propantheline bromide, and tricresyl phosphate reduced reproductive performance but not follicle numbers. Counts were not affected by toxicants for which the susceptible sex could not be determined (bisphenol A, ethylene glycol, oxalic acid). Altered follicle counts without apparent reproductive impairment occurred in CD-1 mice at lower doses of BPD but were not observed for nontoxic chemicals. These data suggest that differential follicle counts (1) are a quantifiable endpoint of ovarian injury in conventional bioassays, and (2) in some instances, may provide a more sensitive indicator of female reproductive toxicity than fertility.