Characterization of a rat in vitro ovarian culture system to study the ovarian toxicant 4-vinylcyclohexene diepoxide

Repeated daily dosing of rats with the occupational chemical 4-vinylcyclohexene diepoxide (VCD) causes selective depletion of the smallest preantral ovarian follicles (primordial and primary). These targeted populations are difficult to study because they comprise very little of the overall mass of ovarian tissue. Additionally, they are randomly distributed throughout the ovary. Therefore, a neonatal rat ovarian culture system containing predominantly primordial and primary follicles was developed and its susceptibility to VCD was assessed. The in vivo sensitivity of neonatal rats to VCD dosing was first confirmed by daily injection of VCD (80 mg/kg/day ip) on postnatal days (PND) 4-19. On PND 19, depletion of primordial and small primary follicles was evident. Ovarian cultures were then established utilizing a floating organ culture system to treat ovaries from PND 4 Fischer 344 rats in vitro. Initial follicle populations in cultured ovaries consisted of primordial (81%) and small primary (19%) follicles, whereas larger-sized preantral follicles had developed after 15 days in culture (67% primordial, 31% small primary, and 2% large primary). Cultured rat ovaries were sensitive to follicle depletion by incubation with VCD (≥30 μM), and follicle loss occurred in a time-dependent manner (8-15 days). Evidence for apoptosis in VCD-exposed ovaries, as demonstrated in vivo, was obtained using immunohistochemistry. There was significantly more staining for apoptosis-associated active caspase-3 and TUNEL in ovaries incubated with VCD (30 μM, 15 days) compared with matched controls. These results demonstrate that small preantral follicles of cultured neonatal rat ovaries are sensitive to VCD exposure. The similarities between VCD's effects in vitro and in vivo demonstrate the usefulness of this system for future mechanistic studies related to ovarian follicle loss induced by VCD or other ovotoxic chemicals.