Reproductive toxicity of cyclophosphamide in the C57BL/6N mouse: 1. Effects on ovarian structure and function

The antineoplastic alkylating agent, cyclophosphamide (CPA) is known to impair normal female reproductive function. We have examined the time- and dose-dependent effects of CPA on the ovary, specifically, its impact on follicle numbers, ovarian morphometrics, and estradiol (E2) production. Female C57BL/6N mice were treated ip with CPA in normal saline at doses of 0, 75, 200, or 500 mg/kg. Ovaries were removed 1 to 14 days following treatment and serial sections were prepared. Differential follicle counts revealed that primordial follicles were most sensitive to CPA (ED50 = 122 mg/kg), followed by antral and growing follicles. Primordial follicles were affected by all doses of CPA and were completely destroyed by 3 days in the 500 mg/kg dose group. The greatest reduction in antral follicles was to 49% and 7% of controls by CPA doses of 200 and 500 mg/kg, respectively. Plasma E2 concentrations correlated best with antral follicle numbers (r2 = 0.94) and antral follicle volume (r2 = 0.88). Growing follicles were least sensitive to CPA and only decreased at 7 and 14 days. Although atretic changes were observed in growing follicles after treatment with CPA, these follicles recovered and progressed into apparently functional antral follicles (that is, they produced E2). Total ovarian volume was significantly reduced (30 to 40%) in the high-dose group on day 1, and remained depressed throughout the experiment. Examination of ovarian morphometrics indicated that this volume loss represented specific temporal changes in corpora lutea (CL), interstitial tissue, growing follicles, and antral follicles. At 1 and 3 days after treatment, the major loss in ovarian volume was due to a reduction in antral follicle and interstitial tissue volumes, while at 7 days the majority of volume loss was accounted for by the absence of CL. It is not known if CL are directly affected by CPA at the early time points, but their absence at 7 and 14 days is probably due to earlier destruction of antral follicles. These results demonstrate that CPA-induced ovarian toxicity is exhibited as temporal changes in both structural and functional features of the ovary, particularly in destruction of primordial and antral follicles and depressed E2 production. Information of this type also gives insight into ovarian response to chemical disruption of folliculogenesis and its recovery process.