Long-term tamoxifen impacts the ovary but not preimplantation embryo development†

Many studies have examined the acute effects of chemotherapies and radiation on the ovary; however, few address the impact of long-term chemotherapeutic agents on future reproduction for cancer survivors. The purpose of our study was to determine the effect of long-term tamoxifen (TAM) treatment on follicle development, ovarian reserve, and embryogenesis in a murine model. Adult female mice were treated with TAM (250 mg/kg fed ad libitum in mouse chow) or control chow for three weeks, followed by 0-, 1-, or 3-weeks of TAM washout. Mice were then superovulated, mated, and their oviducts flushed to determine ovulation, fertilization, and in vitro embryo development rates. At specific timepoints, one ovary from each animal was harvested, sectioned, and stained with H&E, while the other ovary was evaluated for gene expression of targeted genes. Ovarian size was calculated based on average surface area over six slices. Follicles (primordial, primary, secondary, antral / pre-ovulatory), and corpora lutea (CL) were counted and averaged for each slice. All data were analyzed using mixed-effects analysis of variance (ANOVA), with treatment group as the fixed effect and ovary treated as a random effect nested within treatment. Ovaries of TAM treated mice were 36% smaller on average than control ovaries (P = 0.01). Primordial follicle counts were 35% lower than control ovaries (P = 0.007) and there were 3.4x more atretic follicles in the ovaries of TAM treated mice (P < 0.001). There were no significant differences in the number of primary, secondary, antral or pre-ovulatory follicles. The number of oocytes and embryos flushed from control mice (total ovulated) was 1.65x greater than TAM treated mice. However, fertilization rates and in vitro embryo blastulation rates were not significantly different between controls and TAM groups. In summary, for this murine model, long-term TAM negatively impacted ovarian reserve (primordial follicle number) without overtly affecting the in vitro embryo development to the blastocyst.