The meiotic competence of ovine oocytes derived from early antral follicles is influenced by the season

BACKGROUND-AIM In seasonal breeders, the quantity and quality of immature oocytes obtained from the ovaries can vary greatly depending on the season. The present study aimed at evaluating the influence of the season (breeding and nonbreeding) on the in vitro acquisition of meiotic competence of cumulus-oocyte complexes (COCs) derived from sheep early antral follicles (EAFs). METHODS For this purpose, Sarda sheep ovaries were collected from a local abattoir in the breeding (October – November) and non-breeding season (March-May). The COCs were retrieved from EAFs (350-400 µm) by rupturing the follicle wall with a 21-gauge needle. Subsequently, they were washed and cultured individually in a 96-well plate containing TCM199 supplemented with 0.15 μg/mL Zn sulfite, 10-4 IU/ mL FSH, 10 ng/mL estradiol, 50 ng/mL testosterone, 50 ng/ mL progesterone, and 5 μM Cilostamide. After five days long in vitro culture (LIVC), the healthy COCs were selected and subjected to in vitro maturation. RESULTS The results indicated that at the end of the culture period COCs had more intact granulosa cell layers with acceptable morphology (30/72, 41.6%) in the breeding season compared to the non-breeding season (18/67, 26.8%; Chi-squared test P = 0.067). The resumption of meiosis in the collected COCs during the breeding season was higher than in the non-breeding season (13.3 vs 11.3%, respectively: Chi-squared test p<0.001). CONCLUSIONS In conclusion, these results suggest that the LIVC of oocytes derived from sheep EAFs in the non-breeding season have a lower meiotic competence than those collected in the breeding season. Therefore, we recommend the breeding season as a suitable time for the LIVC of oocytes derived from sheep EAFs.