Background: We investigated at what stage early cultured preantral mouse follicles become dependent on a minimal effective FSH dose (10 mIU/ml) and analysed the influence of implementing FSH at several time-points during in vitro culture.
Methods: Two-layered mouse follicles were cultured for 12 days under seven different FSH exposure regimens and ovulated on day 13 by hCG/EGF. Ovulated cumulus-oocyte complexes were fertilized and embryos were cultured up to the blastocyst stage.
Results: When FSH was absent or added only once at the start of culture, follicle survival was significantly reduced (22 and 52% respectively versus 95% when FSH was continuously present, P < 0.01) and oocyte quality was compromised, providing few oocytes for embryo culture (19 and 7% versus 71% in continuous presence of FSH, P < 0.01). Optimal follicle survival rates can be ensured by implementing FSH at the latest from day 4 of culture. By introducing FSH later than day 4, follicle survival rates and number of ovulated oocytes decreased. Estradiol production and luteinization were strongly related to the moment of introducing FSH in culture. Fertilization and preimplantation embryo development rate were found to be highest in cultures where FSH support was implemented during the preantral stage.
Conclusion: Exposure to FSH before formation of the antral-like cavity had a positive effect on follicle survival and oocyte robustness.