Hydrostatic pressure improves in-vitro maturation of oocytes derived from vitrified-warmed mouse ovaries

Background: Cryopreservation has limited successes and in-vitro maturation is used to improve its results. Hydrostatic pressure (HP) plays an important role in follicular development. Objective: This study was designed to examine the effects of HP on in-vitro maturation of oocytes and cell death in cumulus cells derived from vitrified-warmed mouse ovaries. Materials and Methods: Preovulatory follicles were harvested from non-vitrified and vitrified-warmed 6-8 week-old female NMRI mouse ovaries and randomly assigned to following groups: non-vitrified (control), non-vitrified with HP exposure (treatment I), vitrified-warmed (treatment II) and vitrified-warmed with HP exposure (treatment III). The follicles of treatments I and III were subjected to HP (20 mmHg) for 30 min and after that all groups were cultured for 24h and assessed for in-vitro maturation of oocytes. The viability and apoptosis of cumulus cells and oocytes were assessed using supravital nuclear staining and TUNEL assay, respectively. Results: Oocytes harvested follicles in both control and treatment II had a significantly lower percentage of metaphase II oocytes (MII) than the treatment I and III (23.5±3.1, 15.03±4.6 and 32.7±3.2, 25.5±4.6; respectively) (p < 0.05). Viability of the cumulus cells reduced in treatment I, II and III (83.4, 83.3 and 77.7%) compared to control (86.9%), (p < 0.05). The apoptotic index in cumulus and oocyte complexes in treatments I and III (10.7±0.8 and 15.3±0.8) was higher than in control and treatment II (6.7±0.5 and 9.7±0.5) (p < 0.05). Conclusion: These results demonstrate that HP had a mild effect on cell death incidence in cumulus cells without any effect on oocyte. However, it can be used as a mechanical force to improve in-vitro maturation of oocytes derived from vitrified-warmed mouse ovaries.