Assessment of nuclear and cytoplasmic maturation in in-vitro matured human oocytes

Background: With improved prospects for the use of human oocyte in-vitro maturation in assisted reproductive technologies, the need to define more clearly the coordination of nuclear and cytoplasmic maturation has arisen. Methods: Immunofluorescence and confocal microscopy were used to evaluate cell cycle-dependent modifications in chromatin and microtubules in human germinal vesicle oocytes (n=455) undergoing in-vitro maturation. Results: Four distinct classes of germinal vesicle stage oocytes were identified based on the expression of G2/interphase characteristics, but, of these, only one class of oocytes was competent to complete meiotic progression to metaphase-H in vitro. The majority of germinal vesicle stage oocytes resumed meiosis within 6 h (88.9%) of culture and exhibited an accelerated pace of progression to metaphase-II (66.7%) over 24 h, but in general were unable to maintain meiotic arrest and defaulted into interphase within 24 h of polar body emission. Characterization of microtubule dynamics and chromatin phosphorylation demonstrates specific cell cycle deficiencies in in-vitro matured human oocytes. Conclusion: This work forms a basis for future studies aimed at optimizing nuclear and cytoplasmic maturation during in-vitro maturation.