Quantitative morphokinetic parameters identify novel dynamics of oocyte meiotic maturation and cumulus expansion†

We defined baseline morphokinetic parameters of mouse oocyte maturation and cumulus expansion in vitro using a closed time-lapse incubator and validated the findings using tool compounds known to disrupt these biological processes. Meiotic maturation and cumulus expansion are essential for the generation of a developmentally competent gamete, and both processes can be recapitulated in vitro. We used a closed time-lapse incubator (EmbryoScope+(TM)) to establish morphokinetic parameters of meiotic progression and cumulus expansion in mice and correlated these outcomes with egg ploidy. The average time to germinal vesicle breakdown (GVBD), time to first polar body extrusion (PBE), and duration of meiosis I were 0.91 +/- 0.01, 8.82 +/- 0.06, and 7.93 +/- 0.06 h, respectively. The overall rate of cumulus layer expansion was 0.091 +/- 0.002 mu m/min, and the velocity of expansion peaked during the first 8 h of in vitro maturation (IVM) and then slowed. IVM of oocytes exposed to Nocodazole, a microtubule disrupting agent, and cumulus oocyte complexes (COCs) to 4-methylumbelliferone, a hyaluronan synthesis inhibitor, resulted in a dose-dependent perturbation of morphokinetics, thereby validating the system. The incidence of euploidy following IVM was >90% for both denuded oocytes and intact COCs. No differences were observed between euploid and aneuploid eggs with respect to time to GVBD (0.90 +/- 0.22 vs. 0.97 +/- 0.19 h), time to PBE (8.89 +/- 0.98 vs. 9.10 +/- 1.42 h), duration of meiosis I (8.01 +/- 0.91 vs. 8.13 +/- 1.38 h), and overall rate and kinetics of cumulus expansion (0.089 +/- 0.02 vs 0.088 +/- 0.03 mu m/min) (P > 0.05). These morphokinetic parameters provide novel quantitative and non-invasive metrics for the evaluation of meiotic maturation and cumulus expansion and will enable screening compounds that modulate these processes.

Automated summary

This study defined the baseline morphokinetic parameters of mouse oocyte maturation and cumulus expansion using a closed time-lapse incubator and verified the results using tool compounds. The morphokinetic parameters provide novel quantitative and non-invasive metrics for evaluating meiotic maturation and cumulus expansion.