Cytoplasmic movement velocity in unfertilized mouse oocytes: a supportive but not definitive marker of embryo quality

In vitro fertilization (IVF) is an essential method not only in the treatment of infertility but also in livestock breeding and in the management of zoo populations and endangered species. The success of IVF depends on oocyte quality, necessitating reliable methods for assessing female gamete competence. Here, we investigated biomechanical properties - particularly cytoplasmic movement velocity (CMV), known to be dependent on cytoskeletal functionality - as noninvasive markers of oocyte quality. We used mouse immature (GV) and mature (MII) oocytes as an experimental model. Cortical tension and cellular stiffness were measured using micropipette aspiration, and CMV was assessed with time-lapse imaging combined with Particle Image Velocimetry analysis. Our data indicate that transcriptionally quiescent GV oocytes (SN) exhibit higher CMV than transcriptionally active GV oocytes (NSN). Moreover, maternal and postovulatory aging, cryopreservation, and in vitro maturation significantly affect the CMV of MII oocytes. However, CMV in freshly ovulated MII oocytes does not correlate with their ability to develop to the blastocyst stage. We also show that changes in CMV are typically accompanied by opposite changes in cellular stiffness, with one exception - the transition from GV to MII stage, during which both CMV and stiffness decrease. Our results suggest that CMV may facilitate oocyte quality assessment, particularly in immature oocytes, by supporting the selection of SN-type oocytes. However, CMV measured in MII oocytes alone is not sufficient to predict quality of the resulting embryo. Further research is needed to validate these findings in target species and to optimize relevant IVF protocols.