Time-lapse confocal imaging-induced calcium ion discharge from the cumulus-oocyte complex at the time of cattle oocyte activation

Oocyte activation, the dynamic transformation of an oocyte into an embryo, is largely driven by Ca2+ oscillations that vary in duration and amplitude across species. Previous studies have analysed intraoocyte Ca2+ oscillations in the absence of the oocyte's supporting cumulus cells. Therefore, it is unknown whether cumulus cells also produce an ionic signal that reflects fertilisation success. Time-lapse confocal microscopy and image analysis on abattoir-derived cattle cumulus-oocyte complexes coincubated with spermatozoa revealed a distinct discharge of fluorescence from the cumulus vestment. This study demonstrated that this Ca2+ fluorescence discharge was an artefact induced by the imaging procedure independently of oocyte activation success. The fluorescence discharge was a direct result of cumulus cell membrane integrity loss, and future studies should consider the long-term effect of fluorescent labels on cells in time-lapse imaging. However, this study also demonstrated that the distinctive pattern of a coordinated fluorescence discharge was associated with both the presence of spermatozoa and subsequent embryo development to the morula stage, which was affected by Ca2+ chelation and a reduction in the active efflux of the fluorophore. This indicates that the cumulus vestment may have a relationship with oocyte activation at and beyond fertilisation that requires further investigation.