Mitochondrial Patterns in Bovine Oocytes with Different Meiotic Competence Related to Their in vitro Maturation

This study was designed to specify chromatin and mitochondrial patterns in bovine oocytes with different meiotic competence in relation to maturation progress, resumption of meiosis, MII onset and completion of maturation. Oocytes with greater or lesser meiotic competence, recovered separately from medium (MF) and small follicles (SF), were categorized according to morphology. Four oocyte categories, healthy and light‐atretic MF and healthy and light‐atretic SF oocytes were matured and collected at 0, 3, 7, 16 and 24 h of maturation. Specific differences in terms of chromatin and mitochondrial patterns were found among the maturing oocyte categories. Resumption of meiosis was accelerated in light‐atretic oocytes, as compared with healthy oocytes, regardless of their meiotic competence. More competent oocytes activated mitochondria twice during maturation, before resumption of meiosis and before completion of maturation, while less competent oocytes did it only once, before completion of maturation. Changes in mitochondrial activity differed in light‐atretic compared with healthy in both more and less competent oocytes. Healthy meiotically more competent oocytes formed clusters and produced ATP for the whole time of maturation until its completion, while light‐atretic more competent oocytes and healthy less competent oocytes reduced these activities earlier, at MII onset. Contrary to these oocyte categories, light‐atretic less competent oocytes increased cluster formation significantly before resumption of meiosis. It can be concluded that bovine oocytes with different meiotic competence and health differed in the kinetics of mitochondrial patterns during maturation.