The completing of the second meiotic division by MII mouse oocytes correlates with the positioning of F-actin and mitochondria in the ooplasm.

Actin filaments play an essential role in the process of oocyte maturation and completion of meiosis. However, whether the localization of F-actin in the ooplasm is associated with normal completion of the second meiotic division remains unclear. Mitochondrial distribution is another important parameter correlating directly with MII oocyte capacity to finalize meiosis. Our objective was to examine the role of actin microfilaments in the distribution of mitochondria and, respectively, Metaphase II (MII) oocytes meiotic potential. We show monoclonal antibody-mediated inhibition of actin polymerization in young mouse oocytes, reduction of the amount of F-actin, and induction of mitochondrial clustering induced by antibody treatment. Similar phenotype, even in untreated eggs, was observed in in vitro oocyte aging experiments. Observed changes correlate with reduced ability of MII oocytes to extrude the second polar body and form the pronuclei. Changes in colocalization of F-actin and mitochondria likely resulted from disturbed cytoskeleton architecture. The perturbations in the amount of F-actin and its distribution largely coincide with mitochondrial redistribution. Based on these data, we suggest actin microfilament's participation in redistribution of mitochondria during MII oocyte aging in vitro. Accordingly, patterning of F-actin is indicative of high rate of the completed second meiotic division. These results help evaluating oocyte's quality and choosing optimal time between placement into culture and in vitro fertilization.