Correlation of abnormal mitochondrial distribution in mouse oocytes with reduced developmental competence.

Selection of good quality oocytes is important for improvement of assisted reproductive technology. Here, we studied the relationship of the mitochondrial distribution in metaphase II stage (MII) oocytes with fertility, since mitochondria in ooplasm are essential for energy production required for fertilization and embryo development. To observe mitochondria non-invasively, we used oocytes from a transgenic mouse, in which enhanced green fluorescent protein is targeted to the mitochondrial matrix and thus fluorescence is observed exclusively in the mitochondria. Control oocytes with mitochondria distributed around the nucleus showed normal embryo developmental competence, whereas oocytes with abnormal diffuse and fragmented mitochondria showed a significantly lower rate of embryo development after activation by intracytoplasmic sperm injection or strontium, which is a very effective agent for activation of mouse oocytes. Also, we showed that the reduced developmental competence of oocytes with diffuse and fragmented mitochondria caused by vitrification and thawing is similar to that of oocytes with abnormal mitochondrial foci obtained naturally. These findings suggest that abnormal mitochondrial distribution in oocytes at MII is a cause of developmental retardation and therefore normal mitochondrial distribution could be used as a criterion for selection of good oocytes.