Sequential observation of mitochondrial distribution in mouse oocytes and embryos

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Abstract
The purpose of this study was to elucidate changes in the distribution of mitochondria through the cell cycle. Mouse oocytes and embryos were recovered sequentially from mice and stained with the vital fluorescent mitochondrial stain rhodamine 123. Mitochondrial staining pattern were classified into three types: aggregation (Ag), homogeneous (H), and perinuclear accumulation (PA). Sequential observations revealed that mitochondria of oocytes and embryos grown in vivo translocated in the cytoplasm during the cell cycle, showing the H pattern be fore human chorionic gonadotropin (hCG) administration, the PA pattern 8–9 hr post-hCG, the H pattern again 10–14 hr post-hCG, and the PA pattern again 24 and 31–32 hr post-hCG following fertilization. In the twocell stage, the Ag pattern was shown 35 hr post-hCG, the H pattern was observed 40 hr post-hCG, and the PA pattern was found 48 hr post-hCG. In the embryos cultured in vitro and showing developmental block, mitochondrial translocation was shown to be inhibited after they aggregated in the early two-cell stage (35 hr post-hCG). Moreover, the translocation of mitochondria was restored by the addition of superoxide dismutase or thioredoxin to the culture medium. Both of these enzymes have already been shown to have the ability to overcome developmental block. The present study revealed that mitochondria translocated in the cell cycle and suggested that there is a close relationship between mitochondrial translocation and developmental arrest.