Data from our laboratory using hamsters and rhesus monkeys show that during fertilization, active mitochondria become unequally distributed in the oocyte cytoplasm. In general, active mitochondria relocate to surround the pronuclei (Barnett et al., 1996, 1997; Squirrell et al., 2003). In hamster oocytes, active mitochondria were homogeneously distributed in the oocyte cytoplasm before activation by spermatozoa, but a few hours later they began to translocate to the peripronuclear region (Squirrell, 2002; Squirrell et al., 2003). This localization occurs naturally and is tightly correlated with embryo development competence in vitro, because culture conditions that disturb or block embryo development in vitro also disrupted the normal mitochondrial localization profile (Barnett et al., 1997; Squirrell et al., 2001; Ludwig et al., 2001). Although we do not know whether this is a cause or effect relationship, the observation does suggest that caution is needed in designing culture conditions for IVP of embryos in ART.
We next studied rhesus monkey oocytes using multiphoton laser scanning microscopy (MPSLM; Squirrell et al., 2003). During fertilization, oocytes exhibited a peripronuclear clustering of mitochondria, but this accumulation was not present throughout the entire period prior to syngamy. The duration ranged from <1 hr to 10 hr, with a mean of 3 hr. However, unlike in hamsters, the distribution of mitochondria became more homogeneous after fertilization. The localization profile of active mitochondria differed among oocytes from the same cohort. To understand possible relationships between mitochondrial distribution, especially pronuclear localization, and subsequent embryo development, macaque oocytes were labeled, then imaged, inseminated, imaged again and allowed to develop in separate culture drops. Mitochondrial localization in the oocytes was consistent with their competence to develop into blastocysts. Similar studies revealed a relationship between mitochondrial localization and competence in cattle oocytes (Krisher and Bavister, 1997, 1998). Mitochondria also relocalize to the peri-nuclear region in fertilized human oocytes, but data are insufficient to establish a link with development (Van Blerkom et al., 2000; Sathananthan and Trounson, 2000).
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