| Abstract
| - BACKGROUND. Intracytoplasmic sperm injection (ICSI) is a technique in which sperm are injected directly into unfertilized oocytes, whereby offspring can be obtained even with dysfunctional sperm. Despite its advantages in human and animal reproductive technology, the low rate of resultant live offspring is perturbing. One major cause is thought to be embryonic chromosomal abnormalities. However, there is no direct evidence of how these occur or how they affect pregnancy outcomes. METHODS. Chromosomal dynamics during the first mitotic division of mouse embryos were analyzed using a new live-cell imaging technology. After imaging, the embryos' developmental capacities were determined. RESULTS. When ICSI-generated embryos were monitored for their chromosome integrity, some embryos with apparent normal morphology seen by conventional light microscopy had abnormal chromosome segregation (ACS) at the first mitotic division. Chromosomal fragments were misaligned during the first metaphase and formed micronuclear-like structures at the interphase of the 2-cell stage. Similar ACS was also found in mouse embryos produced by microinjecting round spermatids, with even higher frequency. Giemsa staining and immunostaining revealed that these fragments were derived from double-strand DNA breaks in the paternal genome. About half of the embryos with ACS developed into normal-looking morulae or blastocysts and implanted, but almost all of them aborted spontaneously before embryonic day 7.5. CONCLUSIONS. ACS during first mitosis appears to be a major cause of early pregnancy losses in ICSI-generated mouse embryos. Moreover, this novel imaging technology could be applicable as a method for the assessment of embryo quality.
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