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A number of research and clinical studies have confirmed high frequency of cytogenetic abnormalities in human oocytes and embryos. Chromosomal imbalance in oocytes can result from loss or gain of individual chromatids or whole chromosomes. The underlying causes remain to be fully elucidated, but important modifying factors include the number and location of chiasmata and maternal age. Aneuploidy is a major cause of congenital abnormalities, mental retardation, and miscarriage. However, most of the chromosome abnormalities detected in human embryos are likely to be lethal at very early embryonic stages, and are probably incompatible with the formation of a clinical pregnancy. Many fertility clinics now screen the embryos produced during in vitro fertilization (IVF) cycle in order to identify those that are chromosomally normal. Screening the oocytes/embryos from these patients for aneuploidy using PGS may be particularly beneficial in terms of IVF outcome.
Laser-assisted polar body biopsy is best accomplished when the oocyte is affixed to the holding capillary with the first polar body at 12 O'clock position and the second polar body located right of the first one but in the same focal plane. In contrast to embryonic blastomeres, polar bodies are rather small and do not require any special pretreatment like hypo-osmotic swelling or pronase digestion. The size and position of openings drilled in the zona pellucida (ZP) can influence further embryonic development and in particular the mode of hatching at the blastocyst stage. The presence of a cytoplasmic bridge between the first as well as the second polar body and the oocyte can impose during biopsy. A frequent problem in analysis of the first polar body is high degree of fragmentation observed in human first polar bodies.
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