Preimplantation development of manipulated mouse zygotes fused with the second polar bodies: a cytogenetic study

Immediately after fertilization one chromatid of each maternally-derived chromosome is extruded into the second polar body (2PB). We tested the ability of these "extra" chromosomes to support preimplantation development. Micromanipulation and electrofusion techniques were used to fuse 2PBs with diploid, haploid, or enucleated mouse zygotes. Androgenetic haploids, intact embryos, and digynic triploids served as the controls for the reconstructed embryos. Androgenetic haploid zygotes developed to the blastocyst stage only when fused with the 2PBs. This result demonstrates that even when extruded into the 2PB, chromosomes retain their ability to support normal preimplantation development. However, 2PB fusion with diploid zygote impaired preimplantation development. Normal development of experimentally produced digynic triploids (zygotes with one extra maternal pronucleus) indicated that developmental arrests, caused by the 2PB fusion, were not the result of triploidy or micromanipulation procedures. Cytogenetic studies showed that developmental failures of the reconstructed embryos were caused by premature chromosome condensation of the polar body chromosomes. This result indicates that 2PB must be removed from the zygotes' perivitelline space during animal cloning experiments. In addition, we showed that 2PB fusion with enucleated zygote is a reliable method for 2PB karyotyping and may be used in the studies of mammalian meiosis.