Formation of tetraploid mouse blastocysts following blastomere fusion with polyethylene glycol.

A rapid method to produce tetraploid embryos by blastomere fusion using polyethylene glycol (PEG) has been developed. Individual four-cell stage blastomeres were aggregated into pairs with phytohemagglutinin (PHA) and then treated with 45% (w/v) PEG (MW 1,000). 56.8% (804/1,416) of treated blastomere pairs fused. Tetraploid blastomeres were aggregated into quartets or doublets to restore cell number equivalent to that of whole or half diploid embryos, respectively. Best development was obtained with quartets, 72.9% (78/107) of which cavitated. 46.4% (13/28) of doublets and 31.7% (13/41) of singly cultured tetraploid blastomeres formed cavitated structures. Uniform tetraploidy was confirmed by fixing cavitated embryos and analyzing their metaphase plates. The fusion method described here replaces Sendai virus with a chemically defined fusogen and confirms earlier observations (Snow, '73; Graham, '71) that tetraploidy does not prohibit preimplantation development. This fusion method obviates some of the drawbacks of cytochalasin B-induced tetraploidy, namely the blocking of cleavage and the potential of diploid/tetraploid mosaicism.