A method for obtaining chimaeric mouse blastocysts with two separate inner cell masses: a preliminary report.

Pairs of zona-free mouse blastocysts aggregated in the presence of inactivated Sendai virus and subsequently cultured in vitro will fuse to form a chimaeric blastocyst with one common blastocoelic cavity. Depending on the relative position of the inner cell masses in the apposed 'parental' blastocysts, the resulting chimaeric blastocyst contains either a single inner cell mass (ICM) of dual origin or two discrete ICMs each originating from one embryo. In the present experiments, fusion between the two aggregated blastocysts occurred in 23% of the pairs and 64% of these chimaeric blastocysts contained two ICMs. Blastocysts of the latter type could potentially give rise to pairs of embryos which as regards the topography of the foetal membrane would resemble spontaneous identical twins, although they would be genetically dissimilar. Possible applications of the described method are discussed.