Monozygotic twin formation in mouse embryos in vitro.

Monozygotic twins developed from cultured murine blastocysts at the ratio of approximately 1:100. The locus at which the denuded blastocysts attached to the culture dish was usually a random section of their mural trophoblasts, in which case single egg cylinders developed unilaterally. However, in those few blastocysts attaching with their antipolar mural trophoblasts, the inner cell mass became subdivided into two parts because of restrictions imposed on its growth by the apically situated polar trophoblasts and the plastic substrate. Each subdivision apparently incorporated totipotent cells, resulting in the bilateral formation of two egg cylinders sharing the same ectoplacental cone.