The nature of intercellular coupling within the preimplantation mouse embryo.

The changing nature of intercellular coupling during the 4- and 8-cell stages of mouse early development has been investigated by iontophoretic injection of carboxyfluorescein, horseradish peroxidase and current into individual blastomeres in either the intact embryo or after their disaggregation and reaggregation into pairs. Coupling junctions that allowed only molecules of low molecular weight (putative gap junctions) were found not to appear until 2-5 h beyond the 3rd cleavage division (8-cell stage). However, intercellular junctions that were not size selective were detected in intact embryos only throughout the 4- and 8-cell stages. It is proposed that this junctional communication results from the persistence of midbodies through all or part of the two, and in a few cases the three, cell cycles following their formation at the first and second cleavage divisions. We conclude that the cells of the early mouse embryo may be linked in a more extensive syncytial network than was hitherto suspected.