Ultrastructural studies of early mouse embryos obtained by oocyte fusion

Oocyte fusion induced by inactivated Sendai virus results in the production of 'zygotes' that are able to undergo the first stages of embryonic development. The oocyte fusion products (OFP) obtained follow a morphological developmental pattern equivalent to that of control embryos, at least up to the 8-cell stage. The percentage of OFP that reach the 8-cell stage is extremely low (3%) compared with control embryos cultured in vitro (95%). On light microscopy, the OFP obtained show morphological characteristics identical to control embryos, although their cell diameters are larger. The cortical reaction, meiotic reactivation, extrusion of second polar bodies and pronucleus formation take place as observed in controls. The ultrastructural characteristics of oocyte fusion products at the 1-, 2-, 4- and 8-cell stages are analogous to those of controls, including the presence of structures related to the activation of the embryo genome. However some differences concerning cell ultrastructure, mainly in the nucleus, are observed and discussed in the text.