A spontaneous ovarian teratocarcinoma was isolated from a LT/Sv mouse female and converted into an ascites tumor from which embryonal carcinoma (EC) cells were dissociated. Non-enucleated and enucleated, activated oocytes were fused with EC cells and either cultured in vitro or transferred into ligated oviducts of Swiss/A females. The nucleocytoplasmic hybrids cultured in vitro up to 22 h were examined cytologically at various time intervals. EC nuclei showed morphological remodelling in the foreign cytoplasm. EC chromosomes and female pronuclear chromosomes together formed a common metaphase. The nucleocytoplasmic hybrids developed in vivo were analyzed cytologically between the first and third day after oviduct transfer. The majority of embryos developed abnormally and, in a few instances, they had passed several cleavage divisions and reached, at best, a developmental stage resembling a premature morula. Fertilized, enucleated eggs were fused with EC cells or microinjected with EC nuclei. The resulting nucleocytoplasmic hybrids were either cultured in vitro or in vivo up to the fourth day. Enzyme tests were carried out on the nuclear transplant embryos, using electrophoretic variants of glucose phosphate isomerase (GPI) in order to distinguish between EC nuclei (GPI-A) and recipient eggs (GPI-B). The EC-specific GPI could be detected in about one third of the embryos analyzed and, in several instances, also together with the egg-specific GPI. Most of them were arrested during early cleavage divisions. Some embryos cleaved abnormally or mimicked normal embryogenesis. In a few instances, development resulted in embryos that resembled late preimplantation embryos.

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