Ultrastructure of cell hybrids between mouse oocytes and blastomeres

Cell hybrids were formed between mouse ovarian oocytes and blastomeres from 2‐, 4‐ and 16‐cell stage embryos. No centrioles were located at the spindle pole, no matter what the origin of the blastomere, but microtubule organizing centers (MTOCs) of varying size were regularly present. After fusion the arrangement of the two chromosome sets varied. In some cases the meiotic chromosomes formed metaphase plate I, while the chromosomes of the blastomere under‐went premature condensation and were either scattered on the spindle surface or among the spindle microtubules. In other cases the meiotic and mitotic chromosomes intermingled at random within a large cytoplasmic islet, while a true spindle did not form. None of the hybrids cleaved, although on occasion the first polar body was extruded. The two hemispheres composing the hybrid and originating from the oocyte and blastomere could be recognized due to the presence or absence of cortical granules in the cell cortex.