Ultrastructure of cell fusion and premature chromosome condensation (PCC) of thymocyte nuclei in metaphase II mouse oocytes.

Following PEG (polyethylene glycol) treatment of ovulated metaphase II mouse oocytes aggregated with thymocytes, fusion of cell membranes occurs. Prerequisites for cell fusion are: close apposition of lectin-agglutinated (phytohemagglutinin-treated) membranes of both cells, formation of firm punctual adhesion sites, and expansion of adhesion sites over a certain area. Establishment of the firm cell-cell contact is associated with development of actin-like filaments along both of the adhering plasma membranes. Membrane fusion occurs at single or multiple sites, and is followed by internalization of thymocyte-oocyte membrane complexes decorated with actin filaments into the hybrid cell cytoplasm. A filamentous actin layer forms also along the inner surface of newly formed hybrid oocyte-thymocyte plasma membrane. Thymocyte nuclei incorporated into oocyte cytoplasm undergo nuclear envelope breakdown and premature chromosome condensation (PCC) leading, eventually, to formation of single chromatids complete with kinetochores. Concomitantly with chromatin condensation an extensive polymerization of microtubules starts in the center of the chromatin mass which leads to the formation of an apparently non-functional spindle-like structure.