Behaviour of hamster and mouse round spermatid nuclei incorporated into mature oocytes by electrofusion

Round spermatids of the hamster and mouse were electrofused with homologous mature oocytes to examine the behaviour of their nuclei within the ooplasm. A single spermatid was inserted in the perivitelline space of a mature oocyte and an electric fusion pulse given. In the hamster, the best spermatid-oocyte fusion took place when the oocytes were pretreated with neuraminidase, subjected to 30 s AC (2 MHz, 20 V/cm) followed by a single fusion DC pulse (3000 V-cm, 10 microseconds) and another 30 s AC current. Inclusion of micromolar Ca2+ and Mg2+ in the fusion medium was essential for oocyte activation. Under these conditions all oocytes were activated and 20-40% fused with spermatids. Of these fused oocytes only 5-10% had fully developed spermatid (male) and oocyte (female) pronuclei. In the rest the spermatid-derived pronuclei remained small throughout the pronuclear stage. However, nucleolus-like structures appeared de novo and DNA synthesis occurred in these small pronuclei. Regardless of the size of male pronuclei, chromosomes from the spermatid and oocyte appeared to mingle and participate in the first cleavage. About 70% of the fused oocytes developed into the 2-cell stage. Electrofusion of mouse oocytes with spermatids was less efficient. Even under the best conditions tested, less than 10% of the oocytes fused with spermatids. Here again, most spermatid nuclei remained small throughout the pronuclear stage.