INTRACYTOPLASMIC INJECTION OF HUMAN SPERMATOZOA INTO MOUSE OOCYTES - A USEFUL MODEL TO INVESTIGATE THE OOCYTE-ACTIVATING CAPACITY AND THE KARYOTYPE OF HUMAN SPERMATOZOA

When intracytoplasmic sperm injection (ICSI) is performed, it is important to know the capacity of sperm cells to activate the oocytes, although knowledge of their ability to fuse with the oocytes is not vital, Hamster oocytes are not suitable for this purpose because they are easily activated by the injection procedure itself. We therefore investigated whether mouse oocytes could be used to assess the activation properties of human spermatozoa. Mouse oocytes were randomized for injection with initially motile spermatozoa, medium, heat-treated or salt-extracted sperm-matozoa, and the survival and activation rates were examined. About half of the mouse oocytes survived the intracytoplasmic injection of a human sperm cell. Unlike hamster oocytes, the rate of activation provoked by the injection procedure itself was acceptably low (20%), resembling in this respect the behaviour of human oocytes. Following the injection of initially motile human spermatozoa, all mouse oocytes were activated. The injection of heat-treated or salt-extracted human spermatozoa resulted in activation rates of 14 and 15% respectively, comparable with the results following sham ICSI. These data support the hypothesis of a sperm-associated oocyte activation factor, In most activated oocytes, the human sperm nucleus decondensed to form a male pronucleus. Cytogenetic analysis at the first metaphase revealed that human sperm chromosomes were able to undergo replication in a heterologous environment. From our data we concluded that human spermatozoa can be injected successfully into mouse oocytes, resulting in a reasonable survival rate, and that mouse oocytes provide a useful model for both the assessment of the sperm-associated oocyte activation factor and the cytogenetic analysis of human spermatozoa.