Pretreatment of hamster oocytes with Ca2+ ionophore to facilitate fertilization by ooplasmic micro-injection

The fertilization rate after ooplasmic micro-injection in mammals is low. In hamsters, micro-injection of sperm nuclei into oocytes resulted in the release of the second polar body and the formation of female and male pronuclei, but disappearance of the cortical granules was minimal. These unfavourable results seem to be associated with insufficient activation of oocytes by the current techniques of micro-injection. Therefore, we attempted to increase the fertilization rate by pretreating the oocytes with a Ca2+ ionophore, A23187 (10 microM, 3 min). Sperm nuclei were then micro-injected into the treated oocytes and 5 h later the percentage of oocytes at the pronuclear stage was examined: 53.1% for untreated controls and 85.7% for the treated group (P less than 0.05). However, when the number of pronucleate oocytes was divided by the number of A23187-activated oocytes, the percentage was 81.0% for the controls and 85.7% for the treated group. These findings suggest that pretreatment of oocytes with ionophore does not facilitate fertilization of activated oocytes, but that it indirectly enhances their development to the pronuclear stage by increasing the efficiency of egg activation.