The ability to generate normal Ca transients in response to spermatozoa develops during the final stages of oocyte growth and maturation

Intracellular Ca oscillations at fertilization are responsible for triggering egg activation. The aim of this study was to examine the effect of the age of the oocyte donor and in-vitro maturation on the generation of Ca transients at fertilization. The results show that <10% of in-vivo and in-vitro matured oocytes from 19-day old mice develop to the blastocyst stage in vitro. In contrast, 43% of in-vivo and 25% of in-vitro matured oocytes from 24-day old mice developed to the blastocyst stage. In parallel experiments, intracellular Ca was monitored at fertilization. Oocytes from 19-day old mice generate significantly fewer transients than oocytes from 24-day old mice. In-vitro maturation significantly decreased the ability of oocytes from 19-day old mice but not 24-day old mice to generate Ca transients in response to spermatozoa. Furthermore, we investigated the effect of oocyte maturation on Ca signalling. Immature oocytes generated fewer Ca oscillations and ceased oscillating earlier than mature oocytes. These studies suggest that the ability to generate Ca transients in response to spermatozoa increases in the final stages of oocyte development and during oocyte maturation. This may contribute to the acquisition of developmental competence in the final stages of oogenesis.