Dissociation between intracellular calcium elevation and development of human oocytes treated with calcium ionophore

Objective: To develop an acceptable model system to study calcium activation of human oocytes. Design: Study of oocyte development and intracellular calcium [ca] i dynamics of activated oocytes. Setting: Research center affiliated with infertility service. Main Outcome Measure: Morphologic evidence of meiotic maturation and cell division under high-power Hoffman optics with an inverted microscope. Meiotic maturation was determined by the number of polar bodies or the presence of a pronucleus, and cell division was determined by evidence of a cleavage furrow or presence of blastomeres. To monitor the effect of calcium ionophore on [Ca] i levels, oocytes were incubated with fura-2 (2μM) for 30 minutes and [Ca] i was determined by ratioing the emission fluorescence (510-nm long-pass filter) during simultaneous excitation at 340 and 380 nm with a microspectrofluorimeter. Result(s): All oocytes loaded with fura-2 and then exposed to ionophore exhibited an isolated elevation of [Ca] i , followed by prompt return to baseline levels. None of the oocytes showed signs of cleavage or of meiotic maturation after treatment with calcium ionophore. Conclusion(s): Human oocytes activated with calcium ionophore A23187 or ionomycin exhibited elevated [Ca] i but remained resistant to subsequent meiotic maturation and cleavage. Our results differ from some reports of parthenogenetic activation of human oocytes. These differences may result from different activation protocols or culture conditions. Because none of the 126 oocytes cleaved after the activation protocols used in these experiments, this approach should provide an ethically acceptable model system to study calcium dynamics in human oocytes.