Fig. 1. PLC-induced Ca²⁺ oscillations are cell cycle-dependent. PLC cRNA was injected into MII-arrested mouse eggs and at two different stages of the first cell cycle following parthenogenetic activation: interphase G1 and late G2. Eggs were microinjected with Fura-dextran and PLC cRNA. Intracellular Ca²⁺ was monitored by changes in fluorescence excitation ratio. Time zero indicates approximately the point at which PLC was microinjected. (a) A series of Ca²⁺ oscillations occurs during MII that ceases as the egg enters interphase (n=20). (b) MII eggs were activated with Sr²⁺ in the presence of cytochalsin D. 2 hours after pronuclear formation the eggs were injected with Fura-dextran and PLC. No Ca²⁺ transients were detected in these parthenotes (n=20). (c) Parthenogentically activated eggs were cultured until interphase (G2), then 2-6 hours before NEB was due to occur embryos were injected with Fura-dextran and PLC cRNA (n=15). Ca²⁺ oscillations again occur.