A transient rise in the concentration of Ca in the cortex upon fertilization was demonstrated in medaka eggs injected with aequorin. Detection of the aequorin luminescence with an ultra-high sensitivity photonic microscope system revealed a wave of increased Ca concentration starting at the site of sperm entry (animal pole) and being propagated along the cortex of the egg toward the antipode. The wave traversed the entire egg surface within 2-3 min. The peak value of the aequorin luminescence, and therefore the peak value of the Ca transient, was generally higher at the site of sperm entry than in other regions. The peak values of the luminescence (and therefore of the Ca concentration in the cortex) remained fairly constant during propagation of the wave. Microinjection of Ca into the cortex also induced a Ca wave. When the egg was stimulated by microinjection of Ca at the equatorial region, the Ca wave was propagated at a fairly constant speed over the egg surface, except at the region near the vegetal pole where the wave was retarded. Simultaneous recording of the Ca wave and the wave of cortical change (breakdown of cortical alveoli) in eggs during fertilization revealed that the Ca wave preceded the wave of cortical change. A Ca wave was also demonstrated in sand dollar eggs, although due to their smaller size the phenomenon was not as clear as in medaka eggs.
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