Abstract
A transient rise in the concentration of Ca2+ 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 Ca2+ 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 Ca2+ transient, was generally higher at the site of sperm entry than in other regions. The peak values of the luminescence (and therefore of the Ca2+ concentration in the cortex) remained fairly constant during propagation of the wave. Microinjection of Ca2+ into the cortex also induced a Ca2+ wave. When the egg was stimulated by microinjection of Ca2+ at the equatorial region, the Ca2+ 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 Ca2+ wave and the wave of cortical change (breakdown of cortical alveoli) in eggs during fertilization revealed that the Ca2+ wave preceded the wave of cortical change. A Ca2+ 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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