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Fig. 1 Calcium release following fertilization of the zebrafish oocyte. Changes in cytosolic free calcium were detected by calcium green-dextran fluorescence imaged by confocal fluorescence. The oocyte was immobilized in a 750 μm well with the animal pole oriented to the upper right. Unfertilized oocytes were injected with calcium green dextran to achieve a final intracellular concentration of 500 nM, and allowed to recover for 10 min before photography began. Calcium release was initially observed within a thin layer of cortical cytoplasm near the animal pole, and subsequently spread laterally to involve the entire oocyte cortex. The cytoplasm of the blastodisc was highly active in calcium signaling during the later stages of oocyte activation and cleavage. Images were taken with a 4X objective just before addition of sperm + aquarium water, time=0 (A) and at different times; 1 min (B), 12 min (C), 35 min (D), 45 min (E) and 55 min (F) after insemination.