CALCIUM TRANSIENTS ACCOMPANY OOPLASMIC SEGREGATION IN ZEBRAFISH EMBRYOS

Through the injection of f-aequorin (a calcium-specific luminescent re porter), and the use of an imaging photon detector, transient localize d elevations of free cytosolic calcium in the forming blastodisc (BD) and animal hemisphere cortex were visualized that correlated with oopl asmic segregation. The introduction of an appropriate concentration of the weak (K-D = 1.5 mu mol/L) calcium buffer 5,5'-dibromo-BAPTA resul ts in the dissipation of these calcium domains, and inhibits cytoplasm ic streaming and the subsequent formation of a ED at the animal pole. These inhibitory actions are dependent on the final cytosolic concentr ation of buffer within the egg: greater than or equal to 1.3 mmol/L bl ocks ooplasmic streaming; < 1.3 mmol/L eggs segregate normally. Inject ion of 5,5'-dimethyl-BAPTA (K-D = 0.15 mu mol/L) to a final concentrat ion of 1.5 mmol/l as a control has no effect on ooplasmic streaming. T hese results suggest that localized domains of elevated free cytosolic calcium are essential for ooplasmic segregation in zebrafish. Further more, a hypothetical model is presented linking these calcium transien ts to the contraction of a cortically located actin microfilament netw ork as a possible mechanism providing the driving force for segregatio n.