Visualization of stochastic Ca2+ signals in the formed somites during the early segmentation period in intact,normally developing zebrafish embryos

Localized Ca²⁺ signals were consistently visualized in the formed somites of intact zebrafish embryos during the early segmentation period. Unlike the regular process of somitogenesis, these signals were stochastic in nature with respect to time and location. They did, however, occur predominantly at the medial and lateral boundaries within the formed somites. Embryos were treated with modulators of Ca²⁺]_i to explore the signal generation mechanism and possible developmental function of the stochastic transients. Blocking elements in the phosphoinositol pathway eliminated the stochastic signals but had no obvious effect, stochastic or otherwise, on the formed somites. Such treatments did, however, result in the subsequently formed somites being longer in the mediolateral dimension. Targeted uncaging of buffer (diazo‐2) or Ca²⁺ (NP‐ethyleneglycoltetraacetic acid EGTA]) in the presomitic mesoderm, resulted in a regular mediolateral lengthening and shortening, respectively, of subsequently formed somites. These data suggest a requirement for IP₃ receptor‐mediated Ca²⁺ release during convergence cell movements in the presomitic mesoderm, which appears to have a distinct function from that of the IP₃ receptor‐mediated stochastic Ca²⁺ signaling in the formed somites.