A localized elevation of cytosolic free calcium is associated with cytokinesis in the zebrafish embryo

Cytokinesis, a key step in cell division, is known to be precisely regulated both in its liming and location. At present, the regulatory mechanism of cytokinesis is not well understood, although it has been suggested that calcium signaling may play an important role in this process. To test this notion, we introduced a sensitive fluorescent Ca²⁺ indicator into the zebrafish embryo and used confocal microscopy to measure the spatiotemporal variation of intracellular free Ca²⁺ concentration ([Ca²⁺](i)) during cell cleavage. It was evident that a localized elevation of [Ca²⁺](i) is closely associated with cytokinesis. First, we found that during cytokinesis, the level of free Ca²⁺ was elevated locally precisely at the cleavage site. Second, the rise of free Ca²⁺ was very rapid and occurred just preceding the initiation of furrow contraction. These observations strongly suggest that cytokinesis may be triggered by a calcium signal. In addition, we found that this cytokinesis-associated calcium signal arose mainly from internal stores of Ca²⁺ rather than from external free Ca²⁺; it could be blocked by the antagonist of inositol trisphosphate (InsP₃) receptors. These findings suggest that the localized elevation of [Ca²⁺](i) is caused by the release of free Ca²⁺ from the endoplasmic reticulum through the InsP₃-regulated calcium channels.