LOCALIZED CALCIUM SIGNALS IN EARLY ZEBRAFISH DEVELOPMENT

Activation of the phosphoinositide (PI) pathway has been shown to be i nvolved in the compaction of blastomeres in mouse embryos and in embry onic axis formation in Xenopus and in zebrafish embryos. Here we inves tigate Ca2+ signals in individual blastomeres of zebrafish embryos wit h the goal to better understand the role of PI and Ca2+ signaling for early vertebrate embryogenesis. Initial studies showed that the inosit ol 1,4,5-trisphosphate (IP3) concentration increases after the 32-cell stage of development, suggesting that IP3-mediated Ca2+ signals may b e present during the blastula stage. Ca2+ signals were measured by ide ntifying individual cells using confocal imaging of a nuclear localize d Ca2+ indicator. Using this in situ indicator, changes in Ca2+ concen tration were measured over several hours in each cell of a series of s ections through the developing embryo. Transient increases in Ca2+ con centration that lasted 20-50 sec (Ca2+ spikes) were first triggered du ring the 32- to 128-cell stage in cells of the outer embryonic cell la yer. These cells develop epithelial characteristics and specialize int o the enveloping layer (EVL). No Ca2+ activity was observed during the earlier cleavage cycles or in deep blastomeres. Ca2+ spikes remained restricted to the EVL until the end of the blastula stage. Ca2+ spikes in neighboring EVL cells often occurred in the same short time interv al, indicating that small groups of EVL cells can synchronize their ac tivity. When averaged over several cell cycles, Ca2+ activity showed a n even distribution in the EVL and did not indicate future polarities. (C) 1995 Academic Press, Inc.