Time-lapse confocal imaging of calcium dynamics in starfish embryos

During fertilization and cleavage, embryos undergo transient rises in their intracellular free calcium levels that are postulated to provide essential signals enabling normal development to proceed. In order to analyze the spatiotemporal patterns and possible biological significance of these calcium transients, time-lapse confocal microscopy was used to monitor starfish embryos during normal development and following experimental manipulations that disrupted cleavage and/or the release of calcium ions from internal stores in the embryo. For such analyses, oocytes were co-injected with dextran-conjugated forms of the calcium-sensitive fluorochrome calcium green (CG) and the calcium-insensitive dye rhodamine (Rh) for dual-channel confocal ratioing. Based on CG/Rh ratioed images obtained every 15 sec for the first few hours of development, no prominent calcium spikes were typically evident at the onset of the first cell cycle as hormone-treated oocytes resumed maturation and underwent germinal vesicle breakdown (GVBD). Subsequently, fertilizations of post-GVBD oocytes caused a single prolonged calcium wave that reached relatively uniform amplitudes throughout the ooplasm. Within 90 min after fertilization, most starfish zygotes began to display clusters of repetitive calcium oscillations that typically-but not invariably-preceded nuclear envelope breakdown, anaphase onset, and the formation of the first cleavage furrow. Rapidly decaying calcium oscillations continued through at least the first five cleavages in specimens that developed into normal blastulae, and unlike fertilization-induced calcium waves, such spikes tended to be more pronounced in the cortical cytoplasm during early cleavages. Thus, three different types of calcium dynamics--no marked transients, a single nonoscillating wave, and repetitive oscillations--were observed as normally developing starfish underwent prefertilization maturation, fertilization, and cleavage, respectively, suggesting that the spatiotemporal patterns of calcium spiking can change during starfish early development. In specimens microinjected with colchicine, calcium transients were also visible in the absence of cell divisions, indicating that calcium spiking can be uncoupled from cytokinesis. To assess whether calcium fluxes are required for normal development, oocytes were also treated with heparin to block calcium release mediated by inositol 1,4,5-trisphosphate (IP3). Injections of heparin, but not the control molecule de-N-sulfated heparin, caused abnormal fertilization-induced calcium dynamics in a dose-dependent fashion and typically abolished marked postfertilization calcium oscillations and normal cleavage. Based on correlative studies using caged IP3, heparin interfered with IP3-mediated calcium release, suggesting that such release is involved in the production of the free calcium elevations that occur in normally developing starfish embryos.