Bilateral asymmetry of the inositol trisphosphate-mediated calcium signaling in two-cell ascidian embryos

In ascidian oocytes, numerous calcium signaling events occur at fertilization which contribute to resume and complete meiosis, and determine the three embryonic axes. The main ooplasmic and intracellular calcium channels at work in the calcium signaling of the one-cell embryo have different roles and fates when the first mitosis begins. By whole-cell patch-clamp recording, we observed different families of these calcium channels in the blastomeres of Phallusia mammillata two-cell ascidian embryos. Membrane capacitance has been measured to evaluate the oocyte and blastomere surface area, allowing certification of the exact time of cell division. At the two-cell stage, no difference was observed in the density of voltage-dependent calcium channels in each blastomere, or in the ryanodine-sensitive calcium stores. In contrast, a bilateral asymmetry was recorded for the ooplasmic channels responsible for calcium entry after calcium store depletion: they could be activated only in the blastomere not wearing the polar bodies. The same laterality was observed in the InsP(3)-induced internal calcium release. Moreover, this asymmetry included a one-way communication in the InsP(3)-dependent calcium signaling between the two blastomeres. These results enhance the understanding of the early steps of development, and underscore the interest for ascidians in studies of polarity patterning. (C) 2000 Editions scientifiques et medicales Elsevier SAS.