Periodic calcium activity correlates temporally with the onset of gene expression in the embryo, suggesting a causal relation between these two events. Calcium transients are elicited by the action of fibroblast growth factor (FGF) through the activation of phospholipase C. In this work, we present a reaction-diffusion model that extends our previous results on the generation of calcium oscillations for a single and two coupled blastomere cells to a meridian of the Xenopus embryo at the mid-blastula transition.
INTRODUCTIONIn the Xenopus embryo, a periodic calcium signal initiates at a narrow zone of the presumptive dorsal ectoderm at the beginning of the mid-blastula transition stage (MBT). According to Leclerc et al. (2000), at this stage (MBT or stage 8), the signal consists of transients produced in groups of four or more cells with a period of approximately 200 -300 sec. The source of these calcium transients is restricted to this zone (Fig. 1a), and no experimental data about the mechanisms for their propagation is available. At the MBT, gap junctions electrically interconnect the blastomeres (Slack, 2001;Wolpert et al., 2002) and allow the flow of the Ca 2ϩ ion and small molecules such as IP 3 (Slack, 2001;Höfer et al., 2001). This finding suggests that the calcium transients can spread through these channels from the source to other parts of the embryo (Guthrie, 1984;Landesman et al., 2000Landesman et al., , 2003.The signal produced at the dorsal ectoderm may influence not only the cells of the prospective ectoderm but also the prospective mesoderm ones.At the MBT stage, the embryo is a relatively undifferentiated sphere in which the ectoderm, mesoderm, and endoderm are not yet well defined. The onset of a center of intense periodic calcium activity at the prospective dorsal ectoderm is likely to have a central role for the regional differentiation of the Xenopus blastula. Periodic calcium activity starts at the MBT, just when zygotic genetic activity begins. As the calcium ion is a second messenger that can influence the genetic activity of cells (Dolmetsch et al., 1998) 302 DÍAZ ET AL.pression at the zone of influence of the periodic signals becomes a plausible mechanism for regional differentiation.We have proposed previously that the primary mechanism for the generation of this periodic signal is the action of fibroblast growth factor (FGF) on its specific receptor (FGFR; Díaz et al., 2002), in agreement with experimental data from this system (Ryan et al., 1998). This action could be further enhanced by the signal from the Wnt5/Ca 2ϩ pathway (Saneyoshi et al., 2002). The action of the Wnt5 molecule on its specific receptor induces the liberation of calcium from the endoplasmic reticulum (ER) stores through the specific activation of phospholipase  (Saneyoshi et al., 2002), whereas FGF acts on its specific receptor and induces the liberation of calcium from the ER stores through the specific activation of phospholipase C␥1 (Ryan et al., 1998, Browaeys-Poly et al., 1998. Both phospholipases can a...