During vertebrate embryonic development, somites are formed from the unsegmented presomitic mesoderm (PSM) by a highly regulated process. They are formed in pairs in an anterior to posterior progression on either side of the notochord. Ca²⁺ signals, in the form of localized elevations of [Ca²⁺]_i were visualized from three separate embryological domains during somitogenesis: the notochord, PSM and maturing somites. Unlike the ordered morphological process of somitogenesis, Ca²⁺ signals were found to be chaotic with respect to time and location within individual embryos, and when comparing embryos. They did, however, display common characteristics that provided clues to their embryological function and significance. Signals generated by the maturing somites took the form of intercellula...[ Read more ]

During vertebrate embryonic development, somites are formed from the unsegmented presomitic mesoderm (PSM) by a highly regulated process. They are formed in pairs in an anterior to posterior progression on either side of the notochord. Ca²⁺ signals, in the form of localized elevations of [Ca²⁺]_i were visualized from three separate embryological domains during somitogenesis: the notochord, PSM and maturing somites. Unlike the ordered morphological process of somitogenesis, Ca²⁺ signals were found to be chaotic with respect to time and location within individual embryos, and when comparing embryos. They did, however, display common characteristics that provided clues to their embryological function and significance. Signals generated by the maturing somites took the form of intercellular Ca²⁺ waves generated by Ca²⁺ release from the ER via IP₃Rs. They occurred predominantly at the medial or lateral extremities of maturing somites and evidence, resulting from manipulation of [Ca²⁺]_i, indicates that they may play a role in establishing and maintaining these essential boundaries. Signals visualized from the PSM were also stochastic with respect to location and time, and again took the form of localized intercellular Ca²⁺ waves generated by Ca²⁺ release via IP₃Rs. They were predominantly generated in a region of the PSM that displayed some correlation with the expression pattern of the zebrafish homologue (her1) of the Drosophila pair-rule gene Hairy, which is involved in generating periodicity during somitogenesis and has been suggested to be linked to Ca²⁺ signaling. Experimentation indicated, however, that there appeared to be no linkage between the PSM Ca²⁺ transients visualized and her1 expression patterns. Signals generated by the notochord were also chaotic and generated by Ca²⁺ release via IP₃Rs. Their biological function remains unclear. It is thus suggested that the transients generated by these three embryological domains represent a new class of stochastic developmental Ca²⁺ signaling, where Ca²⁺ is but one of several potential developmental regulators and the variability in detecting a Ca²⁺ transient reflects the degree that this signaling mechanism is utilized during specific developmental processes in individual embryos.