Imaging of multicellular large-scale rhythmic calcium waves during zebrafish gastrulation

A clock-work somite

Somites are transient structures which represent the most overt segmental feature of the vertebrate embryo. The strict temporal regulation of somitogenesis is of critical developmental importance since many segmental structures adopt a periodicity based on that of the somites. Until recently, the mechanisms underlying the periodicity of somitogenesis were largely unknown. Based on the oscillations of c-hairy1 and lunatic fringe RNA, we now have evidence for an intrinsic segmentation clock in presomitic cells. Translation of this temporal periodicity into a spatial periodicity, through somite formation, requires Notch signaling. While the Hox genes are certainly involved, it remains unknown how the metameric vertebrate axis becomes regionalized along the antero-posterior (AP) dimension into the occipital, cervical, thoracic, lumbar, and sacral domains. We discuss the implications of cell division as a clock mechanism underlying the regionalization of somites and their derivatives along the AP axis. Possible links between the segmentation clock and axial regionalization are also discussed. BioEssays 22:72-83, 2000.

Study of calcium signaling in non-excitable cells

The fundamental importance of calcium signaling in the control of cellular physiology is widely recognized. A dramatic illustration of this is the fact that a Medline search for review articles containing the word "calcium" in the title reveals 4,629 hits, whereas the whole body of calcium signaling literature (approximately 2 x 10(6) pages) is more than enough to fill a decent-sized library. Most of this literature deals with calcium signaling in excitable cells types (mainly neurons and muscle cells), but non-excitable cell types are capable of calcium signaling as well. Although calcium fluxes in the latter cell types have attracted much less interest, the literature involved is still vast. Nevertheless, in this review article we hope to contribute some valuable insights to the field. First we shall discuss the experimental techniques available to the researcher interested in calcium signaling in non-excitable cell types with special attention to patch clamp electrophysiology. Subsequently, we shall review some of the results obtained with these techniques by focussing on the calcium-regulating mechanisms in non-excitable cells and discussing the importance of these mechanisms for physiology.

Calcium signaling in the developing Xenopus myotome

Embryonic Xenopus myocytes generate spontaneous calcium (Ca(2+)) transients during differentiation in culture. Suppression of these transients disrupts myofibril organization and the formation of sarcomeres through an identified signal transduction cascade. Since transients often occur during myocyte polarization and migration in culture, we hypothesized they might play additional roles in vivo during tissue formation. We have tested this hypothesis by examining Ca(2+) dynamics in the intact Xenopus paraxial mesoderm as it differentiates into the mature myotome. We find that Ca(2+) transients occur in cells of the developing myotome with characteristics remarkably similar to those in cultured myocytes. Transients produced within the myotome are correlated with somitogenesis as well as myocyte maturation. Since transients arise from intracellular stores in cultured myocytes, we examined the functional distribution of both IP(3) and ryanodine receptors in the intact myotome by eliciting Ca(2+) elevations in response to photorelease of caged IP(3) and superfusion of caffeine, respectively. As in culture, transients in vivo depend on Ca(2+) release from ryanodine receptor (RyR) stores, and blocking RyR during development interferes with somite maturation.

Organization of early development by calcium patterns

This survey focuses on early or primitive developmental phenomena for which the location of a steady high calcium region or the direction of a calcium wave is critical and calcium is more than a trigger. It starts with the long studied roles of calcium in fucoid eggs and in Dictyostelium and progresses to newer work on high calcium regions in medaka fish, zebrafish, and Drosophila eggs. It then proposes that propagated, ultraslow developmental waves in six diverse systems indicate a new and important class of calcium waves. These include the morphogenetic furrow in Drosophila eye discs, floret formation in sunflowers, DNA replication waves in protozoan macronuclei, growth-cone like waves in hippocampal neurons, and two others. It then considers the possible organizing roles of slow calcium waves. Here, it emphasizes surface contractile waves during primary neural induction and elsewhere as well as the possibility of cellular peristalsis. Finally, it reviews the organizing roles of fast calcium waves in ascidian eggs.