Calcium Signaling in the Developing Xenopus Myotome

Abstract

Embryonic Xenopus myocytes generate spontaneous calcium (Ca2+) 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 Ca2+ dynamics in the intact Xenopus paraxial mesoderm as it differentiates into the mature myotome. We find that Ca2+ 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 IP3 and ryanodine receptors in the intact myotome by eliciting Ca2+ elevations in response to photorelease of caged IP3 and superfusion of caffeine, respectively. As in culture, transients in vivo depend on Ca2+ release from ryanodine receptor (RyR) stores, and blocking RyR during development interferes with somite maturation.