CALCIUM OSCILLATIONS AND CALCIUM WAVES COORDINATE RHYTHMIC CONTRACTILE ACTIVITY WITHIN THE STELLATE CELL LAYER OF MEDAKA FISH EMBRYOS

During gastrulation and body axis formation in medaka (Oryzias latipes) fish embryos, rhythmic contraction waves sweep across a tissue sheet known as the periderm, which covers both the developing embryo and its associated yolk cell. Cope et al. ([1990] J. Exp. Zool., 254:270-275) have found that these contractions are generated by a layer of stellate-shaped cells which is closely apposed to the inner surface of the periderm. In this paper, we show that rhythmic contractions in the stellate cell layer are correlated with elevations of intracellular [Ca2+]. Medaka periderms were first explanted and secured to microscope coverslips using a modified plasma clot culture technique. The explants were then loaded with the calcium indicator dye Flue-SAM and observed with time-lapse epifluorescence confocal microscopy. Rises of intracellular [Ca2+] were observed to propagate from cell to cell in the stellate cell layer, in the form of intercellular waves. After a latency of a few seconds, a coordinated contraction within the stellate cell layer ensued. Rhythmic Ca spikes were also observed within individual stellate cells. In many cases, rises of intracellular [Ca2+] preceded cellular contraction in these individual cells by several seconds. The Ca oscillations and Ca waves that occur in medaka stellate cells represent one of the earliest expressions of Ca-based excitability and contractility within the blastoderm of post-fertilization vertebrate embryos. (C) 1995 Wiley-Liss, Inc.