| The role of calcium has been investigated in the initiation and continuation of cell movement in amphibian gastrulation. A systematic study of and changes in calcium distribution was done of specific developmental stages and particular embryonic regions of Xenopus laevis. Measurements of calcium content were made by atomic absorption spectroscopy, X-ray microanalysis, and X-ray fluorescence spectroscopy. Ionic calcium levels of specific embryonic areas were determined by the GBHA {glyoxal bis (2-hydroxyanil)} histochemical test.;Embryos in media with increased calcium concentration initiated gastrulation about an hour before those in standard media. The ionophore A23187 greatly speeded up the beginning of gastrulation and the expansion of the dorsal lip of the blastopore, possibly by activating a cortical microfilament system. Doubling the concentration of the ionophore speeded up the formation of the dorsal lip even more. Results show that an increase in free calcium concentration can control the time of initiation of gastrulation. Calcium release from internal stores is sufficient to initiate gastrulation. Cytochalasin B delayed the start of gastrulation, and doubling its concentration resulted in a further delay. In vitro and in vivo studies demonstrated calcium binding by cytochalasin B. It is proposed that this might be the mechanism of action of cytochalasin B.;Stage and area specific differences in calcium content were observed by all the techniques used. Calcium content of whole embryos (ppm/mg wet weight) increased during gastrulation. It is suggested that high levels of total calcium, which includes bound as well as free calcium, may be related to future movements, accumulate prior to actual movement. Increases in the levels of ionic and total calcium corresponded to the beginning and continuation of invagination at the blastopore. The results indicate mediation of lip formation by calcium. In all stages examined, blastoporal lip formation is associated with release of calcium from a bound to a free form. |
