LAUSR

interior has been suggested to function in embryo morphogenesis by providing space and extracellular matrix components for proper cell movements during gastrulation. In contrast, surprisingly little is yet known about the formation and function of IF during zebrafish embryogenesis. Using a combination of cell biological and imaging experiments, we have begun analyzing IF accumulation in blastula-togastrula stage embryos and the potential role these accumulations might play in embryo patterning and morphogenesis during gastrulation. Our preliminary observations suggest that IF begins to accumulate between blastomeres at early cleavage stages, even before tight junction assembly at the epithelial cells of the blastoderm surface becomes apparent. Moreover, pharmacological inhibition of Na+/K+ ATPase strongly reduced IF accumulation without impairing surface epithelial integrity, pointing to the possibility that proper ion transport and thus the regulation of osmolarity between the inside and outside of the embryo is critical for IF accumulation. Finally, we found that in Na+/K+ ATPase inhibited embryos with reduced IF accumulation mesoderm and endoderm internalizationwas defective, suggesting that IF accumulation is required for proper gastrulation movements.