Fluid transport across the epiblast of the early chick embryo

A simple method is described which allows quantitation of the rate of fluid transport across the isolated epiblast of the early chick embryo. This method consists of allowing the tissue to form spheres, which then spontaneously undergo a large volume increase. The rate of fluid uptake into the spheres can be estimated by measuring the dimensions of the spheres. Pharmacological and electrophysiological studies were performed on the spheres to determine the mechanisms of fluid transport. It was found that fluid is driven into the interior of the spheres by the osmotic gradient generated by unidirectional sodium transport and to a lesser extent by another mechanism, as yet unknown. We discuss possible candidates for this mechanism, and consider the significance of these findings to early development.