Sodium transport and the control of epiblast polarity in the early chick embryo

The sodium transport properties of chick epiblast during gastrulation were studied by various techniques. It was found that the epiblast is capable of unidirectional apical tobasal sodium transport, in towards the underlying intraembryonic space. The Na-K-ATPasewas localized by [ 3 H]ouabain binding and autoradiography near the basal surfaces of the cells, and the number of pump sites was quantified. The transport rate of sodium was determined with 22 Na. Electrophysiological studies on embryos at primitive streak stages showed a transepithelial potential of about + 16 ± 5mV (basal side positive) which was sensitive to strophanthidin. Applying similar voltages but of reverse polarity to isolated sheets of epiblast caused a rapid reversal of some oftheir morphological polarity markers as well as some of their physiological functions. The relevance of these results to development is discussed.