Ion transport across the early chick embryo: I. Electrical measurements, ionic fluxes and regional heterogeneity.

The chick blastoderm at the stage of late gastrula is a flat disc formed by three cell layers and exhibiting epithelial properties. Blastoderms were cultured in miniature chambers and their electrophysiological characteristics were determined under Ussing conditions. Under open-circuit condition and identical physiological solutions on both sides, spontaneous transblastodermal potential difference (Voc) of -7.5 +/- 3.3 mV (ventral side positive) was measured. Under short-circuit condition (transblastodermal delta V = 0 mV), the blastoderm generated short-circuit current (Isc) of 21 +/- 8 microA/cm2, which was entirely dependent on extracellular sodium, sensitive to ouabain applied ventrally and independent of extracellular chloride. The net transblastodermal Na+ flux fully accounted for the measured Isc, both under control conditions and with ouabain. The total transblastodermal resistance (Rtot) was 390 +/- 125 omega cm2. Frequently, the Voc, Isc and Rtot showed spontaneous oscillations with a period of 4-5 min. Removal of endoderm and mesoderm did not significantly affect the electrical properties, indicating that the electrogenic sodium transport is generated by the ectoderm. The Voc and Isc measured in the area pellucida (-1.3 +/- 0.8 mV, 9.3 +/- 4.4 microA/cm2) and extraembryonic area opaca (-7.8 +/- 1.1 mV, 31.2 +/- 12.7 microA/cm2) were significantly different. Such a heterogeneous distribution of electrical properties can explain the presence in the blastoderm of extracellular electrical currents found by using a vibrating probe.