Endogenous electrical currents and voltage gradients in Xenopus embryos and the consequences of their disruption

Abstract

Starting early in development, the Xenopus embryo maintains an inwardly positive transepithelial potential (TEP) across its ectoderm. This "ectodermal battery" drives ionic current out of the embryo through low-resistance regions. The blastopore represents one such region, as we detected currents leaving this region starting at stage 14 (2 microA/cm2) and peaking at stage 22 (100 microA/cm2). The addition of the epithelial Na(+)-channel blocker, benzamil, to the bathing solution reduced these currents to 18% of their previous levels. The blastopore current generates an intraembryonic voltage gradient that is most negative at the blastopore. Along the rostral-caudal axis on the ventral side of the embryo, the gradient averaged 27 +/- 4 mV/mm between the blastopore and a point 400-600 microns rostral to the blastopore. Up the flank along the dorsal-ventral axis, the gradient averaged 40 +/- 11 mV/mm between the blastopore and a point 250 microns dorsal. We also found that the anterior neural folds at stage 17 are sites of small (2 microA/cm2 average) outward current. This current also generates a significant internal electrical field. We hypothesized that the steep gradients we detected in the Xenopus embryo play a causal role in development. To test this, we impaled stage 14-16 embryos with glass microelectrodes filled with agarose-gelled saline and passed currents through these electrodes that nulled or reversed the endogenous currents through the blastopore. Twenty of 23 embryos treated with currents of 100 nA or greater for 9 to 11 hr showed developmental abnormalities that were apparent externally. These abnormalities included the formation of ventral pigmented bulges, failure of the anterior neural tube to close, reduced head development and retarded eye formation, the extrusion of cells from the blastopore into the bath, and a failure to form functional cilia. Only one of 14 control embryos (those treated with 10 nA or no current) developed abnormally. Five embryos were treated with 100 nA of current that augmented the endogenous current. One of those exhibited abnormal head development while the other 4 appeared normal.