The neural tube of the Xenopus embryo maintains a potential difference across itself

In Xenopus embryos, the ectodermal epithelium generates a substantial transepithelial potential (TEP) during certain periods of early development. In this study, we have found that the neural tube (which is derived from the embryonic ectodermal epithelium) of stage 21-25 Xenopus embryos also maintains a potential across itself, with the lumen being, on average, 18 +/- 1 mV negative relative to the interstitial spaces. This transneural tube potential (TNTP) declines gradually from a maximum of -21 +/- 2 mV at stage 23 to a minimum of -14 +/- 2 mV at stage 25. Vibrating probe measurements on transected embryos suggest that the neural tube is capable of driving a current. Large outward currents ranging from 10 to 26 microA/cm2 were detected just dorsal to the center of wounds in transected stage 21-24 embryos, but near the dorsal margin of the wound, in the region corresponding to the cut face of the neural tube, outward current densities were less than half the maximum, ranging from 3 to 9 microA/cm2. The reduced outward current near the dorsal margin suggests a locus of inward current in this region that is subtracted from the much larger outcurrents. Such greatly reduced outward currents were not detected near the ventral margin of the wound.