ENDOGENOUS IONIC CURRENTS AND VOLTAGES IN AMPHIBIAN EMBRYOS

Using a noninvasive vibrating electrode for the measurement of extrace llular current, we show that a polarized ionic current traverses the e mbryo for many hours in the anuran, perhaps days in the urodele follow ing gastrulation. The voltage driving these ionic currents is an inter nally positive transepithelial potential (TEP) normally expressed acro ss embryonic integuments. Current is driven out of the lateral walls o f the neural folds and the blastopore and enters most of the rest of t he embryo's body surface. The magnitude of the TEP is transitorily dep endent on external sodium and can be reduced by the embryo's immersion in Na+ depleted media or by treatment with 50 mu M amiloride. Both tr eatments fail to chronically reduce externally detected currents, howe ver. The pattern of currents traversing the embryo suggests they would be associated with rostral-caudal and medial-lateral gradients of vol tage within the embryo. By sampling the distribution of TEPs in axolot l embryos, we provide measurements of the former-an internal, caudally negative, potential gradient beneath the neural plate ectoderm. The m agnitude of these endogenous fields is on the order of 10 to 20 mV/mm and is within a range of potential known to affect the shape and migra tion of a variety of embryonic cell types in vitro. We suggest that en dogenous currents and voltages in the vertebrate embryo may provide gr oss cues for cell movement and emerging developmental pattern. (C) 199 4 Wiley-Liss, Inc.