WEAK APPLIED VOLTAGES INTERFERE WITH AMPHIBIAN MORPHOGENESIS AND PATTERN

We have imposed steady DC electric fields over developing gastrula and neurula stage axolotls, Ambystoma mexicanum. These applied voltages w ere meant to confound cues provided by endogenous currents and fields that we have measured and believe to be controls of pattern and morpho genesis during early development. Applied voltages in the physiologica l range (25-75 mV/mm) cause severe disruption of development when impo sed over neurulae whose orientation within the field is random or fixe d. In the latter case, developmental defects are more likely to occur at that end of the neurula (rostral or caudal) that faces the cathode, or negative pole, of the applied field. In neurulae whose orientation within the field was fixed, the lowest magnitude producing developmen tal abnormality was between 5 and 25 mV/mm. Physiological measurement of the embryonic transepithelial potential (TEP) when perturbed by art ificially applied voltages demonstrates that ectoderm facing the catho de is hyperpolarized, while ectoderm facing the anode is depolarized a t all fields strengths tested. These data show that the electrical pol arity of embryonic ectoderm is predictably disrupted by applied voltag es. Though applied voltages exert this same effect on the ectoderm of gastrulae, exposure only during gastrulation does not lead to developm ental abnormality. This observation demonstrates that the applied elec tric field does not harm the embryo in some non-specific way and furth er emphasizes the stages of neurulation as those most sensitive to art ificially applied or endogenous voltages. These data strongly support the notion the polarized natural voltages within amphibian embryos are controls of their emerging pattern. (C) 1994 Wiley-Liss, Inc.