The presence and possible role of endogenous ionic currents and voltage gradients in the development of Ambystoma mexicanum embryos

Mary Elizabeth Mourey Metcalf, Purdue University

Abstract

Using a noninvasive vibrating electrode for the measurement of extracellular current, polarized ionic current was observed to traverse the Ambystoma mexicanum embryo throughout the period of neurulation. The voltage driving these ionic currents is an internally positive transepithelial potential (TEP) normally expressed across embryonic integuments. Current is driven out of the lateral walls of the neural folds and the blastopore and enters most of the rest of the embryo's body surface. By sampling the distribution of TEPs in axolotl embryos, preliminary evidence for an internal, caudally negative, potential gradient of 5-23 mV/mm is provided. In addition, steady DC electric fields were imposed over developing axolotl gastrulae and neurulae. These applied voltages were meant to confound developmental cues provided by endogenous currents and fields. Applied voltages in the physiological range (25-75 mV/mm) cause severe disruption of development when imposed over neurulae whose orientation within the field is random or fixed. 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 of field producing developmental abnormality was between 5 and 25 mV/mm. Physiological measurement of the TEP within the applied field showed that ectoderm facing the cathode was hyperpolarized, while ectoderm facing the anode was depolarized. These data show that the electrical polarity of embryonic ectoderm is predictably disrupted by applied voltages. Though applied voltages exert this same effect on the ectoderm of gastrulae, exposure only during gastrulation does not lead to developmental abnormality. This observation demonstrates that the applied field does not harm the embryo in some nonspecific way, and further emphasizes the stages of neurulation as those most sensitive to artificially applied or endogenous voltages. These data strongly support the notion that the polarized natural voltages within amphibian embryos are controls of their emerging pattern.

Degree

Ph.D.

Advisors

Borgens, Purdue University.

Subject Area

Biology|Neurology|Biophysics

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