The properties and propagation of a cardiac-like impulse in the skin of young tadpoles

Summary

The contact relationships of skin cells in late embryos and young larvae of Xenopus laevis are described. Superficial cells are joined by ‘tight’ or ‘gap’ junctions at their outer periphery but elsewhere ‘simple appositions’ are found. All-or-none impulses are evoked in the skin by electrical or mechanical stimuli (Fig. 3). Evidence is presented in favour of the view that these impulses are generated by the majority of skin cells and not by some neuronal element in the skin. The impulse propagates throughout the skin from any stimulated point (at average speed of 7.7 cm/sec) even when the animal is in distilled water. However, removal of Na⁺ from solutions bathing the inner skin surface or treatment with Tetrodotoxin abolishes the impulse indicating that it is Na⁺ dependent. Current injected into skin cells spreads to others so it is suggested that the impulse propagates by direct current flow from cell to cell. The ‘neuroid’ conduction system in the skin of Xenopus tadpoles is compared to similar systems in coelenterates and to the propagation of the vertebrate cardiac impulse.