The Genesis and Transmission of Epidermal Potentials of Newt Embryonic Cells in Vivo and in Vitro

The genesis and transmission of action potentials in epidermal cells of a newt ( Cynops pyrrhogaster ) embryo were investigated quantitatively in vivo during development and in vitro in the absence of nerve cells. Typical action potentials, composed of a fast spike followed by a slow action potential, can be recorded from any of the epidermal cells from Stage 24/25 to 35/36. The potential is graded with current intensity, and only the slow component induces transmission to other epidermal cells. The fast spike is found in all epidermal cells from Stage 24/25 to Stage 50; it is abolished by Stage 52. The slow potential disappears at Stage 38 just before or after hatching. The cultured epithelioid explants (epithelioid aggregate) and cultured monolayer cells taken from the presumptive epidermal tissue of the ectoderm of the pregastrula, indicate that sequential changes in the genesis of the dual action potentials are similar to those of the intact embryo. In monolayer cell culture devoid of nerve cells, the epidermal cells, also generate a two-step action potential. Such two-step potentials are characteristic of both ciliated and non-ciliated epidermal cells and occur even during mitotic activity. In contrast, cultured neural plate cells isolated from the neurula generate typical spike-like action potentials.