Epithelial cells of Hydra are dye-coupled

In the past decade, a strong correlation has been established between gap junctions, seen in cell ultrastructure studies, and cell coupling (ionic, metabolic or dye coupling) assayed physiologically(1). In Hydra, ultrastructural analyses have indicated that the epithelial cells of both cell layers are connected extensively by gap junctions; gap junctions have also been observed between the two layers(2-7). On the basis of these results, one would expect electrical and dye coupling between epithelial cells of Hydra. However, de Laat et al.(8) reported that these cells were neither dye- nor electrically coupled, which was unexpected as cells in another coelenterate have been shown to be coupled ionically(9). Cell-cell coupling in Hydra is particularly interesting because extensive experiments on head regeneration in this coelenterate have led to well-defined models of patterning that require communication between cells of the type that may be provided by gap junctions(10-14). We have re-examined dye coupling in Hydra and we report here that, after injection of Lucifer yellow into single epithelial cells, neighbouring cells were observed to contain the dye. We conclude that the epithelial of cells of Hydra are indeed dye-coupled.