Fig. 8. Two models for the mechanism behind generation of EP. The `single-cell model' hypothesizes that the Na<sup>+</sup> conductance of the basolateral membranes of marginal cells (blue lines) generates a large positive membrane voltage, which is the source for the positive EP (90 mV; red zone). In this model, TJs in marginal cells (green) are thought to be essential for EP generation. In the `two-cell model', the K⁺ conductance in the inner membranes (blue lines) of basal cells and of intermediate cells, which are connected to basal cells through gap junctions (GJ), are assumed to generate the source of EP (90 mV; red zone). In this model, the involvement of marginal cells in the generation of EP was limited to the maintenance of the low K⁺ concentration in the intrastrial space and TJs in basal cells (green) play a crucial role.