A correlative freeze-etch and electrophysiological study of communicating junctions in crystalline lenses

We have conducted a correlative electrophysiological and morphological study of cell to cell coupling in frog and rat lenses. Electrical impedance measurements from frog and rat lenses were curve fit to a model of lens structure to obtain a value for internal resistivity (Ri). The mean and standard deviation of Ri was 550 +/- 190 ohm-cm (n = 7) in rat lenses and 3400 +/- 340 ohm-cm (n = 10) in frog lenses. These results indicate that the extent of cell to cell coupling is far more extensive in rat lenses than in frog lenses and therefore suggest that rat lens fiber cells are conjoined by greater numbers of communicating junctions than frog lens fiber cells. Freeze-etch replicas were made of fiber cells from rat and frog lenses of comparable size and from a comparable area (intermediate cortex) as that used in the electrophysiological study. A total of 987 and 1,393 square microns of replicated membrane were examined in rat and frog lenses, respectively. 1,573 communicating junctions were counted in rat lens replicas cumulatively measuring 313 square microns or 31.7% of the total membrane area. 604 communicating junctions were counted in frog lens replicas cumulatively measuring 163 square microns or 11.7% of the total membrane area. These results demonstrate that the amount of communicating junction predicted to be necessary to account for the more extensive electrotonic coupling between fiber cells in rat lenses than in frog lenses is qualitatively confirmed by morphological analysis.