Cell-to-cell fusion of lens fiber cells in situ: correlative light, scanning electron microscopic, and freeze-fracture studies.

We have discovered cell-to-cell fusion between fiber cells of adult frog lenses in situ. Stereo scanning electron microscopy (SEM) revealed fusion between neighboring fiber cells in radial cell columns (RCCs) and in the same growth ring, respectively. Cell-to-cell fusion of fiber cells in the lens produced fusion zones that in cross-section were larger and of different polygonal shapes than unfused fiber cells. The shape and sizes of fiber cells surrounding fusion zones and the alignment of RCCs were also altered. Serial sectioning through fusion zones confirmed that they were areas of cell-to-cell continuity established by the union of neighboring fiber cells as seen by SEM. Fusion zones represent a previously unrecognized intercellular pathway in the adult frog lens. Although numerous fusion zones were seen throughout the lens cortex and nucleus, cell-to-cell fusion was rarely observed to have occurred between elongating fiber cells. Interestingly, communicating junctions with an unusual ultrastructure that closely resembles the appearance of membranes in the process of fusion demonstrated in other systems were frequently seen in the region of the superficial cortex where fusion zones were most numerous. The fact that such unusual communicating junctions were not found in any other region of the lens leads us to speculate that structural changes in fiber cell communicating junctions may herald the formation of fusion zones and that the initial site of cell-to-cell fusion between fiber cells may be within communicating junctional plaques.