Summary

Results of electrical, dye-coupling and morphological studies have previously suggested that gap junctions mediate communication between the anterior epithelium of the lens and the underlying lens fiber cells. This connection is believed to permit ‘metabolic cooperation’ between these dissimilar cell types and may be of particular importance to the fiber cells, which are thought incapable of autonomous ionic homeostasis. We reinvestigated the nature of the connection between epithelial and fiber cells of the embryonic chicken lens using fluorescence confocal microscopy and freeze-fracture analysis. In contrast to earlier studies, our data provided no support for gap-junction-mediated transport from the lens epithelium to the fibers. Fluorescent dyes loaded biochemically into the lens epithelium were retained there for more than one hour. There was a decrease in epithelial fluorescence over this period, but this was not accompanied by an increase in fiber cell fluorescence. Diffusional modeling suggested that these data were inconsistent with the presence of extensive epithelium-fiber cell coupling, even if the observed decrease in epithelial fluorescence was attributed exclusively to the diffusion of dye into the fiber mass via gap junctions. Furthermore, the rate of loss of fluorescence from isolated epithelia was indistinguishable from that measured in whole lenses, suggesting that decreased epithelial fluorescence resulted from photobleaching and leakage of dye rather than diffusion, via gap junctions, into the fibers. Analysis of freeze-fracture replicas of plasma membranes at the epithelial-fiber cell interface failed to reveal evidence of gap-junction plaques, although evidence of endocytosis was abundant. These studies were done under conditions where the location of the fracture plane was unambiguous and where gap junctions could be observed in the lateral membranes of neighboring epithelial and fiber cells. Paradoxically, tracer molecules injected into the fiber mass were able to pass into the epithelium via a pathway that was not blocked by incubation at 4 degrees C or by treatment with octanol and which excluded large (approximately 10 kDa) molecular mass tracers. Together with previous measurements of electrical coupling between fiber cells and epithelial cells, these data indicate the presence of a low-resistance pathway connecting these cell types that is not mediated by classical gap junctions.