Cytoskeletal components of the adherens junctions between the photoreceptors and the supportive müller cells

The outer limiting "membrane" (OLM) of the vertebrate retina comprises a series of heterotypic adherens junctions between the photoreceptors and the supportive Müller cells. These junctions appear to support the photoreceptors, which in teleosts, anurans, and birds are motile, and thus help them maintain their orientation with respect to incoming light. In an unusual role for this type of junction, they also provide a semipermeable barrier, preventing the diffusion of some proteins out of the extracellular space that surrounds the inner and outer segments of the photoreceptors. Using immunoelectron microscopy, we examined the association of actin, myosin, alpha-actinin, and vinculin with the OLM junctions of the adult chicken retina. Vinculin was detected close to the plasma membrane in the cytoplasmic plaques of the junctions, as it was in the adherens junctions of the retinal epithelium. Labelling of actin, myosin, and alpha-actinin was spread more throughout the plaques and was distributed unevenly about the junctions; labelling was much more extensive in the Müller cells than in the photoreceptors. Thus, the junctions of the OLM show similarity to the other adherens junctions in that their cytoplasmic plaques contain actin, myosin, alpha-actinin, and vinculin. But the large aggregation of actin, myosin, and alpha-actinin in the Müller cells, and their resulting asymmetrical distribution about the junctions, is unusual, and possibly an adaptation for the special function of the OLM junctions, in providing both structural support for the motile photoreceptors and a semipermeable barrier.