FINE-STRUCTURE AND PLASMA-MEMBRANE ORGANIZATION OF RETINAL MULLER CELLS IN 2 REPTILES

The fine structure of retinal Muller cells was examined in two reptiles with conventional electron microscopy and freeze-fracture technique. The results obtained by examining ultrathin sections and Pt/C replicas showed that the distribution of both cytoplasmic organelles and intramembrane particles is highly polarized. On the basis of cytoplasmic organization, three main cytotopographical regions were identified. The apical region, which contacts the subretinal space, has microvilli, intermediate junctions and most of the mitochondria. The intermediate region, spanning from the outer plexiform layer to the ganglion cell layer, contains the nucleus, Golgi complex, smooth endoplasmic reticulum and a longitudinally oriented system of microtubules. The basal region, formed by terminal feet that contact the vitreous body, contains large amounts of intermediate filaments. At this level, the plasma membrane domains facing the vitreal basal lamina are characterized by the presence of orthogonal arrays of particles. The present results demonstrate that both the ultrastructure and membrane morphology of reptilian Muller cells are similar to those reported for other vertebrates that possess avascular retinas and are consistent with the functions proposed for this glial cell type in the different retinal layers.