Neuro-glial relationships at the external limiting membrane of the newt retina

Abstract

The ultrastructure of the external limiting membrane of the adult newt retina and of the seleral, outer portions of the Müller cells in this region, has been examined in detail. The radial fibers of Müller expand in a conical fashion at their scleral ends to compensate for the narrowing of the photoreceptor myoids in this region. The Müller cell conical expansions are divided into two regions, a scleral mitochondria-free zone and a more vitreal mitochondrial zone. Apical Müllerian microvilli, contain fine filaments which are continuous with the filamentous mitochondria-free zone, where coated vesicles and multivesicular bodies are found. Lateral extensions of the mitochondria-free zone appear in longitudinal sections to overlap, and in tangential sections to interlock with adjacent Müller cells and so fit together like the pieces of a jig-saw puzzle. The external limiting membrane is composed of both interglial and neuroglial junctions. The most scleral component of the interglial junction consists of large gap junctions which form an extensive network between the interlocking apical processes of the Müller cells and are consistently observed in a plane perpendicular to the long axis of the Müller fibers. A second interglial junctional component, resembling a zonula adherens, is found vitreal to the interglial gap junction network. The neuroglial junctions resemble zonulae adherentes but display a frequent narrowing of the intercellular cleft and gap junctions were never observed between these cells. A second population of large interglial gap junctions are observed but these are distinguished from gap junctions of the external limiting membrane, by their proximity to the mitochondria-filled portions of the conical Müller cell expansions. Glio-glial and neuro-glial relationships are discussed with regard to the nutritional role of the Müller cell in this avascular retina.