Fine structure and radioautography of rabbit photoreceptor cells

The retina of the adult rabbit contains two types of photoreceptors. Type I predominates and is a typical rod. The identity of the type II cell has been uncertain until now. This study demonstrates that the type II cell has certain conelike morphologic features including a pale nucleus, complex synaptic pedicel, and multiple wrappings of the outer segment by microvilli of the pigment epithelium. The type II outer segment is also conelike in that some intradisc spaces are patent to the extracellular space, that the pattern of renewal is diffuse as demonstrated radioautographically with sugar and amino acids as precursors, and that the few examples of distal tip shedding found in this study occur mainly in the early part of the dark cycle. The interior of the type II outer segment is labeled selectively with 3H-fucose, whereas the interior of the type I outer segment remains unlabeled with this sugar at all time intervals examined. The label probably reflects the presence of a fucose-containing glycoprotein in the conelike outer segment which is lacking in the rod outer segment and may represent a visual pigment or other intrinsic molecule(s). An amorphous material which forms a symmetric sleeve around the photoreceptor connecting cilium has been demonstrated after intravitreal injections of horeseradish peroxidase. This cilium-associated material is labeled specifically by 4 hr following intravitreal injection of 3H-fucose, before the interphotoreceptor matrix shows significant labeling, suggesting that it is composed of a sugar-containing material. Possible functions of the cilium-associated material, including structural support and ionic regulation, are discussed.