Lectin and antibody labelling of developing rat photoreceptor cells: an electron microscope immunocytochemical study

Lectin and rhodopsin antibody binding sites were studied in developing and adult rat photoreceptors in order to compare changes in the total carbohydrate pool with the movement of a known glycoprotein rhodopsin. Electron microscope immunocytochemical techniques utilizing modified colloidal gold methods were used. At birth, all three lectins - Concanavalin A (ConA), Ricinus communis agglutinin II (RCA II) and wheat germ agglutinin (WGA) - showed heavy labelling of the photoreceptor surface scleral to the outer limiting membrane. At the same age, a monoclonal antibody against rhodopsin, RET-P1, revealed sparse labelling of only occasional immature photoreceptor surfaces. At postnatal day 4(P4), all three lectins showed variable binding to the inner segment and along the length of the newly forming connecting cilium. There was generally a region of more intense label at the base of the cilium. RET-P1 binding to P4 retina showed a discontinuous distribution, with heavily labelled inner segments being adjacent to unlabelled inner segments. This pattern indicates that the initial expression of rhodopsin is not a coordinate event but occurs in discrete cells, possibly related to the end of mitosis. RET-P1 binding at this age was reduced or absent from the proximal connecting cilium. AT P7, when the outer segments are beginning to develop, all the lectins and RET-P1 showed reduced binding to the inner segment plasma membrane and heavy labelling of the outer segment surface. In favourable sections, heavy labelling of the photoreceptor cell body plasma membrane by ConA and RCA II was also observed, terminating abruptly at the outer limiting membrane. The variation in ligand binding between different cellular compartments which are all formed from a continuous plasma membrane may indicate the presence of special barriers to diffusion of membrane components. This labelling pattern persisted into maturity. RET-P1 and lectin binding did not always correspond in developing retina, indicating that at least part of the observed lectin label must be due to other glycoproteins or glycolipids. Post-embedding thin section labelling of adult rat retina revealed a uniform binding pattern across the outer segment for ConA, WGA and RET-P1. However, RCA II exhibited labelling only along the basal edge of outer segments. Labelling of isolated, opened discs from bovine rod outer segments revealed binding to a single surface for ConA, WGA and RET-P1, but RCA II only labelled a small amount of membrane. Hence RCA II seems to recognize a determinant present only on the outer segment plasma membrane.