Ultrastructural visualization of primate cone photoreceptor matrix sheaths.

Glycoconjugates, including glycolipids, glycoproteins, and proteoglycans, are present in the plasma membrane of photoreceptor cells and in the interphotoreceptor matrix surrounding photoreceptor cell ellipsoids and outer segments. Although the precise function of these molecules is unknown, they may be important in mediating photoreceptor-pigment epithelial cell interactions, outer segment membrane assembly, and/or disc shedding. Lectins, affinity ligands for defined carbohydrate sequences, have proven particularly useful in studying the glycoconjugate composition of the interphotoreceptor matrix. The peanut lectin selectively binds to domains of the interphotoreceptor matrix surrounding cone ("cone matrix sheaths"), but not rod inner and outer segments. This is evidence for the existence of chemical and structural heterogeneity within the interphotoreceptor matrix. The studies described herein utilized ultrastructural pre-embedding histochemical labeling to assess whether, in addition to the surrounding interphotoreceptor matrix, peanut lectin binding is associated directly with that plasma membrane of cone inner and outer segments. This study confirms that ferritin-conjugated peanut agglutinin binds to cone matrix sheaths, and, in addition, provides ultrastructural evidence for the presence of binding to the plasma membrane surrounding cone inner and outer segments. The data suggest that cone membrane-associated peanut agglutinin-binding molecules may differ from those located within cone matrix sheaths.