Origin and organization of pigment epithelial apical projections to cones in cat retina.

The apical surface of the retinal pigment epithelial cells (RPE) in the cat extend long sheetlike membranes that wrap concentrically above and around cone outer segments forming the cone sheath. The origin and organization of these sheetlike projections were studied in serial sections by electron microscopy. The apical surface of the RPE cells was found to consist of a thin zone of anastomosing ridges, or microplicae, from which longer projections extend. The lamellar projections forming the cone sheath originate from the microplicae as small cytoplasmic tabs that rapidly expand into broader sheets. Growth of individual sheets to their final size and shape continues by lateral and longitudinal expansion, fusion, and subdivision of the membrane. The small area of connection to the cell body allows the lamellae to overlap and interdigitate in forming the complex organization of the sheath. Microfilaments but not microtubules extend into the apical processes. RPE cilia (9 + 0 microtubules) with associated basal bodies, striated rootlets, and microtubules mark the location of retinal cones. These structures may be part of a microtubule organizing center that participates in morphogenesis of the cone sheath. They also may be involved in anchoring the apical projections forming the sheath, or in the movement of apical projections during the phagocytosis of outer segment discs shed from cone tips.