FIGURE 24.13 • Electron micrograph of the retinal pigment epithelium in association with the outer segments of rods and cones. Retinal pigment epithelial cells (RPE) contain numerous elongated melanin granules that are aggregated in the apical portion of the cell, where the microvilli extend from the surface toward the outer segments of the rod and cone cells. The retinal pigment epithelial cells contain numerous mitochondria and phagosomes. The arrow indicates the location of the junctional complex between two adjacent cells. 20,000. (Courtesy of Dr. Toichiro Kuwabara.)
The outer limiting membrane (layer 3) is formed by a row of zonulae adherentes between Mler’s cells.
The outer limiting membrane is not a true membrane. It is a row of zonulae adherentes that attaches the apical ends of Mler’s cells (i.e., the end that faces the pigment epithelium) to each other and to the rods and cones (see Fig. 24.10). Because Mler’s cells end at the base of the inner segments of the receptors, they mark the location of this layer. Thus, the supporting processes of Mler’s cells, on which the rods and cones rest, are pierced by the inner and outer segments of the photoreceptor cells. This layer is thought to be a metabolic barrier that restricts the passage of large molecules into the inner layers of the retina.
The outer nuclear layer (4) contains the nuclei of the retinal rods and cones.
The region of the rod cytoplasm that contains the nucleus is separated from the inner segment by a tapering process of the cytoplasm. In cones, the nuclei are located close to the outer segments, and no tapering is seen. The cone nuclei stain lightly and are larger and more oval than rod nuclei. Rod nuclei are surrounded by only a thin rim of cytoplasm. In contrast, a relatively thick investment of cytoplasm surrounds the cone nuclei (see Fig. 24.11).
The outer plexiform layer (5) is formed by the processes of the photoreceptor cells and neurons.
The outer plexiform layer is formed by the processes of retinal rods and cones and the processes of horizontal, interplexiform, amacrine, and bipolar cells. The processes allow the electrical coupling of photoreceptor cells to these specialized interneurons via synapses. A thin process extends from the region of the nucleus of each rod or cone to an inner expanded portion with several lateral processes. The expanded portion is called a spherule in a rod and a pedicle in a cone. Normally, many photoreceptor cells converge onto one bipolar cell and form interconnecting neural networks. Cones located in the fovea, however, synapse with a single bipolar cell. The fovea is also unique in that the compactness of the inner neural layers of the retina causes the photoreceptor cells to be oriented obliquely. Horizontal cell dendritic processes synapse with photoreceptor cells throughout the retina and further contribute to the elaborate neuronal connections in this layer.
The inner nuclear layer (6) consists of the nuclei of horizontal, amacrine, bipolar, interplexiform, and Mler’s cells.
Mler’s cells form the scaffolding for the entire retina. Their processes invest the other cells of the retina so completely that they fill most of the extracellular space. The basal and apical ends of Mler’s cells form the inner and outer limiting membranes, respectively. Microvilli extending from their apical border lie between the photoreceptor cells of the rods and cones. Capillaries from the retinal vessels extend only to this layer. The rods and cones carry out their metabolic exchanges with extracellular fluids transported across the blood–retina barrier of the RPE.
The four types of conducting cells—bipolar, horizontal, interplexiform, and amacrine—found in this layer have distinct orientations (see Fig. 24.9).