This chapter deals with the interface between the photosensitive outer segments of photoreceptor cells and the retinal pigmented epithelium (RPE). At this interface, photoreceptor cells and cells of the RPE, both highly polarized, abut one another. The photoreceptor cells are responsible for converting light into electrical signals through the process of transduction, a subject beyond the scope of this chapter; however, there are a number of excellent reviews that provide detailed descriptions of the process.^59,110,116 Photoreceptor cell outer segments, which contain the photosensitive visual pigments, are continually renewed through the addition of new membrane basally and intermittent (daily) shedding of old membrane from their apical tips. Shed outer segment membrane is ingested by the simple, cuboidal RPE cells, which are located directly adjacent to the photoreceptors and separate them from the choroidal vasculature. Interspersed between these two retinal layers is the interpho-toreceptor matrix, a unique extracellular matrix that fills the “subretinal” space (figures 4.1 and 4.3). The matrix is composed of molecules that appear to play a role in mediating biochemical and physical interactions among the retina, RPE, and choroidal vasculature. Thus, the photoreceptor–RPE interface is an area of crucial importance to proper retinal function.

Figure 4.1.  

Light micrograph of a section of central retina from a monkey eye depicts the relationship between the choroid (C), retinal pigmented epithelium (RPE), interphotoreceptor matrix (asterisks), and neural retina. The neural retina is composed of a defined number of cell types arranged in a precise lamellar configuration. The apical surface of the neural retina contains highly polarized photoreceptor cells that abut the apical surface of the retinal pigmented epithelium. Interspersed between the apicies of these two retinal layers is the interphotoreceptor matrix (asterisks), a unique extracellular matrix that fills the subretinal space. Two types of photoreceptor cells can be identified morphologically. Cone photoreceptor inner segments (CI) are large in diameter, and the outer segments (arrows) are broader basally and tapered toward their apical tips. In contrast, rod photoreceptor inner (RI) and outer (arrowheads) segments retain a relatively uniform diameter that is smaller than that of cone photoreceptors (OLM = outer limiting membrane; ONL = outer nuclear layer (contains photoreceptor cell nuclei); OSL = outer synaptic layer; INL = inner nuclear layer [contains nuclei from Müller, amacrine, bipolar, and horizontal cells]; ISL = inner synaptic layer; GC = ganglion cell; NFL = nerve fiber layer; ILM = inner limiting membrane).