Docosahexaenoic Acid Uptake and Metabolism in Photoreceptors: Retinal Conservation By an Efficient Retinal Pigment Epithelial Cell-Mediated Recycling Process

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Abstract
The vertebrate retina is compartmentalized into two chambers, each of which is isolated from the other and from the circulatory system by tight junctions (Rodriguez de Turco, et al., 1991b). These barriers occur between the cells of the retinal pigment epithelium (RPE), the distal tips of the Müller cells and photoreceptors (forming the outer limiting membrane), and the proximal ends of the Müller cells (forming the inner limiting membrane). The inner chamber contains the elements of the neural retina, while the outer compartment, the interphotoreceptor matrix, surrounds the photoreceptor outer and inner segments (Fig. 1). The synthesis and packaging of photoreceptor membrane phospholipids take place within the endoplasmic reticulum and Golgi apparatus of the inner segments; these phospholipids are subsequently targeted for either the synaptic terminals or the light-sensitive outer segments. Following assembly of the photosensitive membranes, the newly formed discs are added basally to the stack of discs already within the outer segment, resulting in continual elongation. This constant biogenesis of disc membranes is offset by shedding and phagocytosis of outer segment tips by the RPE cells. In addition to their involvement in this daily membrane degradative process, RPE cells participate actively in the synthesis of new photoreceptor membranes. They efficiently take nutrients from the choriocapillaris and deliver them to the photoreceptor cells through the interphotoreceptor matrix.