Oil droplets of the retinal epithelium of the rat

Oil droplets are observed in the retinal pigment epithelium (RPE) of both light- and dark-adapted rats. The accumulation of these droplets is strongly correlated with the clearance of large phagosomes that are found on a circadian basis in the RPE. Although strongly correlated with phagosomes in this way, the composition of the oil droplets does not resemble the lipid composition of rod outer segments (ROS). The important differences are that most of the 18:0 fatty acid (stearic) and virtually all of the polyunsaturated ones (principally docosahexaenoic, 22:6) are absent from the droplets. The major (greater than 95%) constituents of the droplets are triglycerides admixed with small amounts of vitamin A esters (less than 5%). This result suggests that certain of the ROS lipids, especially the polyunsaturated fatty acids, are cleared quickly from the RPE and are, perhaps, recycled to the neural retina. Such a process would account for the known ability of the retina to conserve the polyunsaturates despite dietary deficiencies.