Pathogenesis of drusen in the primate.

Two monkey eyes that showed clinical evidence of drusen were studied by light and electron microscopy. The drusen-like spots had several different morphological patterns: the appearance of typical drusen, budding retinal pigment epithelial (RPE) cells, and vacuolation of retinal pigment epithelial cells. Several stages of budding were seen. In some lesions, part of the RPE cell protruded into the sub-RPE space. The upper portion of the budding cell was connected to the cytoplasm of the parent RPE cell and was surrounded by basement membrane of the RPE cell. These budding cells had plasma membranes, cytoplasm that contained organelles, and a nucleus. Disconnected buds, separate from the parent RPE cell, were also seen; these showed degeneration. Finally, an accumulation of vesicular, granular, tubular and linear material was found in the nodular space beneath the RPE cell. It is suggested that this budding of RPE cells is the initial event in drusen-formation.