An antibody against
recoverin, the
calcium-binding protein, labels photoreceptors, cone bipolar cells, and a subpopulation of cells in the
ganglion cell layer. In the present study, we sought to establish the origin and identity of the cells expressing
recoverin in the
ganglion cell layer of the rat retina. By double labeling with
rhodopsin, we demonstrate that early in development some of the
recoverin-positive cells in the
ganglion cell layer are photoreceptors. During the first postnatal week, these
rhodopsin-positive cells are eliminated from the
ganglion cell layer, but such neurons remain in the inner nuclear layer well into the first postnatal month. Another contingent of
recoverin-positive cells, with morphological features equivalent to those of bipolar cells, is present in the postnatal retina, and approximately 50% of these neurons survive to maturity. The incidence of such cells in the
ganglion cell layer was not affected by early transection of the optic nerve, a manipulation that causes rapid loss of retinal ganglion cells. These
recoverin-positive cells were not double-labeled by cell-specific markers expressed by photoreceptors, rod bipolar cells, or horizontal and amacrine cells. Based on their staining with
recoverin and salient morphological features, these ectopic profiles in the
ganglion cell layer are most likely cone bipolar cells. Collectively, the results provide evidence for photoreceptors in the
ganglion cell and inner nuclear layers of the developing retina, and a more permanent subpopulation of cone bipolar cells displaced to the
ganglion cell layer.