Ectopic Photoreceptor Cells and Cell Death in the Developing Rat Retina

To confirm the identity of presumed photoreceptor-like neurones displaced from their normal location in the developing retina we have examined their morphology and extent of occurrence in the Long-Evans hooded rat aged one to six weeks postnatally. Displaced photoreceptor cells (PR) in the inner nuclear layer showed changing nuclear chromatin patterns during retinal development akin to those occurring in PR cells in the outer nuclear layer. PR cell cytoplasmic specializations included outer segments in various stages of formation and presynaptic terminal features including synaptic ribbons and vesicles. Processes abutting on PR cell terminals did not have postsynaptic specializations. Displaced PR cells may have arisen from PR progenitors which failed to retain a foothold at the retina's ventricular surface. The incidence of displaced PR cells determined from quantification of their planimetric densities decreased from 18% of the INL cell population at day 9 postnatally to less than 2% at day 15. A few such cells remained even at 36 days. Their fate appeared to be migration to the ONL and, or, in situ degeneration. Counts of necrotic cells carried out at ages preceding, during, and following the period during which ectopic PR cells were most numerous indicated that the decline in numbers of displaced PR cells coincided temporally with the period during which cell degeneration in the INL was most prominent. Degeneration of cells in the INL, including ectopic PR cells, was sufficient to account for a considerable proportion of the retinal thinning that occurred during development. Results suggest that future studies of retinal development in genetically or experimentally manipulated animals should consider abnormalities in cell migration and death.