Rod photoreceptor differentiation in fetal and infant human retina

Human rods and cones are arranged in a precise spatial mosaic that is critical for optimal functioning of the visual system. However, the molecular processes that underpin specification of cell types within the mosaic are poorly understood. The progressive differentiation of human rods was tracked from fetal week (Fwk) 9 to postnatal (P) 8 months using immunocytochemical markers of key molecules that represent rod progression from post-mitotic precursors to outer segment-bearing functional photoreceptors. We find two phases associated with rod differentiation. The early phase begins in rods on the foveal edge at Fwk 10.5 when rods are first identified, and the rod-specific proteins NRL and NR2e3 are detected. By Fwk 11–12, these rods label for interphotoreceptor retinoid binding protein, recoverin, and aryl hydrocarbon receptor interacting protein-like 1. The second phase occurs over the next month with the appearance of rod opsin at Fwk 15, closely followed by the outer segment proteins rod GTP-gated sodium channel, rod arrestin, and peripherin. TULP is expressed relatively late at Fwk 18–20 in rods. Each phase proceeds across the retina in a central-peripheral order, such that rods in far peripheral retina are only entering the early phase at the same time that cells in central retina are entering their late phase. During the second half of gestation rods undergo an intracellular reorganization of these proteins, and cellular and OS elongation which continues into infancy. The progression of rod development shown here provides insight into the possible mechanisms underlying human retinal visual dysfunction when there are mutations affecting key rod-related molecules.