Disruption of developmental timing in the albino rat retina

We have examined the spatial and temporal gradients of two developmental processes in albino and pigmented rats: outer plexiform layer (OPL) development, and rate of cell production. The OPL first appears as a thin, discontinuous break in the cytoblast layer that is frequently interrupted by the profiles of migrating neuro- and glioblasts. In both strains, this occurs in an area temporal to the optic disc that corresponds to the eventual site of peak ganglion cell density, but is not located along the line of nasotemporal division. The OPL is first evident at P5 in pigmented animals, but its appearance in albino animals is delayed approximately 30 hours, and its development appears to follow a flatter spatial gradient than in pigmented animals. In pigmented animals OPL formation is complete over most of the retina by P10, but in albino animals at this age it is yet to be completely formed at any retinal location. Reductions in mitotic activity are also first evident in temporal retina, but unlike OPL development, appear to follow the same temporal-spatial gradient in both strains. Reductions in temporal retina are obvious by P4, and mitotic activity has ceased altogether in midtemporal retina by P6 and throughout most remaining retinal regions by P8. Thus, the initial reduction of mitotic activity precedes the onset of OPL formation in both strains, but OPL development lags behind the reduction of mitotic activity to a greater extent in albino than in pigmented animals. Some aspects of differentiation within the inner nuclear layer (INL) were also examined. Just prior to the time of the onset of OPL formation, three distinct sublaminae are apparent in the INL. Cells in the innermost sublamina appear to be in an early stage of differentiation. Cells in the middle sublamina appear to be postmigratory, but have not yet begun to differentiate. Cells in the outermost sublamina have the appearance of migrating neuroblasts. At least some of these outer cells appear to migrate across the developing OPL to the outer nuclear layer, since the outermost sublamina becomes thinner and eventually disappears at the same time that the OPL becomes a continuous, uninterrupted plexiform layer. Cells of the middle sublamina apparently begin differentiation at about the time that this migration is complete. Although this sequence is the same in both albino and pigmented strains, its onset is delayed in albino animals by the same amount as the onset of OPL formation.(ABSTRACT TRUNCATED AT 400 WORDS)