Patterns of cytogenesis in the developing retina of the wallaby Setonix brachyurus

Patterns of mitosis were examined during development from embryonic day (E) 19, 9 days before birth, for retinae of the wallaby Setonix brachyurus, using cresyl violet stained material. For neural retina, mitosis took place at the ventricular surface from the earliest stage of eye formation until postnatal day (P) 100. Numbers of mitotic figures reached a peak of approximately 12,000 by P43. Average densities ranged between 300/mm2 and 600/mm2 up to P12 and then fell to below 50/mm2 by P25 before reaching a second peak of over 400/mm2 at P43. Up to P50, mitoses were present across the entire retina. By P60, a 'cold spot' lacking mitotic activity had formed in temporal retina and progressively extended to reach peripheral regions by P100; no mitoses were seen at P150. The timing and location of the 'cold spot' coincided with our previous description of the appearance of an area centralis in the ganglion cell layer (Dunlop and Beazley 1985). Inner and outer plexiform layers (IPL and OPL) formed between P24-40 and P50-100 respectively and were seen first in temporal retina. Furthermore, the extent of the OPL matched the mitotic 'cold spot'. By contrast to neural retina, mitosis in the pigment epithelium was panretinal and was largely complete by P3. The data suggest that cell addition to the inner or outer nuclear layers contribute to differential retinal expansion and the establishment of cell density gradients in the ganglion cell layer.