Changing distribution of retinal ganglion cells during area centralis and visual streak formation in the marsupial Setonix brachyurus.

Injections of the axonal marker horseradish peroxidase (HRP) were made into optic tracts and/or visual centres of Setonix brachyurus, a small wallaby (quokka) during development and in adults. Distributions of HRP-labelled and unlabelled cells in the retinal ganglion cell layer were estimated from sections or wholemounts counterstained with cresyl violet. Between 20 and 40 days postnatal we did not observe an area centralis or visual streak in either the labelled or unlabelled cell populations. These regional specialisations arose between 60 and 80 days within the labelled cell population, while unlabelled cells remained approximately evenly distributed. Our findings suggest there is no 'hidden' area centralis and visual streak within the evenly distributed total cell population of the retinal ganglion cell layer at stages before cells can be classified on morphological grounds.