DEVELOPMENT OF THE RABBIT RETINA, .3. DIFFERENTIAL RETINAL GROWTH, AND DENSITY OF PROJECTION NEURONS AND INTERNEURONS

To provide a quantitative description of postnatal retinal expansion in rabbits, a new procedure was developed to map the retinae, which cover the inner surface of hemispheres or parts of rotation ellipsoids, in situ, onto a single plane. This method, as well as the known distribution of Muller cells per unit retinal surface area, were used to estimate the redistribution of specific subpopulations of Muller cells within different topographic regions of the retinae. Muller cells are known to exist as a stable population of cells 1 week after birth and can therefore be used as ''markers'' for determining tissue expansion. Our results show that differential retinal expansion occurs during development. Peripheral retinal regions expand at least twice as much as the central ones. Furthermore, there is a greater vertical than horizontal expansion. This differential retinal expansion leads to a corresponding redistribution of 5-hydroxytryptamine (5-HT) accumulating amacrine cells. Differential retinal expansion, however, does not account for all of the changes in the centro-peripheral density gradient of cells in the ganglion cell layer (GCL)-mostly retinal ganglion cells-during postnatal development. The changes in the ganglion cell layer were evaluated in Nissl-stained wholemount retinal preparations. Additionally, the difference between expansion-related redistribution of cells in the GCL and Muller cells was confirmed in wholemount preparations where Muller cells (identified as vimentin positive) and cells in the GCL (identified by fluorescent supravital dyes) were simultaneously labeled. It is assumed that many of the ganglion cells within the retinal center are not translocated during retinal expansion, possibly because their axons are fixed. In contrast, 5-HT accumulating amacrine cells-which are interneurons without a retinofugal axon-display a passive redistribution together with the surrounding retinal tissue.