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 i n rabbits, a new procedure was developed to map the retinae, which cov er the inner surface of hemispheres or parts of rotation ellipsoids, i n situ, onto a single plane. This method, as well as the known distrib ution of Muller cells per unit retinal surface area, were used to esti mate the redistribution of specific subpopulations of Muller cells wit hin different topographic regions of the retinae. Muller cells are kno wn to exist as a stable population of cells 1 week after birth and can therefore be used as ''markers'' for determining tissue expansion. Ou r results show that differential retinal expansion occurs during devel opment. Peripheral retinal regions expand at least twice as much as th e central ones. Furthermore, there is a greater vertical than horizont al expansion. This differential retinal expansion leads to a correspon ding redistribution of 5-hydroxytryptamine (5-HT) accumulating amacrin e 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 ev aluated 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 wher e Muller cells (identified as vimentin positive) and cells in the GCL (identified by fluorescent supravital dyes) were simultaneously labele d. It is assumed that many of the ganglion cells within the retinal ce nter are not translocated during retinal expansion, possibly because t heir axons are fixed. In contrast, 5-HT accumulating amacrine cells-wh ich are interneurons without a retinofugal axon-display a passive redi stribution together with the surrounding retinal tissue.