Regional specialization of the chick retina as revealed by the size and density of neurons in the ganglion cell layer.

On the basis of both morphological criteria and survival after ganglion cell axotomy, three populations of cells can be recognized in the ganglion cell layer of the chick retina. These are: 1) Irregularly shaped cells, which are not affected by axotomy and lack Nissl stain. They comprise about 5% of the total cell population and are probably glial cells. 2) Small cells which have a distinctive teardrop-shaped perikarya, stain for Nissl substance, and are not affected by axotomy. They occur with a uniform density of about 4,000 cells/mm2 across the entire retina and comprise 30-35% of the total number of cells in the ganglion cell layer. It is suggested that these cells are displaced amacrine cells. 3) Cells which stain strongly for Nissl substance and disappear after axotomy. These cells comprise 60-65% of the population of cells in this layer. The density of the latter cells varies throughout the retina. A high-density region in the central area extends into the superior-temporal retina. This high-density region corresponds to a position in the lateral visual field extending into the infero-frontal field. The total number of these cells agrees with the reported number of the optic nerve fibres in the chick, (Rager and Rager, '78); therefore they are presumably ganglion cells. The size distribution of the presumptive ganglion cells varies in different parts of the retina. The only previous study of the distribution and size of cells in the ganglion cell layer of the avian retina did not distinguish between ganglion cells and displaced amacrine cells (Binggeli and Paule, '69). The present results are therefore likely to be a more accurate description of the total number of ganglion cells, the regional variations in their density, and the characteristics of their size than previously reported.