We report a quantitative analysis of the major populations of cells present in the retina of the C57 mouse. Rod and cone photoreceptors were counted using differential interference contrast microscopy in retinal whole mounts. Horizontal, bipolar, amacrine, and Mü ller cells were identified in serial section electron micrographs assembled into serial montages. Ganglion cells and displaced amacrine cells were counted by subtracting the number of axons in the optic nerve, learned from electron microscopy, from the total neurons of the ganglion cell layer. The results provide a base of reference for future work on genetically altered animals and put into perspective certain recent studies. Comparable data are now available for the retinas of the rabbit and the monkey. With the exception of the monkey fovea, the inner nuclear layers of the three species contain populations of cells that are, overall, quite similar. This contradicts the previous belief that the retinas of lower mammals are "amacrine-dominated", and therefore more complex, than those of higher mammals.

The retina, like many other central nervous system structures, contains a huge diversity of neuronal types. Mammalian retinas contain approximately 55 distinct cell types, each with a different function. The census of cell types is nearing completion, as the development of quantitative methods makes it possible to be reasonably confident that few additional types exist. Although much remains to be learned, the fundamental structural principles are now becoming clear. They give a bottom-up view of the strategies used in the retina's processing of visual information and suggest new questions for physiological experiments and modeling.