THE NUMBER OF UNIDENTIFIED AMACRINE CELLS IN THE MAMMALIAN RETINA

The three largest known populations of amacrine cells in the rabbit re tina were stained with fluorescent probes in whole mounts and counted at a series of retinal eccentricities. The retinas were counterstained using a fluorescent DNA-binding molecule and the total number of nucl ei in the inner nuclear layer were counted in confocal sections. From the total number of inner nuclear layer cells and the known fraction o f them occupied by amacrine cells, the fraction of amacrine cells made up by the stained populations could be calculated. Starburst cells ma de up 3%, indoleamine-accumulating cells made up 4%, and AII cells mad e up 11% of all amacrine cells. By referring four smaller populations of amacrine cells to the number of indoleamine-accumulating cells, the y were estimated to make up 4% of all amacrine cells. Thus, 78% of all amacrine cells in the rabbit's retina are known only from isolated ex amples, if at all. This proportion is similar in the retinas of the mo use, cat, and monkey. It is likely that a substantial fraction of the local circuit neurons present in other regions of the central nervous system are also invisible as populations to current techniques.