Association amacrine cells of Ramón y Cajal: rediscovery and reinterpretation

In 1895, by means of the Golgi method, Santiago Ramón y Cajal discovered a cell having a unique morphology in the avian retina. This cell had its cell body in the amacrine cell level of the inner nuclear layer, only a few rudimentary dendrites at the outermost level of the inner plexiform layer (IPL), and a long axon coursing horizontally and terminating in the IPL. Despite having defined amacrine cells as cells without axons, Cajal named this cell type "association amacrine cell" (AAC). This discovery was not confirmed by other investigators for nearly a century. Very recently, however, isthmo-optic target cells (IOTCs), which receive the terminals of centrifugal fibers emanating from the isthmo-optic nucleus, have been identified as one type of AAC. As summarized and discussed in this review, the morphology of the AACs as described by Cajal has been completely confirmed. However, since these cells appear to be classical polarized, monoaxonal neurons and lack the dendritic interactions that are typical of amacrine cells, they should be regarded as a distinct type of retinal interneuron and not as amacrine cells.