We report a survey of the population of ganglion cells in the rabbit retina. A random sample of 301 neurons in the ganglion cell layer was targeted for photofilling, a method in which the arbors of the chosen neurons are revealed by diffusion of a photochemically induced fluorescent product from their somas. An additional 129 cells were labeled by microinjection of Lucifer yellow. One hundred and thirty-eight cells were visualized by expression of the gene encoding a green fluorescent protein, introduced by particle-mediated gene transfer. One hundred and sixty-six cells were labeled by particle-mediated introduction of DiI. In the total population of 734 neurons, we could identify 11 types of retinal ganglion cell. An analysis based on retinal coverage shows that this number of ganglion cell types would not exceed the available total number of ganglion cells. Although some uncertainties remain, this sample appears to account for the majority of the ganglion cells present in the rabbit retina. Some known physiological types could easily be mapped onto structural types, but half of them could not; a large set of poorly known codings of the visual input is transmitted to the brain.

The dendritic structures of retinal ganglion cells in the mouse retina were visualized by particle-mediated transfer of DiI, microinjection of Lucifer yellow, or visualization of green fluorescent protein expressed in a transgenic strain. The cells were imaged in three dimensions and the morphologies of a series of 219 cells were analyzed quantitatively. A total of 26 parameters were studied and automated cluster analysis was carried out using the k-means methods. An effective clustering, judged by silhouette analysis, was achieved using three parameters: level of stratification, extent of the dendritic field, and density of branching. An 11-cluster solution is illustrated. The cells within each cluster are visibly similar along morphological dimensions other than those used statistically to form the clusters. They could often be matched to ganglion cell types defined by previous studies. For reasons that are discussed, however, this classification must remain provisional. Some steps toward more definitive methods of unsupervised classification are pointed out.