Demonstration of cell types among cone bipolar neurons of cat retina.

We identified all the cone bipolar cells (80) in a small patch of one retina and then studied in detail the complete subset (42) that sends axons to sublamina b of the inner plexiform layer. The point was to learn whether the 'types' suggested previously, based on a few examples from a large population, could be substantiated or whether there would be intermediate forms. Tissue from the area centralis (1 degree eccentricity), was prepared as a series of 279 ultrathin sections and photographed in the electron microscope. Thirteen cells were reconstructed completely and parcelled into five categories (b1-b5) based on external morphology. For nine of these cells (two from categories b1-b4 and one from b5) most of the synaptic inputs and outputs were identified. When these nine cells were parcelled according to their synaptic patterns, they sorted into the same five categories. The remaining 29 cells in the population, though not reconstructed, were studied in detail by tracing their processes through the series. Ten of these cells, those near the margin of the series, were incomplete. The other 19 cells had essentially the same distribution of morphologies and synaptic patterns as the subset studied by total reconstruction: when plotted in multiparametric space, they formed distinct clusters corresponding to the five morphological categories. There was no hint of intermediate forms. That all the neurons in the population sort into some cluster (no intermediate forms), and that each neuron sorts into the same cluster by different criteria, argues that the clusters represent natural types. Each type forms a regular array in the region studied with an axonal 'coverage factor' that is close to one.