The ultrastructural differentiation of synaptic sites in the inner plexiform layer of a teleostean retina

The differentiation of axon terminals in the retina inner plexiform layer was studied by electron microscopy, with special reference to synaptic junctions, number and size of synaptic vesicles, dense core vesicles, biogenic amines and ATPase. Five types of synaptic junctions were found, including a ribbon type. They appear on different days of embryonic life and show different patterns of increase. The ultrastructural differentiation of the most frequent type is described in detail. The numbers of synaptic vesicles indicate the existence of three types of axon terminals which appear on different days. These and other data lead to the following model: The contacts between amacrine cells are the first to appear, followed by amacrine--bipolar cell contacts and amacrine--ganglion cell contacts. The latter are the most frequent ones and increase immediately after having appeared, while amacrine--bipolar cell contacts increase only some days later, which is also the case for bipolar--amacrine cell contacts. Biogenic amines and cytochemically detectable ATPase appear along with the formation of synaptic sites.