Synaptic organization of the inner plexiform layer of the retina of Xenopus laevis

Abstract

The inner plexiform layer (IPL) of the retina of the Sough African clawed frog, X. laevis, was studied by EM. Photomicrographs of single sections revealed synaptic morphologies comparable to those in other vertebrate retinae. In a partial serial reconstruction of a bipolar terminal some unusual arrangements were found. The bipolar terminal made some synapses that at 1st examination appeared much like conventional synapses, but subsequent sections always revealed an extremely small ribbon. Many of the ribbon synapses contacted more than 2 postsynaptic processes; up to 6 processes postsynaptic to 1 ribbon contact were seen. A reciprocal synapse was not evident at each ribbon synapse. Montages of the entire width of the inner plexiform layer were constructed from sections cut from 4 different locations across the retina. The numbers of conventional and ribbon synapses per unit volume of tissue were determined. The synaptic densities found in Xenopus were much lower than those reported for other frogs. Differences in synaptic densities from the 4 locations were statistically insignificant. The overall amacrine/bipolar synapse ratio was 6.8/1. The synaptic densities in the inner plexiform layer did not change when the tissue was stained with lead citrate alone rather than with uranyl acetate and lead citrate. The functional significance of the morphological and quantitative synaptic arrangements in Xenopus IPL was discussed, and the synaptic organization was compared to that of other amphibia and vertebrates.