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

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Abstract
The inner plexiform layer (i. p. l.) of the retina of the South African clawed frog, Xenopus laevis , was studied by electron microscopy. Photomicrographs of single sections revealed synaptic morphologies comparable to those in other vertebrate retinae. In a partial serial reconstruction of a bipolar terminal, however, some unusual arrangements were found. The bipolar terminal made some synapses that at first examination appeared much like conventional synapses, but subsequent sections always revealed an extremely small ribbon. Many of the ribbon synapses were found to contact more than two postsynaptic processes; up to six pro­cesses postsynaptic to one 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 four 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 four locations were found to be 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 i. p. l. is discussed, and the synaptic organization is compared to that of other amphibia and vertebrates.