Quantitative synaptology of the inner plexiform layer of the retina of Bufo marinus.

We have performed a quantitative electron microscope study on the inner plexiform layer (IPL) of Bufo marinus retina in order to reveal the types, morphological features, densities and distributions of synapses and compare these parameters to samples derived from the visual streak area and the retinal periphery. The IPL has been subdivided into five sublayers (SLs) and the data were analysed accordingly. The density of synapses was found to be about 140.000/mm2 and was not significantly different between the central and peripheral retina. In both locations, SL3 contained the smallest number of synapses. The amacrine:bipolar presynaptic profile ratio was about 7, both in the centre and at the periphery. Amacrine-amacrine interactions greatly outnumbered other types of synapses and occurred most frequently in SL2 and 4. Bipolar cell outputs are directed mostly to amacrine-amacrine dyads, and the inputs to bipolar terminals from amacrine cells somewhat outnumber those of the outputs. The largest number of synapses directed to ganglion cells was observed in SL1 and derived from amacrine cells. The correlation between the stratification of neurochemically and morphologically characterised amacrine cells, and the morphologically and physiologically characterised ganglion cell types in the IPL are discussed.