A bistratified amacrine cell and synaptic circuitry in the inner plexiform layer of the retina

Retinal amacrine cells rival ganglion cells of the retina in the diversity of their appearance. For a given species and for certain amacrine cells the configuration of their processes seems often to mirror that of ganglion cell dendrites with which they interdigitate in the inner plexiform layer (IPL) of the retina. There are varieties of amacrine cell, however, which have no morphological counterpart among ganglion cells. One of these, a narrow-field, bistratified amacrine cell, is the main subject of this study. For the sake of simplicity, Cajal 7 divided amacrine cells of the vertebrate retina into two classes: 'diffuse' and 'stratified'. The latter term describes cells which 'extend their fiat branches within a definite sublayer of the inner plexiform layer '7. The diffuse amacrine cells Cajal subdivided into large and small, both of which 'distribute terminal ramifications throughout the inner plexiform layer '7. Nevertheless, Cajal hinted that in mammals even 'diffuse' cells terminate preferentially in certain sublayers of the IPL. Comparatively little attention has been given to the morphology of amacrine cells in cat retina4,6,17,22, while amacrine cells of dog 7 and monkey retina a,19 have been illustrated and discussed in some detail. All three species have the same broad classes of amacrine cell. Stratified cells of the narrow-field, 'tufted '2° (Fig. 1C) and wide-field, 'radiate '20 (Fig. 1J and 1K) varieties are common. Truly diffuse amacrines, whether tufted or radiate (Fig. 1L), are infrequently found. Among cells classified by others as small 7 or narrow-field 3, 'diffuse' amacrine cells are a tufted, bistratified subtype found in cat (Fig. 1D), dog (Fig. 1E) and monkey (Fig. IF). In the cat this particular narrowfield, bistratified amacrine cell observed in Golgi preparations (Fig. 2) appears to be identical with a small amacrine cell (Fig. 3) whose ultrastructural features and neural connections are described below.

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