Polyaxonal amacrine cells of rabbit retina: Size and distribution of PA1 cells

Abstract

Type 1 polyaxonal (PA1) amacrine cells have been identified previously in rabbit retina, and their morphological characteristics have been described in detail in the preceeding paper. Like other polyaxonal amacrine cells they bear distinct dendritic and axonal branching systems, the latter of which originates in two to six thin, branching axons which emerge from or near to the cell body. Unlike other types of polyaxonal amacrine cells, however, their branching is stratified at the a/b sublaminar border and their cell bodies are most often displaced interstitially in the inner plexiform layer (IPL). This report emphasizes quantitative features of the population of PA1 cells, documented in Golgi-impregnated and Nissl-stained retinas, and provides further evidence in Nissl preparations for the amacrine-cell nature of polyaxonal amacrine cells. The cell bodies of Golgi-impregnated PA1 amacrine cells are relatively large: 12–15 μm in equivalent diameter over the range extending from the visual streak 6 mm into ventral retina. Over the same range, dendritic trees are 400–800 μm in equivalent diameter, but they are much smaller than the axonal arborizations, which extend up to and perhaps beyond 2 mm from the cell body. Interstitial cell bodies appropriate to PA1 cells have been identified in Nissl-stained, whole-mounted rabbit retinas. In the plane of the retina, these are comparable in area to smaller medium-size ganglion cells, but their very pale Nissl staining, high nuclear/cytoplasmic ratio, and absence of nucleolar staining are all characteristics of amacrine cells. Interstitial displacement of presumed PA1 cells is rare in the visual streak, and the frequency of interstitial cells reaches a peak between 1 and 2 mm ventral to the streak. Counts in Nissl-stained retinas and estimates from nearest neighbor analyses in these and in Golgi-impregnated retinas indicate a density of PA1 cells in the range of 15–16 cells/mm² at about 2 mm ventral to the streak, when an estimated 25% shrinkage of the material is taken into account. Dendritic field overlap, based upon this estimate, is calculated to be about fourfold, while a lower bound to estimates of the overlap of axonal arborizations is nearly an order of magnitude higher. Many similarities are noted in a qualitative and quantitative comparison of PA1 amacrine cells in rabbit and monkey retinas. In assessing the contribution of the structural organization of PA1 amacrine cells to their possible functional role(s), it is notable that their appearance conforms not to amacrine cells as commonly viewed, but to a more conventional model of neuronal dynamic polarization. PA1 amacrine cells have a dendritic tree of limited extent and an axonal arborization capable of influencing a wide retinal area. This structural arrangement could operate to transmit signals generated by focal stimuli impinging upon the dendritic tree to a wide surrounding area of retina where they could modulate these distal regions primarily. A role is suggested for PA1 cells in the retinal mechanisms of neural adaptation.