The type 1 polyaxonal (PA1) cell is a distinct type of axon-bearing amacrine cell whose soma commonly occupies an interstitial position in the inner plexiform layer; the proximal branches of the sparse dendritic tree produce 1-4 axon-like processes, which form an extensive axonal arbor that is concentric with the smaller dendritic tree (Dacey, 1989; Famiglietti, 1992a,b). In this study, intracellular injections of Neurobiotin have revealed the complete dendritic and axonal morphology of the PA1

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... cells in the rabbit retina, as well as labeling the local array of PA1 cells through homologous tracer coupling. The dendritic-field area of the PA1 cells increased from a minimum of 0.15 mm 2 (0.44-mm equivalent diameter) on the visual streak to a maximum of 0.67 mm 2 (0.92-mm diameter) in the far periphery; the axonal-field area also showed a 3-fold variation across the retina, ranging from 3.1 mm 2 (2.0-mm diameter) to 10.2 mm 2 (3.6-mm diameter). The increase in dendritic-and axonal-field size was accompanied by a reduction in cell density, from 60 cells0mm 2 in the visual streak to 20 cells0mm 2 in the far periphery, so that the PA1 cells showed a 12 times overlap of their dendritic fields across the retina and a 200-300 times overlap of their axonal fields. Consequently, the axonal plexus was much denser than the dendritic plexus, with each square millimeter of retina containing ;100 mm of dendrites and ;1000 mm of axonal processes. The strong homologous tracer coupling revealed that ;45% of the PA1 somata were located in the inner nuclear layer, ;50% in the inner plexiform layer, and ;5% in the ganglion cell layer. In addition, the Neurobiotin-injected PA1 cells sometimes showed clear heterologous tracer coupling to a regular array of small ganglion cells, which were present at half the density of the PA1 cells. The PA1 cells were also shown to contain elevated levels of g-aminobutyric acid (GABA), like other axon-bearing amacrine cells.