Synapses of biplexiform ganglion cells in the outer plexiform layer of the retina in Xenopus laevis.

Following retrograde transport of horseradish peroxidase from the optic nerve to the retina in Xenopus laevis, biplexiform ganglion cell were revealed. Their synaptic association with the other elements of the outer plexiform layer (OPL) were studied in serial ultrathin sections. The dendrites in the OPL of two regrogradely labelled biplexiform ganglion cells (GC1 and GC2) were completely reconstructed from the sections. Both cells had similar dendritic field sizes (0.026 and 0.025 m2, respectively) in the OPL, and received a few dozens of synapses (GC1: 39, GC2: 56) synapses. None of the ganglion cell dendrites were found in presynaptic position. Of the synapses, 10 derived from rods, 22 from cones and 7 from horizontal cells to GC1 and 13 from rods, 32 from cones and 10 from horizontal cells to GC2. Synapses were seen neither between interplexiform cell axons and biplexiform ganglion cell dendrites nor bipolar and biplexiform ganglion cell dendrites in the OPL. These observations indicate that the dendrites of biplexiform ganglion cells preferentially but not exclusively receive synapses from photoreceptor cells in the OPL.