A POPULATION OF WIDE-FIELD BIPOLAR CELLS IN THE RABBITS RETINA

We have stained an unusual population of retinal bipolar cells. When t he low molecular weight tracer biocytin was injected into the vitreous body of rabbits, it subsequently accumulated in the somata and proces ses of a population of wide-field bipolar cells. The cells have 2-4 pr imary dendrites. Their dendritic arbors span a field 50 to 200 mu m in diameter. The axonal arbors are sparse and often highly asymmetric. T he longest dimension of the axonal arbor ranges from 100 to 300 mu m. The cells are moderately evenly spaced. They make up less than 1% of t he total population of bipolar cells in the rabbit retina. With the wh ole population stained, regularities in the spatial arrangement of nea rby cells can be recognized. Their dendrites often run to a common poi nt, where they have the appearance of making contact with each other. A similar arrangement is seen for the cells' axonal arbors, so that th e whole population is spatially linked in both the outer retina and th e inner. The exact nature of the points of conjunction cannot be learn ed from light microscopy. One possibility is that the processes run to gether because they contact a common target. If so, the target structu res (one in the outer retina and one in the inner) must be sparse. An alternative is that the points of conjunction represent synapses or ga p junctions among wide-field bipolars of this type. (C) 1995 Wiley-Lis s, Inc.