Synaptic contacts of a two-cone flat bipolar cell in a primate retina.

The Golgi-stained dendrites of a two-cone bipolar cell of a vervet monkey were serially sectioned for electron microscopy and shown to make basal synapses with two neighboring cones. The synapses were all on the cone pedicle membrane adjacent to the bipolar processes invaginating to form the triads. This is the characteristic position for flat midget bipolar cells. The numbers of triads in nine adjacent cone pedicles were not significantly different from those in the cones contacted by the bipolar. Based on this measure the two-cone bipolar does not contact a special population of cones. The advantages and disadvantages of using vertical or horizontal sections for the determination by electron microscopy of the connectivity of the dendrites of Golgi-stained flat bipolar cells are discussed.