CONE SYNAPSES OF A FLAT DIFFUSE CONE BIPOLAR CELL IN THE PRIMATE RETINA

A Golgi-stained flat diffuse cone bipolar cell from a vervet monkey's retina (Cercopithecus aethiops), contacting six cones, was serially se ctioned for electron microscopy (EM) to determine the types of synapse s it made with the cone pedicles. All the synapses were basal (flat) c ontacts. Their distribution and ultrastructural type were similar at e ach pedicle. Approximately half the synapses were definable as triad-a ssociated and the rest were elsewhere on the cone pedicle base. Their ultrastructure is the same regardless of those positions. About 25 syn apses were made with each cone. Thus this type (DB2 of Boycott & Wassl e, 1991) of flat diffuse cone bipolar cell is in contact with six cone s through about 150 synapses. At the eccentricity studied each cone pe dicle probably makes 90-100 basal synapses with between three and four DB2 bipolar cells. This is between two and three times the number tha t are made with all the types of invaginating bipolar cells. A brief r eview of cone photoreceptor synapses with bipolar cells shows that, fo r those so far examined in the primate retina, the dichotomy into two types of bipolar cell invaginating (ribbon-related), with axons ending in the b-layer of the inner plexiform layer (IPL) (hence presumptive On-bipolars) and flat (basal synapses), with axons ending in the a-lay er of the inner plexiform layer (hence presumptive Off-bipolars) is th e rule. But other vertebrate retinae, including that of the cat, also have bipolar cells which vary from this pattern.