A diffuse, invaginating cone bipolar cell in primate retina

A bipolar cell type forming invaginating contacts with many cones was found by light and electron microscopy of Golgi preparations of the rhesus monkey retina. This diffuse, invaginating cone bipolar cell resembles, superficially, rod ("mop") bipolars, and so may correspond to Polyak's "brush" bipolar. However, it differs from rod bipolars in that its dendrites are finer and they end in a single stratum containing cone pedicles in the outer plexiform layer (OPL). In the inner plexiform layer (IPL) its axon terminal is located sclerad (S4) to those of rod bipolars (S5), is thinner, and also more branched, and wider in span than rod bipolars axon terminals. Resectioning of Golgi-impregnated diffuse, invaginating cone bipolars to study their connections in the OPL shows that their dendrites invaginate as many as seven cone pedicles, and terminate as central elements at the ribbon synaptic complex. Thus, the primate retina has multiple (diffuse) and single (midget) cone-contacting bipolar cell pathways in both invaginating as well as flat varieties.