One each of bipolar cell types DB2 and DB4, together with a flat and a
n invaginating midget bipolar cell, were taken from a Golgi-stained rh
esus macaque retina; then serially sectioned for EM examination of the
ir synapses with cone pedicles. The cone input to the dendrites of the
DB2 cell was exclusively at basal junctions; it had a characteristic
distribution. Fifty per cent of the basal synapses were with cone pedi
cle membrane immediately adjacent to the dendrite of a bipolar cell in
vaginating to end opposite the ribbon of a cone triad (this, therefore
, is called triad-associated). The remainder were one or more synapses
distant from the triad-associated position (and, therefore, non-triad
associated). The DB4 cell had both basal (predominantly in the triad-
associated position) and ribbon-related synaptic input. But the basal
to invaginating ratio differed from that of our previously published c
ell; 56% basal, 43% invaginating, as compared with 31% basal and 69% i
nvaginating. Like foveal IMB cells the synapses of the mid-peripheral
invaginating midget bipolar cell were exclusively invaginating; but we
re about 25% more numerous. The flat midget bipolar cell made exclusiv
ely basal synapses. These were 2.5 times more numerous than those of f
oveal flat midget bipolar cells, and 3.5 times the number of invaginat
ing midget bipolar synapses at equivalent eccentricity. The synapses b
etween cones and diffuse and midget bipolar cells are characteristic f
or each particular bipolar cell type, but the details depend on a cell
's distance from the fovea (eccentricity). A rather constant number of
cone pedicle synaptic ribbons 38.6+/-2.5 (n=60) was found across mid-
peripheral macaque and vervet monkey retinae. The smaller mean number
for vervet monkey, 27.4+/-3.5 (n=23), suggests there can also be gener
ic differences in synaptic detail at cone bipolar cell synapses.