An ultrastructural study of interplexiform cell synapses in the human retina.

Using serial sections and electron microscopy, we have found several morphological types of synapses within the outer plexiform layer (OPL) of the human retina. The most conspicuous of these is described in this paper. They have a unique morphology and form synapses with rod and cone bipolar cells in the OPL and onto bipolar and amacrine cell bodies in the inner nuclear layer (INL). Because they occur in processes that extend across the INL, we believe these synapses are made by interplexiform cells (IPCs). These same processes also contact cone pedicles with specialized cell junctions like those made between cones and flat bipolars. These junctions have densification of both cell membranes and widening of the extracellular cleft, but no accumulation of synaptic vesicles. Similar-appearing processes in the inner plexiform layer are thought to belong to IPCs but their contacts were less completely identified. Possible circuitry for these IPCs is described and the possibility that there are different classes of IPCs in the human retina is discussed. The OPL forms in the posterior retina during the tenth fetal week. Our observations suggest that different types of synapses including those of the IPCs are present in this layer from the time of its first appearance.