Retinal structure in the smooth dogfish Mustelus canis: electron microscopy of serially sectioned bipolar cell synaptic terminals

Abstract Portions of axons of bipolar cells in the retina of the smooth dogfish Mustelus canis were sectioned serially and examined by electron microscopy. The studied axons generally could be related to a bipolar cell sub‐type identified by light microscopy. Bipolar cell axons make ribbon synapses onto amacrine processes and ganglion cell dendrites, and onto ganglion cell perikarya. Bipolar cell ribbon synaptic complexes varied as to the number of post‐synaptic processes (1–3) and the orientation of the ribbon with respect to the post‐synaptic membrane. Amacrine processes made numerous conventional synapses onto bipolar cell axons, but reciprocal synapses between amacrine and bipolar cells constituted only 3–25% of all synapses observed. The number of ribbon synapses per unit area of bipolar cell axon membrane differed little among bipolar cell sub‐classes. However, the density of amacrine cell conventional synapses was markedly lower for thin, horizontally‐oriented bipolar cell axons than for axons of other bipolar cell types. Gap junctions were noted between bipolar cell axons of the same sub‐type. They are structurally similar to gap junctions between horizontal cells in Mustelus retina and to those found elsewhere in the nervous system.