The synaptic ultrastructure in the outer plexiform layer of the catfish retina: a three-dimensional study with HVEM and conventional EM of Golgi-impregnated bipolar and horizontal cells.

Synaptic structures between receptors and horizontal and bipolar cells in the outer plexiform layer (OPL) of Golgi-impregnated catfish retina were examined by conventional electron microscopy of serial ultrathin sections and by high-voltage electron microscopy (HVEM) of thick sections. Cone terminals contained multiple synaptic ribbons and rod terminals contained single synaptic ribbons. This observation was used to identify these two types of photoreceptors. The cone horizontal cell, located in the most distal part of the inner nuclear layer (INL), invaginated only cone terminals, whereas the rod horizontal cell, located in the proximal part of the INL, invaginated only rod terminals. Both lateral elements of the triad in the rod terminal originated from a single rod horizontal cell whereas the same structures in the cone terminal were often derived from several cone horizontal cells. Golgi-impregnated catfish bipolar cells were classified into two types based on the differences in their axonal arborization as described by Famiglietti et al. ('77). Axonal endings of type a bipolar cells were located in the distal part, sublamina a, of the inner plexiform layer (IPL), and axonal endings of type b cells were located in the proximal part, sublamina b, of the IPL. Dendrites from type a bipolar cells made direct contact with the synaptic ribbons in both rod and cone terminals whereas those from type b cells made indirect contact with the ribbons in both rod and cone terminals, but rare direct contact with the ribbon in rod terminals were also seen. In addition, bipolar cells made basal junctions or superficial contacts in both rod and cone terminals. The "lateral" processes of bipolar cells invaginating rods penetrated between the rod terminal and rod horizontal cell processes, and made basal junctions with both rod terminals and rod horizontal cells. There was no definitive morphological feature that could be associated with sign-conserving and sign-inverting signal transmission.