Normal and regenerating optic fibers in goldfish tectum: HRP-EM evidence for rapid synaptogenesis and optic fiber-fiber affinity

Abstract

The distribution of normal and regenerating retinal fibers and synapses was studied on tectum in goldfish by light (LM) and electron microscopy (EM). Since labeling of the early regenerating fibers was previously reported to be difficult, a new 'cold-fill' HRP labeling protocol was developed, which labeled regenerating optic fibers and terminals on tectum as early as 14 days after nerve crush when they first arrive on tectum. In order to characterize the laminar distribution of optic afferents in normal fish and in fish regenerating for 14-240 days, EM photomontages of areas 14 microns wide by 160 microns deep through the HRP-labeled primary optic innervation layer (S-SO-SFGS) were constructed. The time points in regeneration that were examined spanned the period in which others have shown that an initially diffuse retinotopic map becomes spatially restricted. At the LM level regenerating optic fibers were restricted to the optic lamina. They reinnervated tectum in an anterior to posterior sequence as previously seen with autoradiography. In addition, at 14 days, some "pioneer" optic fascicles were found to have already grown to posterior tectum where they gave rise to branches with boutonlike terminations and growth-cone-like processes. Form the ultrastructural analysis it was clear that optic fibers and terminals observed strict laminar boundaries as they partitioned themselves in the optic laminae (S, SO and SFGS) in both normal and regenerating fish. The behavior of optic fibers was lamina specific with respect to synapse formation and the orientation of fiber outgrowth. As early as 14 days regeneration, optic fibers made synapses onto the four types of postsynaptic profiles observed in normal fish. Numerous optic terminals were labeled at 14 days, and there appeared to be no waiting period between fiber ingrowth to the SO and synapse formation in the S and SFGS. At 14-60 days, atypical synaptic contacts which appear to be nascent synapses were made by labeled optic fibers in fascicles and by growth-cone-like processes. By 21-30 days, the density of optic terminals was high and there were many more fasciculated optic fibers in the SFGS than normal as late as 350 days. These findings suggest that optic fiber lamination is highly constrained by tectal cues, that fibers rapidly regenerate many synaptic terminals before retinotopic map refinement is complete, and that fibers have a strong affinity for each other.