Retinal terminals in the goldfish optic tectum: identification and characterization.

Retinal terminal profiles in the goldfish optic tectum were identified electron microscopically after (1) labeling with horseradish peroxidase and (2) in the early stages of degeneration in short-term eye enucleates. All labeled terminals shared certain common morphological characteristics which were identical to those of a population of terminals in normal tecta. Terminals of this type disappeared 30 days after enucleation of the contralateral eye. Retinal terminal presynaptic profiles were characterized by (1) round and oval synaptic vesicles; (2) mitochondria with irregular, randomly oriented cristae, large intracristal spaces, dilated membrane spaces, and primarily light matrices; (3) a wide range in profile area, 0.06-6.82 micrometers2; (4) large numbers of synaptic vesicles per profile area (168 +/- 33 synaptic vesicles per micrometers2; (5) asymmetric synapses; and (6) multiple synaptic contacts (1.46 +/- 0.73 per terminal profile). The postsynaptic elements included both dendritic and, less commonly, pleomorphic vesicle-containing profiles. The majority of postsynaptic dendritic profiles were small (0.01-0.40 micrometers2). Serial synaptic contacts were occasionally seen. The combination of vesicular and mitochondrial morphology (1 and 2 above) was necessary and sufficient to establish the retinal origin of a terminal, but use of such criteria would underestimate the number of retinotectal terminals by omitting those which did not have a mitochondrion in the plane of section. The number of such terminals was calculated from independent measurements, and the total number of retinal terminal profiles per area of neuropil was estimated.