A Golgi study of goldfish optic tectum

A study of goldfish optic tectum was performed with rapid-Golgi, Golgi-Kopsch and a modified Golgi-Cox impregnation which proved quite suitable to impregnate cells in the middle tectal layers and to study more closely axonal properties. Fifteen cell types are distinguished, based upon the position of dendritic trees and axonal properties. Two cell types are found with dendrites in the marginal layer: type I with an axon terminating in the central gray layer and type II without an impregnated axon. Three cell types (III, IV and V) have dendrites in a single, specific tectal layer and an axon terminating within the tectum. Five cell types (VI-X) have dendrites in two horizontal planes. Two of them have myelinated axons leaving the tectum, whereas the axons of the remaining three types project to different tectal layers. While these first ten cell types have dendrites almost exclusively in the superficial half of the tectum, the remaining five types have dendrites in deeper layers too. This especially holds for the most conspicuous tectal cells (types XII and XIII), which have dendritic trees branching at three or more horizontal levels and a myelinated axon leaving the tectum, with sometimes a very peculiar course (XIII1). Also type XI has three or more dendritic trees, but its axon was not found. The numerous cells with cell bodies in the deepest tectal layer (type XIV) have dendrites and axonal terminations anywhere in the tectum, except in the most superficial and the deepest layer. However, most dendrites occur in the optic layers, whereas the axons, always originating from the dendritic shaft in the superficial tectal half, generally terminate in the middle tectal layers. Type XV cells have their soma in the deepest tectal layer as well, but their dendrites do not reach the optic layers. Per tectal lobe the following numbers are estimated: type I : 5,000-20,000 neurons; Type III : 2,500-10,000; types IV--XIII : each 500-2,000 and type XIV : 1,000,000-2,000,000. The total number of myelinated tectal efferents is estimated at 2,000-8,000. Comparison with other Golgi studies in teleosts leads to the conclusion that the tecta of these species of fish are basically similar.