Cytoarchitecture of the tectum mesencephali in two types of siluroid teleosts.

The cytoarchitecture of the tectum mesencephali in the siluroid teleosts Bagrus and Ictalurus was studied by means of the Golgi method. These animals are known to have a restricted visual system and it seemed important to study whether this fact would affect the existence or the shape of the main neuron types which have been described for highly visual teleosts. It had been shown for a variety of teleosts that the retinotectal axons and terminals occupy almost exclusively the stratum opticum and the stratum fibrosum et griseum superficiale. The thickness of these strata in Bagrus and Ictalurus was found to be reduced. However, the main types of especially the vertically oriented neurons, such as the pyramidal, fusiform, large pyriform and periventricular neurons which have been described for highly visual species of teleosts, were also found in Bagrus and Ictalurus. Although their shape was somewhat distorted, these neurons, nevertheless, showed processes distributed to the same tectal layers as in highly visual teleosts and are accessible to horizontally distributed fiber systems such as marginal, telencephalotectal and commissural tectal fibers, as well as the retinotectal fibers. Nonvisual inputs appear to be considerably involved in the maintenance of the main neuron types in the siluroid tectum. For example, the pyramidal neuron's apical dendritic tree, which receives the excitatory input from the marginal fibers, is as well developed in siluroids as in highly visual teleosts.