The optic tectum of gymnotiform teleosts Eigenmannia virescens and Apteronotus leptorhynchus: a Golgi study

Golgi, Nissl, Bielschowsky and cholinesterase techniques have been used to analyze the optic tectum of the weakly electric teleost fish Eigenmannia virescens and Apteronotus leptorhynchus. Six layers are readily distinguished: a fairly thick stratum marginale, a narrow stratum opticum and stratum fibrosum et griseum superficiale, a well-developed stratum griseum centrale, a stratum album centrale and a compact stratum periventriculare. Fifty-six neuronal types are present. In regard to comparative aspects of tectal organization, it became apparent that although most neuronal types are similar to those reported in other teleostean fish, there are certain obvious differences such as: pyramidal cell somata not confined to stratum fibrosum et griseum superficiale, but also clustered in the adjacent stratum opticum, presenting stratified or diffuse basilar dendritic arbors; and a change from vertical to oblique and almost horizontal neuronal orientation in the ventral and caudal tectum. The presence of pyramidal cells with aligned and misaligned apical and basal dendritic fields. A cell of stratum griseum centrale with an ascending axon to stratum opticum. A special projection type of fusiform cell of stratum griseum centrale, with an efferent axon of somatic origin. A cell rich stratum griseum centrale, with a wider variety of multipolar and bipolar cell population than reported in other teleosts. Fourteen types of pyriform cells are present, four of which are efferent. Our observations are suggestive of regional differences in regard to the caudalmost tectum in Apteronotus: presumably this is related to the extremely sparse retinal input to this part of the tectum. A close functional correlation has been found between some multipolar and pyriform cells identified in our material with similar cells reported by Rose and Heiligenberg as multisensory cells, following recordings and horseradish peroxidase fillings of these cells. Based on the observation of patchy torus semicircularis input to stratum fibrosum et griseum superficiale, disjunct from the retinal input to this layer, it is proposed that perhaps this arrangement is the result of competition for synaptic targets during development.