Tectal Fiber Connections in a Non-Teleost Actinopterygian Fish, the Sturgeon Acipenser

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Abstract
Tectal fiber connections were studied in members of an early branch of the actinopterygian lineage, the sturgeons Acipenser transmontanus and A. schrenkii, by means of biocytin, HRP, biotinylated dextran amine, and DiI tract tracing methods. The aim of this study is to elucidate the visual pathway via the optic tectum to the thalamus as a part of a series of studies on the visual pathways in sturgeons. After biocytin or biotinylated dextran amine injections to the optic tectum terminals are found bilaterally in the medial and lateral portions of both the dorsal thalamus and ventral thalamus. Ipsilateral projections are much more abundant. Tectal recipient areas in the thalamus overlap in part with the retinal recipient areas. After HRP or DiI injections to the dorsal or ventral thalamus, tectal neurons projecting to the thalamus were labeled in the ipsilateral or bilateral stratum periventriculare. Dendritic morphology of tectothalamic neurons suggests that they receive direct retinal input. These results suggest that visual information passes through the tectum to the thalamic areas which also receive direct retinal projections. In this regard, the visual system of Acipenser resembles that of chondrichthyans (sharks). Other fiber connections of the tectum are also described, which have not previously been studied by tracer methods in a sturgeon.