The cytoarchitecture of the torus semicircularis in the teleost Barbus meridionalis

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Abstract
The torus semicircularis of Barbus meridionalis is composed of two nuclei, the nucleus centralis and nucleus lateralis. Its cytoarchitecture was studied in sections stained by Nissl and Golgi-Colonnier techniques. In the nucleus lateralis two portions were identified: the ‘pars lateralis’ and the ‘pars medialis.’ Cytoarchitecturally, both portions are identical. They exhibit a layered structure in which there is an alternation of cell-poor and cell-rich laminae designated as: (1) the subependymal layer; (2) the layer of small cells; (3) the fibrillar layer; and (4) the layer of disperse cells. The subependymal layer consists of fine fibers and some small rounded-ovoid cells whose dendritic prolongations course horizontally or ventrally. The second layer has small, densely-packed cells with rounded-ovoid and triangular somata and a main dendritic trunk that courses ventrally. The third layer contains dendritic fields of the cells of layer two and of cells from layer four. The fourth layer is composed of fusiform neurons with two dendritic trunks of equal thickness, rounded-ovoid neurons with one or two main dendritic trunks and multipolar triangular stellate neurons with equal dendritic trunks. The nucleus centralis comprises a fibrillar cortex with a structure identical to that of the subependymal layer. There is also a cellular region with the same cell types as those found in the nucleus lateralis. These two nuclei thus compose the torus semicircularis of the barbel. They exhibit the same cytological characteristics and both are differentiated by their cytoarchitectural and functional orders.