Abstract
The cytoarchitecture and neuronal morphology of the torus semicircularis in the redโeared turtle, Chrysemys scripta elegans, were examined in Nisslโstained and Golgiโimpregnated material. The torus semicircularis begins in the caudodorsal mesencephalon and extends rostrally and laterally to end ventrally to the tectal ventricle. The torus semicircularis consists of a central nucleus and a laminar nucleus, which is interposed between the central nucleus and the ventricle.
The central nucleus can be divided into two regions, a small, largeโcelled area, located dorsally, and a larger area of small spherical (6โ17 ฮผm), large spherical (18โ25 ฮผm), triangular (15โ27 ฮผm) and fusiform (10โ26 ฮผm) neurons. The small spherical cells have two dendritic patterns: โradiateโ and โsingle.โ The radiate pattern has a dorsoventral orientation, several secondary branches and few dendritic spines. These cells are usually located in the center of the central nucleus. The single pattern is oriented mediolaterally. This cell type is most often observed at the periphery of the central nucleus. These neurons have few secondary branches and dendritic spines. The large spherical neurons display two dendritic orientations: dorsoventral and mediolateral. All dendritic trees have numerous secondary branches and few dendritic spines. The triangular neurons exhibit primary dendrites projecting from the corners of the somata and have few secondary branches and dendritic spines.
The fusiform neurons have a majority of their dendrites oriented mediolaterally, few secondary branches and a small number of dendritic spines.
The laminar nucleus consists of several layers and three cell types: ovoid (9โ15 ฮผm), triangular (20โ40 ฮผm), and fusiform (20โ40 ฮผm). All neurons have few secondary dendritic branches and few dendritic spines. The dendrites of many neurons course perpendicularly to the long axis of the brainstem and encapsulate the central nucleus. Some ovoid and fusiform neurons display dendrites that enter the central nucleus.