Organization of nucleus rotundus, a tectofugal thalamic nucleus in turtles. I. Nissl and Golgi analyses.

This study consists of a detailed cytoarchitectonic and Golgi analysis of a major tectofugal thalamic nucleus in the red-eared turtle, Pseudemys scripta elegans. Neurons in nucleus rotundus have a unimodal soma size distribution and a common dendritic branching pattern. They have long dendrites which undergo sparse, dichotomous branchings and contribute to dendritic fields that cover a third to half the dimensions of the nucleus. Spicules, 1-2 mu long, and complex appendages, 5-20 mu long, are found with low density on many dendrites in Golgi-Kopsch material. A few cells have beaded dendritic processes. Three cytoarchitectural regions can be differentiated in nucleus rotundus: a shell, a cell-poor region and a core. The shell is a monolayer of somata forming the peripheral boundary of most of the nucleus. The cell-poor region forms a thin zone concentric with and internal to the shell. Shell cells send some of their dendrites concentrically within this zone and others radially into the core region. Core neurons are dispersed within the neuropil of the nucleus and usually have spherical dendritic fields. However, peripheral core neurons have asymmetrical fields, so their dendrites do not extend beyond the shell. Caudomedial and central subregions of the core can be defined on the basis of neuronal density and cytology. Somata in the caudomedial area of the core are densely packed and have slightly darker staining cytoplasm than those in the central subregion. However, their dendrites are similar to those of the central core neurons. There is extensive dendritic overlap between the two subregions.