The Cell Morphology and Ultrastructure of the Nucleus Rotundus in Zebra Finches
The nucleus rotundus (Rt) and ectostriatum (Ec) are the thalamic and telencephalic components, respectively, of the avian tectofugal visual pathway. Previous studies have shown that Rt can be subdivided into at least three hodologically and physiologically distinct areas: anterior, central, and posterior subdivisions. However, the morphology and fine structure of these rotundal subdivisions are not well understood. The present study examined both the morphological and ultrastructural properties of neurons within these Rt subdivisions in zebra finches. First, retrograde tracers were deposited into subdivisions of Ec to visualize cell morphology of Rt neurons that projected to Ec. By examining these retrogradely-labeled neurons, we studied whether there were differences in cell morphology among the three subdivisions. Second, the fine structure of Rt neurons was surveyed using transmission electron microscopy. The first study found that neurons in the anterior and central subdivisions tend to have large-sized neurons, whereas those in the posterior Rt tend to have significantly smaller and rounded neurons. The second study found that synaptic glomeruli were more prevalent in sections taken from the central and posterior Rt than from the anterior Rt. These findings demonstrate that subdivisions of Rt differ in terms of cell morphology and fine structure. Our results, in addition to the hodological and physiological characteristics established in previous studies, suggest that potential parallel processing in Rt subdivisions is reminiscent of the anatomically and functionally distinct modules found in the mammalian extrastriate cortex.