Organization of nucleus rotundus, a tectofugal thalamic nucleus in turtles. II. Ultrastructural analyses

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Abstract
Nucleus rotundus is a large, tectorecipient nucleus in the dorsal thalamus of the pond turtles Pseudemys scripta and Chrysemys picta. Rotundal neurons form a single, morphologically homogeneous population (Rainey, '79) that projects to the dorsal ventricular ridge in the telencephalon. The present paper examines the morphology of and the distribution of synapses upon rotundal neurons. Astrocytes, oligodendrocytes, and neurons can be identified in both 1‐μm sections stained with toluidine blue and electron micrographs of nucleus rotundus. Rotundal neurons contain euchromatic nuclei and the usual complement of mitochondria, rough endoplasmic reticulum, and free ribosomes in their cytoplasm. They are morphologically homogeneous. Two types of terminal boutons can be defined in rotundus. RA boutons contain round synaptic vesicles and form asymmetric synaptic junctions with rotundal dendrites. FS boutons contain small, flattened or pleomorphic vesicles and form nearly symmetric synaptic junctions with rotundal dendrites and somata. RA boutons occasionally form clusters of contiguous boutons that are presynaptic to one or more thin, central profiles. These profiles are probably the dendritic appendages observed on peripheral dendrites in Golgi material. The distribution of RA and FS boutons along dendrites was investigated by a two‐step procedure. First, rotundal neurons were retrogradely solid‐filled with horseradish peroxidase reaction product. Dendritic diameters were measured at 20‐μm intervals along dendritic shafts to produce a plot of dendritic diameter as a function of distance from the soma. Second, the percentage of membrane on dendritic profiles of different diameters that was contacted by RA and FS terminals was determined from electron micrographs. Comparison of the two plots indicates that both bouton types are distributed along the full extent of the dendritic tree, but RA boutons are much more common on the distal two‐thirds of rotundal dendrites. This analysis suggests that rotundal neurons form a single population of cells that are morphologically homogeneous and project to the forebrain. There is no indication of interaction between neurons in nucleus rotundus, either via axonal collaterals or presynaptic dendrites. Boutons are distributed on rotundal neurons such that FS boutons are prevalent on the somata and most proximal segments of the dendritic shafts, while RA boutons are most common on the more distal dendritic shafts. RA boutons also contribute to synaptic clusters that may center around complex dendritic appendages.