Central complex of the primate thalamus: A quantitative analysis of neuronal morphology

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Abstract
Neuronal morphology was analyzed in the central complex (centre median‐parafascicular complex) of macaques and humans. Cell bodies were described from Nissl material. Golgi‐impregnated dendritic arborizations were reconstructed from serial sections and digitized in three dimensions by computer‐assisted microscopy. The central complex was subdivided into three parts on the basis of cytoarchitectonic and hodological criteria: pars parafascicularis (medial), pars media (intermediate), and pars paralateralis (lateral). The mean cross‐sectional areas of cell bodies were identical (181 μm2) in the three parts in macaques. In humans they were larger in the pars parafascicularis (304 μm2) than in the other parts (248 and 240 μm2). Small local circuit neurons were found throughout the complex. Large projection neurons differed statistically in the three parts. In macaques, pars parafascicularis neurons had few dendritic stems and tips (3‐11) and a short total dendritic length (2,000 μm). Pars paralateralis neurons had more ramified (5–60) and longer (5,800 μm) dendrites. They bore numerous axonlike processes. Pars media neurons had intermediate characteristics (5–19; 2,400 μm). In humans, pars parafascicular neurons had similar topological characteristics (3–12) but longer dendrites (3,000 μm) than in the monkey. Pars paralateralis neurons had more branched (6–71) and longer (9,000 μm) dendrites, with more numerous axonlike processes. Pars media neurons also had intermediate characteristics (4–25; 3,800 μm). The present study supports a tripartite subdivision of the primate central complex and demonstrates significant interspecies differences.