Access Type

Open Access Dissertation

Date of Award


Degree Type


Degree Name



Anatomy and Cell Biology

First Advisor

James C. Hazlett


The cytoarchitecture of the substantia nigra pars lateralis (SNl) of the opossum was examined by light and electron microscopic procedures. Nissl-stained, Golgi-impregnated, HRP-reacted and electron microscopic preparations were utilized in this study. SNl neurons were divided into three groups on the basis of cell size; small, medium-sized, and large neurons. Small cells had a nucleus with some clumped heterochromatin, a relatively organelle-free cytoplasm, and long, thin, sparcely branching dendrites. Electron microscopic examination of small cells confirmed the light microscopic observations. Medium-sized neurons possessed a relatively clear nucleus, clumps of Nissl substance, especially at a juxtanuclear position near the emergence of dendrites, and long, rarely branched dendrites that extended for great distances along the surface of and into the substance of the cerebral peduncle. Electron microscopic examination of the medium-sized neurons revealed the usual variety of organelles expected in sensorimotor neurons and confirmed the presence of the large accumulations of Nissl substance in the juxtanuclear position. In Golgi-impregnated material, two populations of medium-sized neurons were observed. The more numerous group possessed a moderate number of dendritic spines while the other was relatively aspinous. It was not possible to further separate these two subgroups. With the exception of size, the largest neurons appeared to be identical to the medium-size moderately spinous neurons. Electron microscopic examination revealed six different types of synapses in the opossum substantia nigra. Four of these synaptic types (A, B, C, and D) form asymmetric contacts on cell bodies, dendrites and spines, while the remaining types (E and F) form symmetric contacts on cell bodies and dendrites alone. Retrogradely labeled neurons were identified in SNl after injections of horseradish peroxidase (HRP) into the neostriatum, thalamus, and tectum. Labeled neurons could be placed into all three size categories. Electron microscopic examination of identified medium-sized nigrotectal neurons revealed that, with the exception of the label, these cells were indistinguishable from other neurons. This report provides evidence for an anatomical substrate which may underlie nigral integration of information processed through the basal ganglia.

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