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This aim of this study was to re-evaluate the types of neurons present in pars compacta of the substantia nigra in the rhesus monkey, and then to determine the effects of aging on the morphology of both the neurons and neuroglial cells. The substantia nigra was therefore examined in Golgi impregnated material, in sections labeled with antibodies to tyrosine hydroxylase and to GABA, and in tissue embedded in plastic for light and electron microscopy. Three types of neurons were encountered: (1) large multipolar neurons with prominent Nissl bodies, (2) bipolar neurons that are medium sized and spindle-shaped, with Nissl bodies confined to the poles of the deeply indented nuclear envelopes, and (3) small multipolar cells with sparse Nissl substance. The large and medium neurons are believed to be dopaminergic and the small neurons GABA-ergic. With age, all of the neurons accumulate lipofuscin, especially the small multipolar neurons. In addition, Marinesco bodies appear within the nuclei of the large multipolar neurons. The dendrites are most severely affected by age; many of them lose organelles and their cytoplasm can become filled with vacuoles, membranous whorls, and dense bodies. In old monkeys many of the astrocytes have inclusions and in the neuropil there is a striking increase in the number of astrocytic processes. Also large spheroids appear. These appear to be derived from astrocytes and they have a core of dense granular material surrounded by a paler peripheral zone of cytoplasm from which processes can extend. Most of the oligodendrocytes have dense inclusions in their cytoplasm and many of the myelin sheaths break down. Microglial cells can become enormously swollen by phagocytosed material. Although both neurons and neuroglial cells are affected by age, no entities that could be construed to be dying neurons were encountered.
Siddiqi et al. (Wed,) studied this question.