Golgi study of the primate substantia nigra. I. Quantitative morphology and typology of nigral neurons.

Neuronal morphology was analyzed in the pars compacta, reticulata, and lateralis of the substantia nigra of humans and macaques. Golgi-impregnated dendritic arborizations, reconstructed from serial sections, were described by using topological, metrical, and geometrical parameters measured in three dimensions. Morphological parameters were statistically analyzed. Cell bodies and axons were also described. The primate substantia nigra comprises few local circuit microneurons. It consists mainly of large projection neurons having large cell bodies and sparsely branched dendritic arborizations. In all subdivisions, "complex endings" and "thin processes" can be found on nigral dendrites. Axons of large neurons occasionally had initial collaterals that never form profuse arborizations. Pars reticulata neurons had a cell body surface of 520 micron2, 4 dendritic stems, and 13 dendritic tips. The total dendritic length (L) was 7,100 micron, the highest dendritic length (Lm) 1,200 micron, and the mean length of dendritic segments 320 micron. Pars lateralis neurons were similar except for their larger cell bodies (650 micron2) and longer dendritic segments (440 micron). Pars compacta neurons had larger cell bodies (860 micron2), thicker and more numerous (5 stems, 19 tips), and longer dendrites (L = 10,500 micron; Lm = 1,400 micron). Large neurons of monkeys had the same topological characteristics as human neurons but shorter dendrites. The overall shape of arborizations was highly variable and not characteristic in any subdivision. A hierarchical typology of nigral neurons is proposed, which comprises two neuronal species, the compacta and reticulata species, and a lateralis subspecies. Pallidal neurons (Yelnik et al., '84) belong to the reticulata species. The position of these species in relation to higher hierarchical levels is discussed.