The neuronal organization of cerebellar lobe C1 in the mormyrid fish gnathonemus petersii (teleostei)

Summary

The corpus cerebelli of mormyrid fishes is strongly developed and differentiated into four lobes: C1−C4. Although all of these lobes contain the characteristic cerebellar layers: granular, ganglionic and molecular, they show distinct architectonic differences. A previous study revealed that the ganglionic layer of C1, in addition to Purkinje elements contains conspicuous giant cells. In the present paper the results of a further analysis of C1 are reported. This analysis is based on serially sectioned brains of Gnathonemus petersii, stained according to Nissl, Bodian and Häggquist. Semi-thin sections were stained with p-phenylenediamine. Routine EM techniques were used to visualize synaptic relations. Mossy fibres and granule, Golgi, Purkinje and stellate cells are located characteristically throughout C1. It appeared that the giant cells of a previous study represent the largest elements of a population which has been termed now the eurydendroid cells. The average size of the latter is somewhat larger than that of the Purkinje cells, but both groups of cells show a considerable overlap in the size of their somata. Purkinje cells and eurydendroid cells are present throughout the ganglionic layer and both have a flattened, sagittally oriented, dendritic tree that extends into the molecular layer. Yet, the eurydendroid cells (EC) display the following characteristics which distinguish them from Purkinje cells (PC): (1) In EC the Nissl substance is dispersed diffusely throughout the soma, whereas in PC it tends to be concentrated around the nucleus, (2) The soma/nucleus ratio for EC is distinctly larger than for PC, (3) The dendritic trees of EC extend over a larger stretch of the molecular layer than those of PC, hence the term EC, (4) The dendrites of EC are more widely spaced and oriented less strictly parallel to each other than those of PC, (5) The dendrites of EC are somewhat irregular in outline and not covered with spines, in contrast to those of PC, (6) The axons of EC are oriented radially and join bundles of coarse fibres which leave the cerebellum whereas the axons of PC extend and ramify within the ganglionic layer, (7) The somata of EC, contrary to those of PC, are enveloped by a dense axonal plexus which forms numerous synaptic terminals on them. The numerical ratio of EC: PC was 1:5.5. The circuitry in C1 and the possible functional roles of its constituent neurons are discussed. It is pointed out that in this lobe the axons of PC impinge on EC and that the latter constitute its output system.