On the development of the cerebellum of the trout, Salmo gairdneri. IV. Development of the pattern of connectivity.

Synaptogenesis has been studied in the corpus cerebelli of the trout Salmo gairdneri, Richardson, 1836. The first synapses are observed in hatchlings and occur between parallel fibres and the shafts of Purkinje dendrites. Subsequently the axosomatic synapses of Purkinje axon collaterals on the neurons of the ganglionic layer appear, and finally the synapses made by climbing fibres and mossy fibres, and by stellate cell axons develop. Young synapses in the cerebellum of the trout resemble the mature structures so closely that the criteria for the identification of the latter can also be applied to the former. The number of parallel fibre synapses and of Purkinje axon collateral synapses increases considerably during development. Eurydendroid cells, the axons of which leave the cerebellum, receive an abundance of Purkinje axon collaterals on their somata and main dendritic trunks. Mossy fibre synapses are numerous in the granular layer. Climbing fibre contacts and synapses of stellate cell axons, both with Purkinje cells, are found occasionally. The following pattern of connectivity is proposed. The main input-output system is formed by the mossy fibres, the granule cells, the Purkinje cells and the eurydendroid cells. Additional pathways are formed by (1) the mossy fibres, granule cells and eurydendroid cells, and (2) the climbing fibres, Purkinje cells and eurydendroid cells. The afferent-efferent systems, mentioned above, are influenced by a number of internuncial elements: (1) The Golgi cells receive their input from the parallel fibres and contact with their axon collaterals the dendrites of granule cells. (2) Axon collaterals of Purkinje cells are in synaptic relation with Golgi cells. (3) Axon collaterals of Purkinje cells impinge upon the somata and main dendrites of other Purkinje cells. (4) Stellate cells, which derive their input from the parallel fibres, synapse with dendrites and somata of Purkinje cells. The possible functional roles of all of these neuronal elements are discussed.