The Purkinje neuron: II. Electron microscopic analysis of the mature Purkinje neuron in organotypic culture.

Purkinje neurons of organotypic cultures were investigated electron microscopically following their analysis with the Golgi technique. The purpose of this study was to critically examine the issue of synaptic specificity in CNS cultures. The unique finding was the synaptic cluster, a terminal which engulfs many Purkinje spines. In the neuropil and on the major dendrites, this synaptic arrangement was interpreted to be a hypertrophic parallel fiber, representing a type of synaptic modulation. The terminals on the somatic spines are also in the form of clusters; some or all of these spines were thought to have developed to form synapses with the climbing fiber. In the absence of this afferent, the parallel fiber--a competing system--takes over the site. This represents a form of synaptic plasticity in these cultures. The inhibitory synaptic relationships were maintained on the soma and dendrites, but it was found that the basket synapses did not quantitatively encase the soma as is seen in the intact animal. Mossy-type terminals were found occasionally synapsing with Purkinje dendritic spines, as has been seen in agranular cerebellum in animals. These mossy terminals are presently thought to originate from the deep cerebellar nuclei within these cultures. Synaptic errors were rarely encountered. It is concluded that this preparation develops in accordance with established neurobiological principles, that the Purkinje neuron reaches a mature state in culture, and that this model has a sound anatomical basis for further experimental work.