Morphological study of cerebellar transplant cocultivated with cerebral cortical graft in the anterior eye chamber. II. Purkinje cells and molecular layer.

Morphological organization of Purkinje cells and of molecular layer of the cerebellar cortex cocultivated intraocularly with cerebral cortex for two months was studied. It was found, that while numerous spines on the main dendritic branches of Purkinje cells in single cerebellar grafts were "vacant" and non-synaptic, dendritic spines of thick Purkinje dendrites in double grafts were covered by large presynaptic bags. The resulting complex synaptic arrangement was strikingly similar although not identical to climbing terminals in normal ("in situ") cerebellar cortex. Three distinct types of large presynaptic climbing-fibre like terminals were distinguished: (a) bouton with dense matrix and small round synaptic vesicles, (b) with large round vesicles and (c) containing ovoid synaptic vesicles. The spines of the thin, presumably tertiary dendritic branches were contacted mostly by one parallel axon varicosity, or--as a contrast to normal conditions--by axon terminal, containing ovoid synaptic vesicles. Irrespective of the shape of synaptic vesicles in the presynaptic terminal, all spine-synapses were of asymmetric type; in contrast, synapses on the dendritic shafts were always symmetric. GABA-immunogold reaction has revealed the presence of this inhibitory transmitter in most axon terminals containing ovoid-pleomorphic vesicles within the molecular layer, including those resembling climbing fiber-terminals. This shows a plasticity of the Purkinje cell dendrites to receive non-specific, "foreign" axons in the absence of specific afferents. Also, the type of synaptic junctions, i.e. whether symmetric or asymmetric, is determined exclusively by the postsynaptic neuron and is independent of the transmitter content of the presynaptic terminal.