Electron microscopic identification of cerebellopontine axon terminals in the opossum

Following unilateral cerebellar nuclear ablation or transection of the brachium conjunctivum, degenerating axon terminals were identified within the pontine nuclei of adult opossums. Most frequently observed was a category of large boutons (1.5-7.5 microns) exhibiting an early filamentous reaction (1-5 days survival) and later becoming electron dense and shrunken (9-12 days survival) while being engulfed by phagocytic elements. Such boutons characteristically were found nestled within a cluster of spine-like projections taking origin from somata as well as proximal and intermediate dendrites. A smaller variety of dark degenerating boutons (0.5-2.0 micron) was observed after survival periods of intermediate length (6-10 days) and although there was some overlap in size with the smallest filamentous boutons, the majority (71%) were clearly less than 1.5 micron in their greatest dimension. The small dark boutons formed synaptic contacts only with the shafts of intermediate and distal dendrites rather than the claw-like dendritic complex apposed to the large filamentous degenerating boutons. Because of this difference in postsynaptic locus and their small size it was suggested that such boutons might represent the terminals of a second population of cerebello-pontine axons. Such observations lead to the hypothesis that the large filamentous endings contacting the distinctive claw-like somal or dendritic projections from axons of relatively large cerebellar nuclear neurons which also project rostrally to the red nucleus and thalamus where they form similar boutons and synaptic complexes. On the other hand, the small dark boutons may have arisen from small projection-type cerebellar nuclear neurons, the majority of whose axons project caudally to the inferior olive after contributing a relativley small number of collateral branches to the pontine nuclei.