Scanning electron microscope recognition of intracortical climbing fiber pathways in the cerebellar cortex

The cerebellar cortex of adult mouse, rat, fish and human has been explored by SEM in order to identify and trace the climbing fiber intracortical pathways and their synaptic relationships. Samples were processed by the freeze-fracture method for SEM using medium and rapid freezing rates. Climbing fibers appeared at the white matter as fine, wavy axonal processes with a characteristic cross-over bifurcation pattern. These features allowed us to differentiate them from the mossy fibers, which are distinguished as thick fibers with a straight course and a dichotomous arborization. In their course through the granular layer the climbing fibers exhibited glomerular and tendril collaterals. The glomerular collaterals appeared covered by the dendritic tips of several granule cells forming thin triangular climbing glomeruli. The tendril collaterals are seen as highly contoured axonal ramifications establishing axo-dendritic connections with the granule cell dendritic digits. Climbing fibers were also observed forming a pericellular plexus around the Golgi cell body. They cross the granular layer and reach the Purkinje cell perikaryon contributing to the formation of infraganglionic plexus and pericellular nest. The climbing fibers showed a typical sagittal compartmentalization and a contoured spiral or zigzag pathway in the course ascending along the Purkinje dendritic branchlets. Fine terminal climbing fiber tendrils synapsing on the tips of Purkinje dendritic spines were observed. The freeze-fracture method for SEM permits us to estimate with a more reliable degree of certainty the amount of branching or lateral collateralization of climbing fibers in the granular and molecular layers.