Three-dimensional morphology of cerebellar climbing fibers. A study by means of confocal laser scanning microscopy and scanning electron microscopy

The intracortical pathway of cerebellar climbing fibers have been traced by means of scanning electron microscpy (SEM?) and confocal laser scanning mi croscopy (CLSM) to study the degree of lateral collateralization of these f ibers in the granular Purkinje cell and molecular layers. Samples of teleos t fish were processed for conventional and freeze-fracture SEM. Samples of hamster cerebellum were examined by means of CLSM using FM4-64 as an intrac ellular stain. High resolution in lens SEM of primate cerebellar cortex was carried out using chromium coating. At scanning electron and confocal lase r microscopy levels, the climbing fibers appeared at the white matter and g ranular layer as fine fibers with a typical arborescence or crossing-over b ranching pattern, whereas the mossy fibers exhibited a characteristic dicho tomous bifurcation. At the granular layer, the parent climbing fibers and t heir tendrils collaterals appeared to be surrounding granule and Golgi cell s. At the interface between granule and Purkinje cell layers, the climbing fibers were observed giving off three types of collateral processes: those remaining in the granular layer, others approaching the Purkinje cell bodie s, and a third type ascending directly to the molecular layer. At this laye r, retrograde collaterals were seen descending to the granular layer. By fi eld emission high-resolution SEM of primate cerebellar cortex, the climbing fiber terminal collaterals were appreciated ending by means of round synap tic knobs upon the spines of secondary and tertiary Purkinje cell dendrites .