Granular layer collaterals of the unipolar brush cell axon display rosette-like excrescences. A Golgi study in the rat cerebellar cortex.

The unipolar brush cell (UBC) is a medium-sized neuron located in the granular layer of the cerebellar cortex and morphologically characterized by a single dendritic shaft terminated by a brush-like tuft [Mugnaini and Floris, J. Comp. Neurol., in press]. This neuronal class is mostly encountered in the vestibular part of the cerebellum, although also present in the rest of the vermal folia. The UBC axon, as seen in Golgi impregnated material, has a tortuous course throughout the granular layer and gives off number of collaterals of variable diameter. Some of the axon collaterals enter the white matter only for a short distance, after which they ascend into the granular layer. Astonishingly, some of the thick granular collaterals of these axons display characteristic expansions resembling the rosette excrescences seen on mossy fibers. In our series these rosettes are mostly of the simple type [Palay, S. and Chan-Palay, V., Cerebellar Cortex. Cytology and Organization, Springer, Berlin, 1974], i.e. short swellings with simple globular contours. The exact projection targets and functional effect of the UBC axon and granular rosettes have still to be demonstrated.