DEVELOPING MOSSY FIBER TERMINAL FIELDS IN THE RAT CEREBELLAR CORTEX MAY SEGREGATE BECAUSE OF PURKINJE-CELL COMPARTMENTATION AND NOT COMPETITION

Many mossy fiber afferent projections to the rat cerebellar cortex ter minate in parasagittal bands. In particular, the anterior lobe vermis of the cerebellum contains alternating bands of mossy fibers from the spinal cord and external cuneate nuclei. The cerebellar cortical effer ents, the Purkinje cells, are also organized in parasagittal bands. Th ese can be revealed by immunochemical staining for the antigen zebrin II, which is selectively expressed by bands of Purkinje cells. In some cases, the boundaries between mossy fiber terminal fields align with identified transitions between zebrin(+)/(-) sets of Purkinje cells, w hereas others are located within apparently homogeneous Purkinje cell compartments. Two theories can explain the terminal-field topography: In one view, mossy fiber terminals segregate during development, becau se growth cones from different sources compete for common territory. A lternatively, mossy fiber growth cones directly recognize chemically d istinct target territories, and activity-dependent mechanisms play onl y minor roles. To explore these issues, two sets of experiments were p erformed. First, the terminal-field map of the neonatal spinocerebella r projection was compared to the Purkinje cell compartmentation as rev ealed by anticalbindin immunocytochemistry. Second, subsets of spinoce rebellar mossy fiber afferents were ablated early in postnatal develop ment, and the consequences for the neighboring cuneocerebellar termina l fields were mapped in the adult with reference to the zebrin II+/(-) compartments. These experiments revealed no evidence that competitive interactions constrain the mossy fiber terminal-field distribution bu t, rather, suggest that the organization of the mossy fiber projection s follows the compartmentation of the Purkinje cells. (C) 1995 Wiley-L iss, Inc.