Path and target finding of afferents in cerebellar anlagen grafted in the cerebellum of adult rats: a Phaseolus vulgaris leucoagglutinin study.

Cerebellar anlagen from rat embryos were grafted into the cerebellum of intact adult rats. Most of the grafts survived and formed 'minicerebella'. The location of the grafts were varied, which provided various types of host/graft interface in laminar configuration: Hw/Gg, Hw/Gp, Hw/Gm, Hg/Gw, Hg/Gg, Hm/Gw, Hm/Gg, Hm/Gp, and Hm/Gm that comprised of the granule cell layer (g), the Purkinje cell layer (p), the molecular layer (m), or the white matter (w) of the host (H) or graft (G). The manner of entrance of mossy and climbing fibers through the host/graft interface and their outgrowth in the graft which has various subset of laminar organization of the cerebellum were examined 28-158 days after grafting by means of anterograde Phaseolus vulgaris leucoagglutinin (PHA-L) labeling. Each cerebellar afferent entered the graft through specific types of interface and grew in specific layers. Mossy fibers passed through Hg/Gg and Hw/Gg, grew in Gg, and mostly terminated there like normal fibers. Fibers in Gg, though rarely, grew further outside Gg like in development. Climbing fibers passed through Hm/Gg, Hw/Gg, and Hw/Gm, proceeded in cortical layers, and terminated in Gm. The outgrowth of climbing fibers in Gm showed selectivity for the direction of Gm with respect to the polarity of Purkinje cells; they permeated Gm to form terminal arbors similar to normal in the direction from the side of Purkinje cell somata to dendrites but not in the reverse direction. Occasionally a single fiber innervated neighboring multiple Purkinje cells. These results indicate that mature cerebellar afferents have potential to regrow and innervate the extraneous cerebellar anlage by finding paths and targets in a manner similar to normal ontogenesis.