Axonal elongation of grafted, embryonic inferior olivary neurons in the adult rat cerebellum and their synaptic formation on host Purkinje cells.

Although the brain grafting of adult mammals had failed to show reinnervation of host targets, embryonic neural tissue grafted to adult brains has been reported to establish new neuronal connections. However, the trajectories of growing axons of grafted neurons in the host brain are poorly understood. To study the morphological aspects of the trajectories of growing axons of grafted neurons in detail, ventromedial medulla containing the inferior olivary nucleus of 14-day rat embryos were grafted into the hemicerebella of 8-week-old adult rats whose inferior olivary neurons and climbing fibers were previously destroyed. The cerebellum and the inferior olivary complex of host rats were histologically examined 1-6 weeks after surgery. The heterotopically grafted neurons sent their axons into the white matter and the molecular layer of the host cerebellum. These growing fibers made plexus formation in the molecular layer, ending with axo-somatic and/or axo-dendritic synapses on host Purkinje cells whose afferent climbing fibers had been eliminated. These results show that the growth cones of growing axons arising from the grafted neuron can find and reach host targets along unusual pathways, and that some of the mature host Purkinje cells retain the ability to form synapses with newly developing axons from the grafted immature inferior olivary neuron.