Intracranial cerebellar grafts: intermediate filament immunohistochemistry and electrophysiology

Pieces of the developing cerebellar anlage were prepared from 13-15 day old rat embryos and transplanted to the cerebellar region of 5-7 and 13-14 day old rat pups. Approximately two months later, sections showed most grafts to consist of both cerebellar cortex, with a typical trilaminar organization, and white matter areas containing large neuronal perikarya. The astrocytic populations were studied using immunohistochemistry with antisera raised against the intermediate filaments, glial fibrillary acidic protein (GFA), and vimentin. The GFA-antiserum revealed a glial interface along most of the border between host brain and graft. Both antisera stained long, slender, although slightly distorted Bergmann fibers spanning the molecular layer. Using GFA-antiserum, star-shaped fluorescent astrocytes were seen in the granular layer and in the white matter. Only in the white matter did the amount of GFA-like immunoreactivity suggest an astrocytic gliosis. With vimentin antiserum fluorescent astrocytes in the white matter were seen. There were no signs of increased amounts of vimentin-like immunoreactivity. Taken together, the amount and distribution of GFA- and vimentin-like immunoreactivity suggests a rather normal astrocytic development in the cerebellar grafts. Using an antiserum against the neurofilament (NF) triplet, delicate immunoreactive fibres were seen in both the molecular and the granular layer. No positive cell bodies could be visualized in the cortical areas. Although the Purkinje cells themselves were negative, fibre baskets around them were intensely stained. In the white matter a high density of NF-positive fibres and some positive perikarya were visualized. Thus the distribution of NF-like immunoreactivity in the grafts corresponded well to the normal NF distribution. The functional maturation of the cerebellar grafts was studied electrophysiologically. A spontaneous mean discharge rate of 19.3 + 1.7 Hz was recorded from the Purkinje cells. This compares with a discharge rate of 26.8 + 1.0 Hz for Purkinje neurons in situ. The difference was at least partly ascribable to the absence of climbing fibre bursts in the grafts. Local stimulation of the graft surface caused both decreased and increased Purkinje cell discharge. In conclusion, these experiments suggest that grafts of fetal cerebellar buds to the young cerebellum develop into cerebellar tissue having both morphological and electrophysiological characteristics quite similar to the normal cerebellum.