The development of cerebellar primordia transplanted to the neocortex of the rat.

Pieces of cerebellar primordia were dissected from the developing fetus at day 18 of gestation in Sprague-Dawley rats and transplanted to the neocortex of a 10- to 12-day-old rat. The histological development of 53 such transplants was analyzed at a series of survival times ranging from 5 min to 426 days. The cerebellar cortex developed much as it does in situ. However, only migratory sequences were strictly followed, while Purkinje cell differentiation and folia formation were initially retarded. In deep parts of the transplant, and throughout transplants confined to deep layers of the neocortex, the external granule layer was associated with penetrating blood vessels. An inverted cytoarchitectural pattern developed as concentric cylindrical layers around these vessels. In contrast, normal lamination and foliation were found only in transplants growing on the neocortical surface. Axons coursing between host and transplant were seen frequently and were especially pronounced at those sites beneath the internal granule layer which appeared to contain deep cerebellar nuclei. At the longest survival time there was preliminary evidence that suggested morphological deterioration of the transplant. Surface transplants, in addition to developing a normal orientation of cells and layers, have the added benefit of being accessible for further experimental manipulations after the transplant has become established.