An autoradiographic study of neuronal development, vascularization and glial cell migration from hippocampal transplants labelled in intermediate explant culture.

Late embryonic and early postnatal rat hippocampal primordia were labelled with [3H]thymidine for varying periods in explant culture before being implanted into hippocampi of adult hosts. The types and distributions of nuclear-labelled cells were determined autoradiographically at 1 month after operation. The labelling of small pyramidal neurons and dentate granule cells was in accordance with their time of normal developmental origin, the dentate granule cells forming distinctive shells with an appropriate gradient of cell accretion. There was a high proportion of labelled glial cells in the transplants, and a massive accumulation around the interface with the host. Labelled glia migrate for up to 2 mm into the host tissue. Labelled endothelial cells occur in vessels in the transplant, in wide-diameter marginal vessels and in two specific types of vascular configuration in the host. Thus the establishment of embryonic hippocampal transplants is associated with a major migration of non-neuronal cells into the host brain, and the formation of specific types of tissue chimaera. This chimaera formation is essential (e.g. in the case of blood vessels) for the survival of the transplant and the routes of glial cell migration may determine the pathways along which transplant nerve fibres can penetrate the host.