Colonization of neural allografts by host microglial cells: relationship to graft neovascularization

In order to illuminate functional roles of microglial cells within neural allografts, we have transplanted both whole and microglial and endothelial cell-depleted E14 neural cell suspensions into the intact striatum of Sprague-Dawley rats. Following posttransplantation times of up to 30 days, the intrastrial allografts were analyzed histochemically using the Griffonia simplicifolia B4 isolectin, a marker for both microglia and blood vessels. Our results indicate that both whole and depleted suspension grafts develop identically in terms of neovascularization and microglial colonization. In both types of transplants microglial cells appeared before any blood vessels were apparent. The main phase of graft vascularization occurred between days 7 and 10 posttransplantation and neovascularization was complete by day 21, as revealed by quantitative image analysis. Microglial cells, which were present as ameboid cells during early posttransplantation times, underwent continuing cell differentiation with time that paralleled graft vascular development. By 30 days posttransplantation microglia within the grafts had assumed the fully ramified phenotype characteristic of resting adult microglia. During graft development and vascularization, microglia were often seen in close proximity to ingrowing blood vessels and vascular sprouts. In conclusion, our study has shown that microglial colonization of grafts and graft vascularization occurs independent of donor-derived microglial and endothelial cells, and suggests that the great majority of microglia and vessels within the graft are host derived. We hypothesize that the host microglia invading the allografts play an active role in promoting graft neovascularization.