The fine structure of vascular-astroglial relations in transplanted fetal neocortex.

The vascular development within allografts of rat fetal neocortex was examined ultrastructurally with particular attention to astroglial-endothelial relationships. Grafts placed in the fourth ventricle exhibited a progressive astrogliosis around the host pial or choroidal vessels incorporated within the transplant which was evident by 1 month postoperative. Immunostaining with antisera to laminin showed intense reactivity around such neovessels at the light microscopic level. Transplants located intraparenchymally within the host parietal cortex also developed reactive astroglial "cuffs" around their marginal vessels by 1 week postoperative, although the degree and location of this reaction varied considerably with time. The origin of the reactive astroglia could not be directly determined from this study, but it is possible that they were stimulated by the collagen and fibroblasts present around vascularizing host pial and choroidal vessels in intraventricular grafts and by meningeal elements that entered the wound created for the intraparenchymal grafts. The marked astroglial reactivity within the grafts raises issues concerning their metabolic activity and their intimate relationship with brain endothelium. The close proximity of reactive astroglia to the graft vasculature would not appear to enhance the blood-brain barrier capabilities of transplant neovasculature, especially in intraventricular transplants, as might be suggested by many in vitro studies.