Vascular and astrocytic reactions during establishment of hippocampal transplants in adult host brain.

Hippocampal primordia were transplanted into the hippocampus or septum of adult rats, and the development of the transplant vasculature studied by light microscopy after India ink perfusion, by immunohistochemistry of collagen type IV (as a marker for basal lamina), and by electron microscopy. From the basal lamina of all vessel types arise streamers of basal lamina which clothe the external plasmalemmal surfaces of adjacent astrocytic processes and invaginate them, suggesting that the astrocytic processess may be involved in directing the assembly of this basal lamina. A few host blood vessels reach the transplants by one day after operation, and probably re-perfuse existing transplant vessels. Two main types of vascular reaction occur over the next week. The first is the formation of wide-diameter reactive vessels with an expanded perivascular space, engorged with mesodermal cells (such as macrophages). The second is the formation of dilated, thin-walled, marginal vessels, from which numerous fine capillaries arise. The formation of astrocyte-apposed basal laminar streamers may be important in the development of channels along which the endothelial cells of the newly growing capillaries later migrate. By one month after operation, the transplant vasculature consists of small capillaries whose size, distribution and density are comparable to those of the host. This paper describes the vascular changes which occur when embryonic central nervous transplants become established in the brain of adult hosts, and indicates that one of the characteristic features is the formation of basal lamina-lined channels invaginating the perivascular astrocytes.