Angiogenesis of the blood-brain barrier in vitro and the function of cerebral pericytes.

Cerebral pericytes constitute an essential component of the blood-brain barrier (BBB) and are involved in blood vessel assembly. Recently, we reported on the induction of a BBB-specific enzyme expressed by cerebral pericytes (pericytic aminopeptidase N/pAPN) in coculture with cerebral endothelial cells. We completed this in vitro BBB system by adding astrocytes to these mixed cultures of endothelial cells and pericytes. Under these triculture conditions, endothelial cells and pericytes reorganize into capillary-like structures (CLSs). Capillary formation can also be achieved by the application of transforming growth factor beta 1 (TGF-b1) in the culture medium of endothelial-pericyte cultures lacking astrocytes. In contrast to the effect achieved by astrocytes, pericytes did not assemble with endothelial cells. In both cases (application of astrocytes or TGF-b1), endothelial cells underwent apoptosis. However, endothelial cells that form CLSs in the presence of pericytes appeared to be resistant to induction of apoptosis. On the basis of these observations, we concluded that astrocytes have a profound influence on the morphogenetic events underlying the organization of the vessel wall; that the effect of TGF-b1 is different from the astrocytic effect because it lacks induction of endothelial-pericyte association; and that pericytes stabilize CLSs formed by endothelial cells in coculture with astrocytes.