Angiogenesis and the blood-brain barrier in solid and dissociated cell grafts within the CNS

Available evidence suggests that blood vessels indigenous to solid CNS and peripheral tissues grafted to the brain are sustained and maintain the morphological and permeability characteristics they manifest in normal life. Furthermore, these vessels of graft origin anastomose (albeit not rapidly) with vessels of the surrounding host tissue predominantly at the host-graft interface and less so, or not at all, within the graft itself. For these reasons, blood-brain and brain-blood barriers, evident in the late fetal and neonatal CNS, can be expected to exist within CNS grafts placed intracerebrally or extracerebrally, providing the graft remains viable. Peripheral neural and non-neural tissues not possessing cellular barriers to circulating macromolecules do not acquire such barriers subsequent to their transplantation within the CNS. The absence of a blood-brain barrier in the adrenal gland grafted intracerebrally may be relevant for the treatment of Parkinson's disease with blood-borne therapeutics. Compared to solid tissue grafts, cell suspension grafts have the potential of becoming vascularized rapidly. That cell suspensions of neurons and of glia are supplied with BBB vessels of host origin and that the permeability characteristics of host BBB vessels are altered by a tumor cell suspension reaffirm the belief that the type of transplanted cell/tissue indeed determines the permeability characteristics of the blood vessels supplying it. The suspected immunologic privilege of the CNS is not absolute. Eventual host rejection of allografts placed within the third ventricle may be a dual consequence of the absence of a BBB at the level of the host median eminence and involvement of the minor histocompatibility complex.