Allografts of CNS tissue possess a blood-brain barrier: I. Grafts of medial preoptic area in hypogonadal mice

Abstract This study represents the first part of a three-part investigation of blood vessels supplying CNS tissue transplanted within the brains of adult mammalian hosts. The results emphasize that blood vessels in solid CNS grafts contribute a blood-brain barrier to that of the host. Neurosecretory cells in basal forebrain grafts placed intraventricularly on the dorsal surface of the host median eminence, a neurosecretory site containing fenestrated blood vessels, do not stimulate similar blood vessels to inhabit the transplanted tissue. Solid grafts of the medial preoptic area containing neurons that synthesize and secrete gonadotropic hormone-releasing hormone (GnRH) were obtained from AKR mice and placed into the third cerebral ventricle of hypogonadal (HPG) mice genetically incapable of synthesizing GnRH. GnRH neurons in the allografts were confirmed immunohistochemically. Blood vessels supplying the host median eminence and the allograft at 10 days to 3 months post-transplantation were analyzed with peroxidase cytochemistry applied in three ways: to HPG mice injected systemically with native horseradish peroxidase; to HPG mice infused into the aorta with peroxidase subsequent to perfusion fixation; and to HPG mice brains fixed by immersion and incubated for endogenous peroxidase activity in red cells retained within blood vessels. The median eminence of the HPG mouse was innervated by GnRH neurons residing within the graft, and blood vessels traversing the median eminence-allograft interface were seen rarely. The allografts contained no fenestrated endothelia, and no extravasations of blood-borne HRP were related directly to leaky blood vessels supplying the grafted tissue. Endothelial cells throughout the CNS grafts were similar morphologically to blood-brain barrier endothelia; they were nonfenestrated, exhibited interendothelial tight junctional complexes and an endomembrane system of organelles, and they endocytosed blood-borne HRP that eventually was sequestered within dense body lysosomes. The results support the belief that blood vessels supplying CNS tissue transplanted to a host brain manifest endothelial characteristics identical to those of the tissue in normal life and to those of the host CNS.