Permeability to blood‐borne protein and 3HGABA in CNS tissue grafts. I. Intraventricular grafts
Authors: Rosenstein Jeffrey M. 
Published: January 2005

In the present study, solid grafts of fetal CNS tissue from the rat neocortex, cerebellum, or ventral mesencephalon were placed into the lateral, III or IV ventricles of young adult hosts. Survival periods ranged from 2 days to 20 months. To study the permeability to protein and potential changes in the blood‐brain barrier (BBB), macromolecules such as HRP, HRP‐human serum albumin, and HRP‐human IgG were administered intravascularly and circulated for periods between 3 minutes and 1 hour. Younger grafts were completely filled with the protein, even at 2 days, when the graft vasculature already contained host macrophages, whereas all older grafts showed variability in permeation with protein ingress initiating at the graft‐host interface and subsequently diffusing through the extracellular spaces. Permeation was from several sources: permeable vessels of the circumventricular organs and the choroid plexus which grew into the grafts, the perivascular spaces surrounding these vessels, or from the normally impermeable vessels of the pia mater, which, because of their engulfment by the graft and subsequent angiogenesis, may have been rendered permanently leaky. Invading vessels were often “cuffed” by lymphocytic cells. Many grafts were only partially filled by the glycoprotein conjugates; ventral mesencephalic grafts allowed the least diffusion even when vascularized by choroidal vessels. Fenestrated vessels were not directly observed even though petechial leaks were evident and vessels indigenous to the CNS grafts retained BBB properties. To determine endogenous protein exudation, noninjected animals were immunocytochemically examined for rat serum albumin (RSA). The distribution of RSA mimicked that of the injected proteins at interface regions, although in most instances the entire graft was filled by a light, diffuse labeling suggesting a steady‐state protein leakage over the life of the graft.

When HRP was delivered intraventricularly, the intraventricular grafts were nearly filled with reaction product by 20 minutes. The depth of penetration in the grafts from the CSF interface was generally threefold greater than in normal brain. The increase in permeation suggests that solutes may flow through these grafts (out of or into the CSF) at an increased rate. Lastly the neurotransmitter tritiated gamma‐aminobutyric acid CHGABA which does not cross the BBB was vascularly administered to hosts bearing neocortical grafts. These experiments not only confirmed the permeability in these grafts but showed that the blood‐borne amino acid could be directly sequestered by grafted neurons or glia.

The results of the present study demonstrate that exogenous or endogenous proteins or a neurotransmitter (3HGABA) can infiltrate both early and long‐term CNS intraventricular grafts, potentially exposing then to an altered extracellular environment. The mechanisms for the observed permeability may include (1) neovascularization by permeable vessels, which, in contrast to prevailing angiogenic concepts, do not change phenotype; (2) a time‐dependent solute diffusion from a permeable CNS source; or (3) vascular cuffing with possible pathology to some vessels.