Correlation of blood-brain barrier function and HT7 protein distribution in chick brain circumventricular organs

The HT7 protein defined by a monoclonal antibody is a specific marker for chick brain endothelial cells (EMBO J., 5 (1986) 3179-3183). In this study, we have investigated the expression of this protein in the brain circumventricular organs which lack a blood-brain barrier. Using immunohistochemical techniques we found that the protein was absent from the vascular system of the pituitary, median eminence, subfornical organ, pineal gland, the organum vasculosum lamina terminalis and the layer of sinusoid blood vessels of the area postrema. In some regions of the median eminence and, more strikingly, in the pineal gland, parenchymal cells expressed the HT7 antigen. Immunoblots of proteins from brain, pituitary, pineal gland and retina showed that the antigen is very abundant in the retina. Lower amounts were present in brain and pineal gland. The glucose transporter was found to be an independent reliable marker for blood-brain barrier endothelium. In the chick brain the distribution of the biochemically distinct proteins was very similar. Using a postembedding technique we have ultrastructurally localized the HT7 protein specifically in blood-brain barrier endothelial cells. Thus, the expression of the HT7 protein and glucose transporter correlated with blood-brain barrier function.