DEVELOPING BRAIN-CELLS PRODUCE FACTORS CAPABLE OF INDUCING THE HT7 ANTIGEN, A BLOOD-BRAIN BARRIER-SPECIFIC MOLECULE, IN CHICK ENDOTHELIAL-CELLS

Homeostasis of the neural microenvironment is maintained by the blood- brain barrier (BBB). To analyze the molecular mechanisms by which the BBB is induced during embryonic development, we have taken advantage o f an in vivo model of BBB induction based on the expression of the HT7 cell surface protein. This protein is a transmembrane glycoprotein of the immunoglobulin superfamily. it is expressed in the chick BBB-form ing endothelial cells, but not in peripheral endothelial cells. Here w e show that the HT7 protein is induced in vessels which had vasculariz ed a quail embryonic brain graft transplanted in the coelomic cavity o f chick embryo. The quail brain graft was vascularized by both chick a nd quail-derived vessels. All chick host-derived vessels in the brain transplant were found to express HT7 while the neighboring chick vesse ls were negative. We conclude that the invading host endothelia! cells differentiated into BBB-forming vessels under the influence of develo ping quail brain cells. This model reproduces the BBB induction during development. It may be useful for further approaches to study the mol ecular mechanisms involved in BBB induction.