International Journal of Molecular Sciences Review Structure and Junctional Complexes of Endothelial, Epithelial and Glial Brain Barriers Mariana Castro Dias *, Josephine A. Mapunda, Mykhailo Vladymyrov and Britta Engelhardt * Theodor Kocher Institute, University of Bern, 3012 Bern, Switzerland;
[email protected] (J.A.M.);
[email protected] (M.V.) * Correspondence:
[email protected] (M.C.D.);
[email protected] (B.E.) Received: 14 October 2019; Accepted: 26 October 2019; Published: 29 October 2019 Abstract: The homeostasis of the central nervous system (CNS) is ensured by the endothelial, epithelial, mesothelial and glial brain barriers, which strictly control the passage of molecules, solutes and immune cells. While the endothelial blood-brain barrier (BBB) and the epithelial blood-cerebrospinal fluid barrier (BCSFB) have been extensively investigated, less is known about the epithelial and mesothelial arachnoid barrier and the glia limitans. Here, we summarize current knowledge of the cellular composition of the brain barriers with a specific focus on describing the molecular constituents of their junctional complexes. We propose that the brain barriers maintain CNS immune privilege by dividing the CNS into compartments that differ with regard to their role in immune surveillance of the CNS. We close by providing a brief overview on experimental tools allowing for reliable in vivo visualization of the brain barriers and their junctional complexes and thus the respective CNS compartments. Keywords: brain barriers; blood-brain barrier; neurovascular unit; blood-cerebrospinal fluid barrier; arachnoid barrier; glia limitans; tight junctions; adherens junctions 1. Introduction The brain barriers established by the endothelial blood-brain barrier (BBB), the epithelial blood-cerebrospinal fluid barrier (BCSFB), the meningeal brain barriers and the blood spinal cord barrier are essential for maintaining central nervous system (CNS) homeostasis [1].