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. 2021 Nov 19;14:752781. doi: 10.3389/fnmol.2021.752781

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Copyright © 2021 Hudson and Campbell.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Cellular and tight junction (TJ) protein composition of the blood- brain barrier (BBB) and inner blood-retinal barrier (iBRB). (A) Schematic of the blood brain barrier (BBB) neurovascular unit (NVU). A single endothelial cell (EC) forms the lumen of the blood vessels surrounded by a pericyte (P) and the basement membrane (BM) containing laminins, nidogens, collagen IV and heparin sulfate proteoglycans. Astrocytes (AS) end-feet ensheath the cell complex with neurone (N) and microglia (M) present in the microenvironment. (B) The iBRB is similar in composition to the BBB (as seen in A) although pericytes (P) are at a ratio of 1:1 with endothelial cells (EC) and Muller cell (MC) processes wrap around the blood vessels along with the astrocytes (AS). The iBRB is found in the retina from the ganglion cell layer (GCL) to the outer nuclear layer (ONL). (C) Schematic of tight junction proteins expressed that join the same endothelial cell or adjacent endothelial cells to one another. Claudin-5 is expressed most abundantly with contribution from claudin 1 and 12 (other family members shown to be expressed in other NVU cells). The TAMPs (occludin, tricelllin) and LSR along with JAM family members (A–C and ESAM) constitute the additional transmembrane proteins. Zonula occludens (ZO) 1 and 2 are expressed cytoplasmic which can form a structural link to the actin cytoskeleton and associate with actin binding proteins.