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The BM is a unique form of the extracellular matrix (ECM) found predominantly underneath endothelial and epithelial cells. It exerts many important functions, including structural support, cell anchoring and signalling transduction.26–28 In the brain, two types of BM are found: an endothelial BM and a parenchymal BM (figure 1), which are separated by pericytes.Under physiological conditions, the two BM layers are indistinguishable and look like one in areas without pericytes (figure 1). Structurally, the BM is a highly organised protein sheet with a thickness of 50–100nm.32–34 Biochemically, the BM consists of four major ECM proteins: collagen IV, laminin, nidogen and perlecan. These ECM proteins are synthesised predominantly by BMECs, pericytes and astrocytes at the BBB. The functional significance of each BM component in BBB integrity is discussed below.  |
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