Blood-Brain Barrier Overview: Structural and Functional Correlation

Abstract

The blood-brain barrier (BBB) is a semipermeable and extremely selective system in the central nervous system of most vertebrates, that separates blood from the brain's extracellular fluid. It plays a vital role in regulating the transport of necessary materials for brain function, furthermore, protecting it from foreign substances in the blood that could damage it. In this review, we searched in Google Scholar, Pubmed, Web of Science, and Saudi Digital Library for the various cells and components that support the development and function of this barrier, as well as the different pathways to transport the various molecules between blood and the brain. We also discussed the aspects that lead to BBB dysfunction and its neuropathological consequences, with the identification of some of the most important biomarkers that might be used as a biomarker to predict the BBB disturbances. This comprehensive overview of BBB will pave the way for future studies to focus on developing more specific targeting systems in material delivery as a future approach that assists in combinatorial therapy or nanotherapy to destroy or modify this barrier in pathological conditions such as brain tumors and brain stem cell carcinomas.

Figure 1

A schematic diagram of brain and simple longitudinal zoom in blood brain barrier (Created by BioRender).

Figure 2

A schematic diagram of transverse section in blood-brain barrier illustrating BBB's cellular structures. (Created by author; BioRender).

Figure 3

A schematic diagram of transverse section in capillary endothelial cells illustrating the structure of tight junction. (Created by author; BioRender).

Figure 4

A schematic diagram of blood brain barrier showing pericyte wrapping around endothelial cells (Created by BioRender).

Figure 5

A schematic diagram of the endothelial cells that form the BBB and their associations with the perivascular end feet of astrocytes showing pathways across the BBB. (a) Generally, tight junctions prevent water-soluble chemicals from penetrating. (b) On the other hand, the enormous surface area of the endothelium's lipid membranes provides an excellent diffusive pathway for lipid-soluble substances. (c) Transporter proteins for glucose, amino acids, purine bases, nucleosides, choline, and other chemicals are found in the endothelium. (d) Specific receptor-mediated endocytosis and transcytosis pick up drugs and particular proteins, such as insulin and transferrin. (e) Adsorptive-mediated transcytosis for transport macromolecules and charged agents to brain. (f) Cell mediated transcytosis pathway depends on leukocytes to pass the BBB. (created by author according to information from [67]; BioRender).