Exploring the Blood Brain Barrier: Structure, Function, and Medical Implications

The blood brain barrier is the brain’s physical defense against pathogens and substances that may alter cerebral homeostasis. It is formed under instruction of neurons via astrocytes, and primarily consists of tightly junctioned brain capillary endothelial cells and pericytes. In this review, we first discuss the structure and function of the blood brain barrier, including the variety of transport methods incorporated to maintain cerebral homeostasis, such as transport channels, membrane transport, carrier mediated transcytosis, receptor mediated transcytosis, adsorption mediated transcytosis, and efflux systems. Then, we focus on the three pathways of pathogenic invasion of the blood brain barrier- paracellular, transcellular, and the “Trojan Horse” mechanism, and elaborate specifically on the capability of the SI protein on the SARS-CoV-2 virus to cross the blood brain barrier, causing encephalitis. Finally, we explore the challenge the blood brain barrier’s extremely low vascular permeability poses on medicine and attempts to overcome these challenges using tDCS and nanocarriers. Thus, the blood brain barrier, its structure and function, and its scientific and medical implications remain intriguing frontiers of science.