Figure 3.

Six major pathways that regulate the transport of molecules across the BBB.^{[73, 76]} (a) RMT is mediated by receptors on the cell surface to transport target molecules. (b) Adsorptive transcytosis occurs when positively charged molecules come in close proximity to the negatively charged membrane. (c) Efflux transporters such as the ABC function through ATP hydrolysis or ATP binding (e.g. P-gp). (d) In carrier-mediated transport, molecules are loaded onto carriers with high specificity at the cell membrane and transported across the EC layer. (e) Passive diffusion is defined as the non-specific transport of small molecules. (f) Unlike the transcellular pathways, paracellular transport involves hydrophilic molecules passing through the space in between adjacent ECs.