Figure 6: Paracellular versus transcellular pathway
Substances and cells that have to cross the BBB have two possible pathways: paracellular and transcellular.
In the paracellular pathway the passage is through the hinges of the cell wall, the tight junctions that are connecting the endothelial cells of the BBB. In the transcellular pathway, the passage is through the endothelial cell wall itself. By mechanisms not fully understood, cells that are crossing the BBB by the transcellular pathway form transient “tunnels” through the endothelial cells, mediating their passage into the brain. In all the rest of the vasculature there is just the paracellular pathway, therefore, the presence of the transcellular pathway makes the BBB even more unique!