Final answer:
Tight junctions between non-fenestrated capillary epithelial cells are the most important element of the blood-brain barrier.
Step-by-step explanation:
The most important element of the blood-brain barrier is tight junctions between non-fenestrated capillary epithelial cells.
Tight junctions are regions where adjacent cells are tightly joined together, forming a barrier that prevents substances from leaking between the cells. In the context of the blood-brain barrier, tight junctions play a crucial role in controlling the transfer of blood components to the brain, thus protecting the brain from potentially harmful substances.