Which type of junctions between endothelial cells form the blood-brain barrier?

• A. Desmosomes
• B. Gap junctions
• C. Intercalated discs
• D. Tight junctions

Answer: (D)

Tight junctions are the structures responsible for forming the blood-brain barrier between endothelial cells in the brain. These junctions create a seal that prevents the passage of substances between the cells, effectively regulating the movement of ions and molecules into and out of the brain tissue. By forming a nearly impermeable barrier, tight junctions protect the brain from potentially harmful substances in the bloodstream while allowing necessary nutrients to pass through selectively.

In contrast, desmosomes provide mechanical strength and stability to tissues, particularly in areas subject to stress, but do not form barriers to substance passage. Gap junctions facilitate intercellular communication, allowing ions and small molecules to pass directly between adjacent cells, but they too do not play a role in creating barriers. Intercalated discs are specialized structures found in cardiac muscle cells that allow for synchronized contractions, and are not found in endothelial cells. Hence, tight junctions are critical for maintaining the unique environment of the brain.