Which statement about tight junctions in the Blood-Brain Barrier is true?

Which statement about tight junctions in the Blood-Brain Barrier is true?

• A. They enable high rates of transcellular transport
• B. They are formed by astrocytic end-feet
• C. They create a seal that restricts paracellular diffusion between endothelial cells
• D. They are unnecessary for barrier function

Answer: (C)

Tight junctions in brain capillary endothelium form a continuous seal between neighboring cells, preventing ions and solutes from slipping between them (paracellular diffusion). Because of this seal, substances must cross the endothelial cell membranes themselves to move from blood into the brain, i.e., via the transcellular route, often using specific transporters, channels, or vesicular mechanisms. The junctions are built from proteins like claudins and occludin and are linked to the actin cytoskeleton by scaffolding proteins such as ZO-1. While astrocyte end-feet help maintain and regulate these junctions as part of the neurovascular unit, the actual tight junctions reside in the endothelial cell membranes. They are essential for barrier function; removing them would compromise the blood–brain barrier. Therefore, the statement that tight junctions create a seal that restricts paracellular diffusion between endothelial cells is the correct description.

Tight junctions in brain capillary endothelium form a continuous seal between neighboring cells, preventing ions and solutes from slipping between them (paracellular diffusion). Because of this seal, substances must cross the endothelial cell membranes themselves to move from blood into the brain, i.e., via the transcellular route, often using specific transporters, channels, or vesicular mechanisms. The junctions are built from proteins like claudins and occludin and are linked to the actin cytoskeleton by scaffolding proteins such as ZO-1. While astrocyte end-feet help maintain and regulate these junctions as part of the neurovascular unit, the actual tight junctions reside in the endothelial cell membranes. They are essential for barrier function; removing them would compromise the blood–brain barrier. Therefore, the statement that tight junctions create a seal that restricts paracellular diffusion between endothelial cells is the correct description.