The blood-brain barrier (BBB) selectively restricts entry of substances into the CNS. The component of the BBB that provides the primary physical barrier to paracellular diffusion of hydrophilic solutes is:
- A Astrocyte end-feet processes surrounding cerebral capillaries
- B Pericytes embedded in the basement membrane of cerebral capillaries
- C The negatively charged glycocalyx on the luminal surface of endothelial cells repelling charged molecules
- D Continuous tight junctions (formed by claudin-5, occludin, and ZO proteins) between adjacent brain capillary endothelial cells ✓
Explanation
The primary physical barrier of the BBB is formed by continuous tight junctions (TJs) between brain capillary endothelial cells. These TJs consist of transmembrane proteins (claudin-5, occludin, junctional adhesion molecules) linked intracellularly to the actin cytoskeleton by scaffolding proteins (ZO-1, ZO-2). Unlike peripheral capillaries with fenestrations and gaps, brain capillary TJs completely seal the paracellular space, forcing all molecules to cross transcellularly. Astrocyte end-feet are critical for BBB induction and maintenance (they signal endothelial cells to form tight junctions) but are not the primary physical barrier. Pericytes regulate BBB permeability and development but are not the barrier itself.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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