Which of the following is NOT true regarding fenestrated capillaries?

• A. They form the blood-brain barrier.
• B. They have pores that increase permeability.
• C. They are commonly found in the kidneys.
• D. They facilitate rapid exchange of substances between blood and tissues.

Answer: (A)

Fenestrated capillaries have pores (fenestrae) in their endothelial lining, which increases permeability and allows rapid exchange of small molecules between blood and surrounding tissues. This makes them well-suited to organs that filter or secrete substances quickly, such as the kidneys (glomerular capillaries), the intestinal mucosa, and many endocrine glands. The brain, however, is protected by a specialized barrier—the blood-brain barrier—which relies on continuous capillaries with tight junctions, a thick basement membrane, and astrocyte endfeet to restrict passage. If fenestrations were present in brain vessels, the barrier function would be compromised, so fenestrated capillaries are not what form the blood-brain barrier. Hence, the statement that fenestrated capillaries form the blood-brain barrier is not true.