Blood brain barrier is formed by?
**Core Concept**
The blood-brain barrier (BBB) is a highly selective permeable barrier that separates the circulating blood from the brain's extracellular fluid in the central nervous system (CNS). It is formed by the tight junctions between the endothelial cells of the brain capillaries, which restrict the passage of ions, metabolites, and other substances into the brain.
**Why the Correct Answer is Right**
The BBB is primarily formed by the tight junctions between the endothelial cells of the brain capillaries, which are composed of a complex network of occluding junctions, adherens junctions, and gap junctions. These tight junctions are formed by proteins such as occludin, claudin, and zonula occludens (ZO)-1, which are embedded in the cell membrane and interact with each other to create a continuous seal. This seal restricts the passage of solutes and ions into the brain, while allowing the free movement of oxygen, carbon dioxide, and other essential substances.
**Why Each Wrong Option is Incorrect**
**Option A:** Astrocytes, microglia, and oligodendrocytes are glial cells that provide support and maintenance functions to the neurons, but they are not directly involved in the formation of the BBB.
**Option B:** The BBB is not formed by the basement membrane, which is a layer of extracellular matrix that surrounds the endothelial cells and supports their growth and function.
**Option C:** While the BBB is a critical component of the CNS, it is not formed by the neurons themselves, but rather by the endothelial cells of the brain capillaries.
**Clinical Pearl / High-Yield Fact**
A key feature of the BBB is its ability to restrict the passage of certain drugs and toxins into the brain, which is why some medications require special formulations or delivery systems to cross the BBB.
**Correct Answer: C. The endothelial cells of the brain capillaries, which are composed of tight junctions formed by proteins such as occludin, claudin, and zonula occludens (ZO)-1.**