**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 a layer of endothelial cells that line the brain's capillaries, which are tightly joined together by tight junctions. This barrier restricts the passage of molecules, including ions, nutrients, and pathogens, into the brain.

**Why the Correct Answer is Right**
Astrocytes play a crucial role in the formation and maintenance of the blood-brain barrier. They form a perivascular glial limiting membrane, which is a layer of astrocytic foot processes that surround the brain's capillaries. This membrane helps to regulate the transport of molecules across the BBB and maintains the integrity of the barrier. Astrocytes also regulate the expression of tight junction proteins, such as occludin and claudin, which are essential for the formation of tight junctions between endothelial cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Microglia are a type of glial cell that act as the main form of active immune defense in the CNS. They are involved in the phagocytosis of pathogens and debris, but they do not play a significant role in the formation of the blood-brain barrier.

**Option B:** Oligodendrocytes are a type of glial cell that are responsible for myelinating axons in the CNS. They play a crucial role in the transmission of nerve impulses, but they are not involved in the formation of the blood-brain barrier.

**Option C:** Ependymal cells line the ventricles and central canal of the spinal cord, and they play a role in the production of cerebrospinal fluid. However, they are not involved in the formation of the blood-brain barrier.

**Clinical Pearl / High-Yield Fact**
Astrocytes are often referred to as the "glial cells of the CNS," and they play a crucial role in maintaining the health and function of neurons. Damage to astrocytes, such as that which occurs in Alzheimer's disease, can lead to disruptions in the blood-brain barrier and the progression of neurodegenerative disease.

**Correct Answer: C. Astrocytes.**