The glial cell primarily responsible for forming the blood-brain barrier is the Astrocyte.

- The blood-brain barrier (BBB) is a highly selective, semipermeable border that separates the circulating blood from the brain and extracellular fluid in the central nervous system (CNS).
- This - barrier plays a crucial role in maintaining the delicate environment required for proper neuronal function.
- Astrocytes are star-shaped glial cells that have numerous extensions called end-feet, which envelop the blood vessels in the brain.
- These end-feet interact closely with the endothelial cells lining the blood vessels and secrete signaling molecules that help induce the formation of tight junctions between these endothelial cells.
- These tight junctions are critical for the selective permeability of the BBB.

Other glial cells serve different functions:
- Oligodendrocytes produce the myelin sheath around CNS axons.
- Microglia act as the resident immune cells of the CNS.
- Ependymal cells line the ventricles of the brain and help produce and circulate cerebrospinal fluid.

Reference: Principles of Neural Science, 5th Edition, by Kandel et al., Volume 1, Chapter 7 - The Blood-Brain Barrier and the Brain’s Microenvironment