The blood-brain barrier is formed by

There are three types of microglia: oligodendrocytes, Schwann cells, and astrocytes. Oligodendrocytes and Schwann cells are involved in myelin formation around axons in the CNS and peripheral nervous system, respectively. Astrocytes, which are found throughout the brain, are fuher subdivided into two groups. Fibrous astrocytes, which contain many intermediate filaments, are found primarily in white matter. Protoplasmic astrocytes are found in gray matter and have a granular cytoplasm. Both types of astrocytes send processes to blood vessels, where they induce capillaries to form the tight junctions making up the blood-brain barrier. Microglia are immune system cells; they are scavenger cells that resemble tissue macrophages and remove debris resulting from injury, infection, and disease (eg, multiple sclerosis , AIDS-related dementia, Parkinson disease, and Alzheimer disease).Ref: Ganong 25th ed, p 85, 86