They enhance the formation and functioning of synapses by keeping in communication with each other and with neurons. Oligodendrocytes Oligodendrocytes form sheaths around the axons of the CNS that serve as insulation. These sheaths are made of myelin , which is a white material that enables the conduction of electrical impulses. Microglia Microglia act as the immune defence cells of the CNS. They are made of the same tissues as monocytes , which are a type of white blood cell that leaves the blood and sets up a front-line defence against invading organisms throughout the body. Ependymal Cells Ependymal cells line the internal cavities of the CNS. The ependymal cells that line the cavities of the brain also contribute to the formation of cerebrospinal fluid (CSF). These cells have tail-like projections called cilia. The beating of this cilia assists the flow of CSF throughout the brain cavities. Ependymal cells also act as stem cells in the brain, and have the potential to form other glial cells and new neurons which are only produced in specific site of the brain. Neurons in most of the brain are considered to be irreplaceable. Schwann Cells Schwann cells are wound repeatedly around nerve fibres in the peripheral nervous system, producing a myelin sheath similar to the membrane produced by oligodendrocytes in the CNS. They also play a role in the regeneration of damaged fibres. Satellite Cells Satellite cells surround the cell bodies of neurons in the ganglia of the PNS. Their function has not