Glial cells or neuroglia are non-neuronal cells in the nervous system that outnumber neurons. These include astrocytes, oligodendrocytes, microglia, and ependymal cells in the CNS, and satellite and Schwann cells in the PNS. Astrocytes are star-shaped cells with cytoplasmic processes that contact neurons, capillaries, and the pia mater— the covering of the brain and spinal cord. Astrocytes support neurons, help to form the blood-brain barrier, regulate the concentration of ions, such as K+, and recycle released neurotransmitters. Oligodendrocytes have flat cytoplasmic processes that wrap around axons, producing myelin, which protects and electrically insulates axons. Microglia are small phagocytic cells that remove dead cells and pathogens from the CNS. Ependymal cells are cuboidal or columnar cells that line the CNS ventricles and central canal. They contribute to cerebrospinal fluid production and monitor its circulation. In the PNS, satellite cells surround and support neuronal cell bodies and regulate their microenvironment. Schwann cells or neurolemmocytes either myelinate single peripheral axons or enclose several unmyelinated axons. They also participate in axon regeneration.