Glial cells are one of the two main types of cells in the nervous system. Glia cells comprise astrocytes, oligodendrocytes, microglia, and ependymal cells in the central nervous system, and satellite and Schwann cells in the peripheral nervous system. These cells do not communicate via electrical signals like neurons do, but they contribute to virtually every other aspect of nervous system function. In humans, the number of glial cells is roughly equal to the number of neurons in the brain.
Glia in the central nervous system (CNS) include astrocytes, oligodendrocytes, microglia, and ependymal cells. Astrocytes are the most abundant type of glial cell and are found in organized, non-overlapping patterns throughout the brain, where they closely associate with neurons and capillaries. Astrocytes play numerous roles in brain function, including regulating blood flow and metabolic processes, synaptic ion and pH homeostasis, and blood-brain barrier maintenance.
Another specialized glial cell, the oligodendrocyte, forms the myelin sheath that surrounds neuronal axons in the CNS. Oligodendrocytes extend long cellular processes that wrap around axons multiple times to form this coating. Myelin sheath is required for proper conduction of neuronal signaling and greatly increases the speed at which these messages travel.
Microglia—known as the macrophages of the CNS—are the smallest glial cell type and specialize in phagocytosis of both pathogens and debris. They protect the CNS from infectious agents and toxins and prune synapses during development. Although microglia are considered glial cells, they have a unique and separate origin compared to other glial cell types. Astrocytes and oligodendrocytes are produced by radial glia, whereas microglia originate from the yolk sac and migrate into the embryo early in embryonic development.
Lastly, ependymal cells are cube-shaped cells with cilia-like protrusions that line the ventricles, where they produce cerebrospinal fluid (CSF). Ependymal cells form a barrier between the brain and the CSF, filtering out potentially harmful substances. Like astrocytes and oligodendrocytes, ependymal cells originate from radial glia found near the lateral ventricles.
In the peripheral nervous system (PNS), similar yet distinct types of glial cells exist. For example, functions performed by CNS astrocytes are accomplished in the PNS primarily by satellite cells, glial cells that provide structure, cushioning, and nutrients to the neuronal bodies they associate with. Another PNS glial cell, the Schwann cell, functions like CNS oligodendrocytes by forming a myelin sheath around neuronal axons. Like myelination in the CNS, PNS axon myelination provides necessary insulation and conductivity for the proper transmission of electrical signals.
Glial cells are critical nervous system protectors and regulators. Not only do they maintain homeostatic conditions and contribute to routine brain function, but they also respond to nervous system injury, infection, and disease. In addition, glia perform critical functions during embryonic development of the nervous system. These cells even contribute to the removal of unnecessary neuronal connections, a process called synaptic pruning. Due to the importance of glia in numerous aspects of brain function, defects in one or more glial cell populations can lead to severe and debilitating neurological conditions, including developmental disorders, Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, and many others.
During development, glial cells provide a scaffolding for neurons to properly migrate on and grow out their axons. Later in life, trauma or neurodegenerative diseases can cause loss of neuronal connections that cannot be regenerated, leading to impaired functioning or paralysis.
