Glial Cells: The Other Half of the Brain

Long dismissed as 'glue', glial cells — astrocytes, oligodendrocytes, Schwann cells, microglia and ependymal cells — insulate, feed, clean, defend and even help shape communication between neurons.

When the German pathologist Rudolf Virchow described the non-neuronal tissue of the brain in the 1850s, he called it Nervenkitt — "nerve putty" — and the name glia, Greek for glue, stuck. For a century glia were thought to be mere scaffolding. We now know they do a great deal.

The main types

Astrocytes — star-shaped cells — regulate the chemical environment around neurons, mop up excess neurotransmitter, supply energy, help form the blood-brain barrier and influence synapses. Oligodendrocytes wrap axons in the brain and spinal cord with myelin; one oligodendrocyte can myelinate segments of dozens of different axons. In the peripheral nerves, Schwann cells do the same job, each wrapping a single segment of one axon. Microglia are the brain's resident immune cells, patrolling for damage and infection and pruning synapses. Ependymal cells line the brain's ventricles, and specialised versions help produce cerebrospinal fluid. There are also oligodendrocyte precursor cells, which keep dividing into adulthood and can make new myelin.

How many?

Textbooks long claimed glia outnumber neurons ten to one. Careful counting suggests the human brain has roughly as many glial cells as neurons overall, though the ratio varies widely between regions.

Active partners

Glia help shape how the brain works and how it fails. Myelin loss causes the symptoms of multiple sclerosis. Microglia are implicated in Alzheimer's disease and in the pruning of synapses during development. Most primary brain tumours arise from glial cells — they are called gliomas. And astrocytes communicate with each other using waves of calcium, raising the question of what role they play in information processing.