Glia

Glia, also called glial cells or neuroglia, are special cells in our bodies that help the brain and nerves work properly. They are found in both the central nervous system — which includes the brain and the spinal cord — and the peripheral nervous system. Unlike nerve cells, glial cells do not send electrical signals, but they are very important for keeping everything running smoothly.

These cells make up more than half of the tissue in our nervous system. They help maintain balance within the brain and nerves, create a protective covering called myelin that insulates nerve fibers, and give support and protection to the main nerve cells, called neurons. There are several types of glial cells, such as oligodendrocytes and astrocytes in the central nervous system, and Schwann cells and satellite cells in the peripheral nervous system, each playing a unique role in keeping our nervous system healthy.

Function

Glia help support and protect brain cells called neurons. They hold neurons in place, give them food and oxygen, and keep them separated from each other. Glia also fight germs and clean up dead neurons.

Glia also help send messages between neurons and can play a part in how we remember things. They were first found in 1856 by a scientist named Rudolf Virchow who thought they acted like glue in the brain.

Types

Glia, also called glial cells, are special cells in the brain and spinal cord that help support and protect nerve cells. They make up more than half of the tissue in the brain.

Macroglia

Macroglia come from a type of tissue called ectodermal tissue.

Microglia

An astrocyte from a rat brain grown in tissue culture and stained with antibodies to GFAP (red) and vimentin (green)

Main article: Microglia

Microglia are special cells that help protect nerve cells in the brain and spinal cord. They can move around and increase in number when the brain is hurt. In a healthy brain, microglia help control reactions to any damage and are linked to some diseases like Alzheimer's disease, Parkinson's disease, and ALS.

Other

There are other types of glial cells, such as pituicytes from the posterior pituitary and tanycytes from the hypothalamus. Even fruit flies, known as Drosophila melanogaster, have glial cells that work in ways similar to those in mammals.

Total number

Glial cells are smaller than nerve cells, but there are about the same number of them in the human brain—around 85 billion. They make up about half of the brain’s total volume. Different parts of the brain have different numbers of glial cells compared to nerve cells. The most common types of glial cells are oligodendrocytes, followed by astrocytes and microglia.

Location	Name	Description
CNS	Astrocytes	Astrocytes (also called astroglia) have a number of projections that link neurons to their blood supply while forming the blood–brain barrier. They regulate the external chemical environment of neurons by removing excess potassium ions and recycling neurotransmitters released during synaptic transmission. Astrocytes may regulate vasoconstriction and vasodilation by producing substances such as arachidonic acid, whose metabolites are vasoactive. Astrocytes signal each other using ATP. The gap junctions (also known as electrical synapses) between astrocytes allow the messenger molecule IP3 to diffuse from one astrocyte to another. IP3 activates calcium channels on cellular organelles, releasing calcium into the cytoplasm. This calcium may stimulate the production of more IP3 and cause release of ATP through channels in the membrane made of pannexins. The net effect is a calcium wave that propagates from cell to cell. Extracellular release of ATP and consequent activation of purinergic receptors on other astrocytes may also mediate calcium waves in some cases. In general, there are two types of astrocytes, protoplasmic and fibrous, similar in function but distinct in morphology and distribution. Protoplasmic astrocytes have short, thick, highly branched processes and are typically found in gray matter. Fibrous astrocytes have long, thin, less-branched processes and are more commonly found in white matter. It has recently been shown that astrocyte activity is linked to blood flow in the brain, and that this is what is actually being measured in fMRI. They also have been involved in neuronal circuits playing an inhibitory role after sensing changes in extracellular calcium. Human astrocytes are larger and more abundant than any other animals'.
CNS	Oligodendrocytes	Oligodendrocytes are cells that coat axons in the CNS with their cell membrane, forming a specialized membrane differentiation called myelin, producing the myelin sheath. The myelin sheath provides insulation to the axon that allows electrical signals to propagate more efficiently.
CNS	Ependymal cells	Ependymal cells, also named ependymocytes, line the spinal cord and the ventricular system of the brain. These cells are involved in the creation and secretion of cerebrospinal fluid (CSF) and beat their cilia to help circulate the CSF and make up the blood-CSF barrier. They are also thought to act as neural stem cells.
CNS	Radial glia	Radial glia cells arise from neuroepithelial cells after the onset of neurogenesis. Their differentiation abilities are more restricted than those of neuroepithelial cells. In the developing nervous system, radial glia function both as neuronal progenitors and as a scaffold upon which newborn neurons migrate. In the mature brain, the cerebellum and retina retain characteristic radial glial cells. In the cerebellum, these are Bergmann glia, which regulate synaptic plasticity. In the retina, the radial Müller cell is the glial cell that spans the thickness of the retina and, in addition to astroglial cells, participates in a bidirectional communication with neurons.
PNS	Schwann cells	Similar in function to oligodendrocytes, Schwann cells provide myelination to axons in the peripheral nervous system (PNS). They also have phagocytotic activity and clear cellular debris that allows for regrowth of PNS neurons.
PNS	Satellite cells	Satellite glial cells are small cells that surround neurons in sensory, sympathetic, and parasympathetic ganglia. These cells help regulate the external chemical environment. Like astrocytes, they are interconnected by gap junctions and respond to ATP by elevating the intracellular concentration of calcium ions. They are highly sensitive to injury and inflammation and appear to contribute to pathological states, such as chronic pain.
PNS	Enteric glial cells	Are found in the intrinsic ganglia of the digestive system. Glia cells are thought to have a number of roles in the enteric system, some related to homeostasis and muscular digestive processes.

Development

Main article: Gliogenesis

Most glia come from ectodermal tissue in a growing embryo, especially from the neural tube and crest. One type, called microglia, comes from hematopoietic stem cells. In adults, microglia can renew themselves and are different from other cells that might enter when there is injury or disease in the central nervous system.

In the central nervous system, glia develop from the ventricular zone of the neural tube. These include cells called oligodendrocytes, ependymal cells, and astrocytes. In the peripheral nervous system, glia come from the neural crest. These include Schwann cells in nerves and satellite glial cells in ganglia.

23-week fetal brain culture astrocyte

Capacity to divide

Glia can still divide into new cells even in adulthood, unlike most neurons. This is seen when the nervous system tries to heal after an injury, like a stroke or trauma, where glia often grow near the damaged area. However, careful studies show that fully developed glia, such as astrocytes or oligodendrocytes, do not divide. Only certain oligodendrocyte precursor cells keep this ability after the nervous system matures.

Glial cells can go through mitosis. Scientists are still learning whether neurons can ever divide. In the past, glia were thought to be simple bystanders in brain signaling. But new research shows they play more active roles than once believed.

Functions

Some glial cells act like a support system for neurons. Others give neurons food and help keep the fluids around the brain balanced, especially near neurons and where they connect with each other.

During the very early growth stages of an embryo, glial cells help guide neurons to move to the right place and make substances that change how neurons and their branches grow.

Neuron repair and development

Glia are very important for the nervous system as it grows and for fixing neurons after they get hurt. In the central nervous system (the brain and spinal cord), glia called astrocytes grow larger and multiply to form a scar. They also make substances that stop a damaged nerve from growing back. But in the peripheral nervous system (the nerves outside the brain and spinal cord), glia called Schwann cells help repair damaged nerves by encouraging them to grow again.

Myelin sheath creation

Oligodendrocytes, found in the central nervous system, look like tiny octopuses with a round body and many arms. Each arm reaches out to a nerve fiber and wraps around it, making a protective coating called myelin. This coating helps signals travel faster along the nerve. In the peripheral nervous system, Schwann cells do the same thing, wrapping around nerve fibers to create myelin, which helps signals move quickly and also helps damaged nerves grow back.

Neurotransmission

Astrocytes play a big role in how signals are sent between neurons. They help clear away certain chemicals from the space where neurons connect, which stops these chemicals from building up to harmful levels. Astrocytes can also release their own signaling chemicals when they get stimulated.

Clinical significance

History

Glial cells and neurons were first seen around the same time in the early 1800s. For a long time, scientists thought glial cells were just like glue holding neurons together. But we now know they are very important.

In 1856, a scientist named Rudolf Virchow first described glial cells. Later studies showed that the famous scientist Albert Einstein's brain had more glial cells in one area than normal brains. Scientists are still learning about how glial cells help the brain work, especially in learning and memory. They used to think these cells just supported neurons, but now we know they play an active role in how our minds work.