Glial cell - AbsoluteAstronomy.com

Glial cells, sometimes called neuroglia or simply glia (Greek

Koine Greek

Koine Greek is the universal dialect of the Greek language spoken throughout post-Classical antiquity , developing from the Attic dialect, with admixture of elements especially from Ionic....

γλία, γλοία "glue"; pronounced in English either /gliːə/ or /glaɪə/), are non-neuron

Neuron

A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

al cells

Cell (biology)

The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

that maintain homeostasis

Homeostasis

Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...

, form myelin

Myelin

Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...

, and provide support and protection for neuron

Neuron

s in the brain, and for neurons in other parts of the nervous system such as in the autonomous nervous system. In the human brain

Human brain

The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...

, there is roughly one glia for every neuron with a ratio of about two neurons for every glia in the cerebral gray matter

Gray Matter

"Gray Matter" is a short story by Stephen King, first published in the October 1973 issue of Cavalier magazine, and later collected in King's 1978 collection Night Shift. It is set in the same area as King's novel Dreamcatcher.-Setting:...

.

As the Greek name implies, glia are commonly known as the glue

Glue

This is a list of various types of glue. Historically, the term "glue" only referred to protein colloids prepared from animal flesh. The meaning has been extended to refer to any fluid adhesive....

of the nervous system; however, this is not fully accurate. Neuroscience

Neuroscience

Neuroscience is the scientific study of the nervous system. Traditionally, neuroscience has been seen as a branch of biology. However, it is currently an interdisciplinary science that collaborates with other fields such as chemistry, computer science, engineering, linguistics, mathematics,...

currently identifies four main functions of glial cells: to surround neurons and hold them in place, to supply nutrients and oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

to neurons, to insulate one neuron from another, and to destroy pathogens and remove dead neurons. For over a century, it was believed that they did not play any role in neurotransmission

Neurotransmission

Neurotransmission , also called synaptic transmission, is the process by which signaling molecules called neurotransmitters are released by a neuron , and bind to and activate the receptors of another neuron...

. That idea is now discredited; they do modulate neurotransmission, although the mechanisms are not yet well understood.

Functions

Some glial cells function primarily as the physical support for neurons. Others regulate the internal environment

Milieu interieur

Milieu intérieur or interior milieu, from the French, milieu intérieur, is a term coined by Claude Bernard to refer to the extra-cellular fluid environment, and its physiological capacity to ensure protective stability for the tissues and organs of multicellular living organisms.-Origin:Claude...

of the brain, especially the fluid surrounding neurons and their synapses, and nutrify neurons. During early embryogenesis

Embryogenesis

Embryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus.Embryogenesis starts with the fertilization of the ovum by sperm. The fertilized ovum is referred to as a zygote...

glial cells direct the migration of neurons and produce molecules that modify the growth of axon

Axon

An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

s and dendrite

Dendrite

Dendrites are the branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project...

s. Recent research indicates that glial cells of the hippocampus

Hippocampus

The hippocampus is a major component of the brains of humans and other vertebrates. It belongs to the limbic system and plays important roles in the consolidation of information from short-term memory to long-term memory and spatial navigation. Humans and other mammals have two hippocampi, one in...

and cerebellum

Cerebellum

The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...

participate in synaptic transmission

Synapse

In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...

, regulate the clearance of neurotransmitters from the synaptic cleft, and release gliotransmitter

Gliotransmitter

Gliotransmitters are chemicals released from glial cells that facilitate neuronal communication between neurons and other glial cells and are usually induced from Ca2+ signaling. [3] While gliotransmitters can be released from any glial cell, including oligodendrocytes, astrocytes, and microglia,...

s such as ATP

Adenosine triphosphate

Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

, which modulate synaptic function.

Glial cells are known to be capable of mitosis

Mitosis

Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...

. By contrast, scientific understanding of whether neurons are permanently post-mitotic

Mitosis

,
or capable of mitosis,
is still developing. In the past, glia had been considered to lack certain features of neurons. For example, glial cells were not believed to have chemical synapses

Synapse

In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...

or to release transmitter

Neurotransmitter

Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...

s. They were considered to be the passive bystanders of neural transmission. However, recent studies have shown this to be untrue.

For example, astrocyte

Astrocyte

Astrocytes , also known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord...

s are crucial in clearance of neurotransmitter

Neurotransmitter

s from within the synaptic cleft, which provides distinction between arrival of action potentials and prevents toxic build-up of certain neurotransmitters such as glutamate (excitotoxicity

Excitotoxicity

Excitotoxicity is the pathological process by which nerve cells are damaged and killed by excessive stimulation by neurotransmitters such as glutamate and similar substances. This occurs when receptors for the excitatory neurotransmitter glutamate such as the NMDA receptor and AMPA receptor are...

). It is also thought that glia play a role in many neurological diseases, including Alzheimer's disease

Alzheimer's disease

Alzheimer's disease also known in medical literature as Alzheimer disease is the most common form of dementia. There is no cure for the disease, which worsens as it progresses, and eventually leads to death...

. Furthermore, at least in vitro

In vitro

In vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...

, astrocytes can release gliotransmitter

Gliotransmitter

glutamate in response to certain stimulation. Another unique type of glial cell, the oligodendrocyte precursor cell

Oligodendrocyte precursor cell

Oligodendrocyte precursor cells in nervous tissue cells precede oligodendrocytes, and may also be able to generate neurons and astrocytes. The principal function of oligodendrocytes is to provide support to axons and to produce the Myelin sheath, which insulates and lowers the effective...

s or OPCs, have very well-defined and functional synapses from at least two major groups of neurons. The only notable differences between neurons and glial cells are neurons' possession of axon

Axon

An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

s and dendrite

Dendrite

s, and capacity to generate action potential

Action potential

In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

s.

Glia ought not to be regarded as "glue" in the nervous system as the name implies; rather, they are more of a partner to neurons.
They are also crucial in the development of the nervous system and in processes such as synaptic plasticity

Synaptic plasticity

In neuroscience, synaptic plasticity is the ability of the connection, or synapse, between two neurons to change in strength in response to either use or disuse of transmission over synaptic pathways. Plastic change also results from the alteration of the number of receptors located on a synapse...

and synaptogenesis

Synaptogenesis

Synaptogenesis is the formation of synapses. Although it occurs throughout a healthy person's lifespan, an explosion of synapse formation occurs during early brain development...

. Glia have a role in the regulation of repair of neurons after injury. In the CNS

Central nervous system

The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

(Central Nervous System), glia suppress repair. Glial cells known as astrocytes enlarge and proliferate to form a scar and produce inhibitory molecules that inhibit regrowth of a damaged or severed axon. In the PNS

Peripheral nervous system

The peripheral nervous system consists of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to connect the central nervous system to the limbs and organs. Unlike the CNS, the PNS is not protected by the bone of spine and skull, or by the blood–brain...

(Peripheral Nervous System), glial cells known as Schwann cell

Schwann cell

Schwann cells or neurolemmocytes are the principal glia of the peripheral nervous system . Glial cells function to support neurons and in the PNS, also include satellite cells, olfactory ensheathing cells, enteric glia and glia that reside at sensory nerve endings, such as the Pacinian corpuscle...

s promote repair. After axonal injury, Schwann cells regress to an earlier developmental state to encourage regrowth of the axon. This difference between PNS

Peripheral nervous system

and CNS

Central nervous system

raises hopes for the regeneration of nervous tissue in the CNS

Central nervous system

. For example a spinal cord may be able to be repaired following injury or severance.

Microglia

Microglia are a type of glial cell that are the resident macrophages of the brain and spinal cord, and thus act as the first and main form of active immune defense in the central nervous system . Microglia constitute 20% of the total glial cell population within the brain...

are like specialized macrophage

Macrophage

Macrophages are cells produced by the differentiation of monocytes in tissues. Human macrophages are about in diameter. Monocytes and macrophages are phagocytes. Macrophages function in both non-specific defense as well as help initiate specific defense mechanisms of vertebrate animals...

s capable of phagocytosis that protect neurons of the central nervous system

Central nervous system

. They are derived from hematopoietic precursors rather than ectodermal

Germ layer

A germ layer, occasionally referred to as a germinal epithelium, is a group of cells, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sponges produce two or three primary tissue layers...

tissue; they are commonly categorized as such because of their supportive role to neurons.

These cells comprise approximately 15% of the total cells of the central nervous system. They are found in all regions of the brain and spinal cord. Microglial cells are small relative to macroglial cells, with changing shapes and oblong nuclei. They are mobile within the brain and multiply when the brain is damaged. In the healthy central nervous system, microglia processes constantly sample all aspects of their environment (neurons, macroglia and blood vessels).

Macroglia

Location	Name	Description >- \| CNS Central nervous system The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...	Astrocyte Astrocyte Astrocytes , also known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord... s	The most abundant type of macroglial cell, astrocytes (also called astroglia) have numerous projections that anchor neurons to their blood supply. They regulate the external chemical environment of neurons by removing excess ion Ion An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a... s, notably potassium Potassium Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are... , and recycling neurotransmitter Neurotransmitter Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to... s released during synaptic transmission. The current theory suggests that astrocytes may be the predominant "building blocks" of the blood-brain barrier Blood-brain barrier The blood–brain barrier is a separation of circulating blood and the brain extracellular fluid in the central nervous system . It occurs along all capillaries and consists of tight junctions around the capillaries that do not exist in normal circulation. Endothelial cells restrict the diffusion... . Astrocytes may regulate vasoconstriction and vasodilation by producing substances such as arachidonic acid Arachidonic acid Arachidonic acid is a polyunsaturated omega-6 fatty acid 20:4.It is the counterpart to the saturated arachidic acid found in peanut oil, Arachidonic acid (AA, sometimes ARA) is a polyunsaturated omega-6 fatty acid 20:4(ω-6).It is the counterpart to the saturated arachidic acid found in peanut oil,... , whose metabolites are vasoactive. Astrocytes signal each other using calcium Calcium Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust... . The gap junction Gap junction A gap junction or nexus is a specialized intercellular connection between a multitude of animal cell-types. It directly connects the cytoplasm of two cells, which allows various molecules and ions to pass freely between cells.... s (also known as electrical synapses) between astrocytes allow the messenger molecule IP3 Inositol triphosphate Inositol trisphosphate or inositol 1,4,5-trisphosphate , together with diacylglycerol , is a secondary messenger molecule used in signal transduction and lipid signaling in biological cells. While DAG stays inside the membrane, IP3 is soluble and diffuses through the cell... 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. The net effect is a calcium wave that propagates from cell to cell. Extracellular release of ATP Adenosine triphosphate Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism... , 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 Functional magnetic resonance imaging Functional magnetic resonance imaging or functional MRI is a type of specialized MRI scan used to measure the hemodynamic response related to neural activity in the brain or spinal cord of humans or other animals. It is one of the most recently developed forms of neuroimaging... . >- \| CNS	Oligodendrocyte Oligodendrocyte Oligodendrocytes , or oligodendroglia , are a type of brain cell. They are a variety of neuroglia. Their main function is the insulation of axons in the central nervous system of some vertebrates... s	Oligodendrocytes are cells that coat axons in the central nervous system Central nervous system The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish... (CNS) with their cell membrane forming a specialized membrane differentiation called myelin Myelin Myelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination... , producing the so-called myelin sheath. The myelin sheath provides insulation Electrical insulation thumb\|250px\|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal insulation... to the axon that allows electrical Electricity Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire... signals to propagate more efficiently. >- \| CNS	Ependymal cells	Ependymal cells, also named ependymocytes, line the cavities of the CNS and make up the walls of the ventricles. These cells create and secrete cerebrospinal fluid Cerebrospinal fluid Cerebrospinal fluid , Liquor cerebrospinalis, is a clear, colorless, bodily fluid, that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord... (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 Radial glial cells are a pivotal cell type in the developing central nervous system involved in key developmental processes, from patterning and neuronal migration to their recently discovered role as precursors during neurogenesis. They arise early in development from neuroepithelial cells...	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 Synaptic plasticity In neuroscience, synaptic plasticity is the ability of the connection, or synapse, between two neurons to change in strength in response to either use or disuse of transmission over synaptic pathways. Plastic change also results from the alteration of the number of receptors located on a synapse... . In the retina, the radial Müller cell is the principal glial cell, and participates in a bidirectional communication with neurons. >- \| PNS Peripheral nervous system The peripheral nervous system consists of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to connect the central nervous system to the limbs and organs. Unlike the CNS, the PNS is not protected by the bone of spine and skull, or by the blood–brain...	Schwann cell Schwann cell Schwann cells or neurolemmocytes are the principal glia of the peripheral nervous system . Glial cells function to support neurons and in the PNS, also include satellite cells, olfactory ensheathing cells, enteric glia and glia that reside at sensory nerve endings, such as the Pacinian corpuscle... s	Similar in function to oligodendrocytes, Schwann cells provide myelination to axons in the peripheral nervous system Peripheral nervous system The peripheral nervous system consists of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to connect the central nervous system to the limbs and organs. Unlike the CNS, the PNS is not protected by the bone of spine and skull, or by the blood–brain... (PNS). They also have phagocytotic activity and clear cellular debris that allows for regrowth of PNS neurons. >- \| PNS	Satellite cells Satellite cells (glial) Satellite glial cells are a type of glial cell that line the exterior surface of neurons in the peripheral nervous system . Satellite glial cells also surround neuron cell bodies within ganglia. They are of a similar embryological origin to Schwann cells of the PNS, as they are both derived from...	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 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.They are thought to have many roles in the enteric system, some related to homeostasis and muscular digestive processes.

Capacity to divide

Glia retain the ability to undergo cell division in adulthood, whereas most neurons cannot. The view is based on the general deficiency of the mature nervous system in replacing neurons after an injury, such as a stroke

Stroke

A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...

or trauma, while very often there is a profound proliferation of glia, or gliosis

Gliosis

Gliosis is a proliferation of astrocytes in damaged areas of the central nervous system . This proliferation usually leads to the formation of a glial scar....

near or at the site of damage. However, detailed studies found no evidence that 'mature' glia, such as astrocytes or oligodendrocyte

Oligodendrocyte

Oligodendrocytes , or oligodendroglia , are a type of brain cell. They are a variety of neuroglia. Their main function is the insulation of axons in the central nervous system of some vertebrates...

s, retain the ability of mitosis. Only the resident oligodendrocyte precursor cell

Oligodendrocyte precursor cell

s seem to keep this ability after the nervous system matures. On the other hand, there are a few regions in the mature nervous system, such as the dentate gyrus

Dentate gyrus

The dentate gyrus is part of the hippocampal formation. It is thought to contribute to new memories as well as other functional roles. It is notable as being one of a select few brain structures currently known to have high rates of neurogenesis in adult rats, .The dentate gyrus cells receive...

of the hippocampus

Hippocampus

and the subventricular zone

Subventricular zone

The subventricular zone is a paired brain structure situated throughout the lateral walls of the lateral ventricles. It has been associated with having four distinct layers of variable thickness and cell density, as well as cellular composition....

, where generation of new neurons can be observed.

Embryonic development

Most glia are derived from ectodermal

Germ layer

tissue of the developing embryo

Embryo

An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

, in particular the neural tube

Neural tube

In the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord...

and crest

Neural crest

Neural crest cells are a transient, multipotent, migratory cell population unique to vertebrates that gives rise to a diverse cell lineage including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia....

. The exception is microglia

Microglia

, which are derived from hemopoietic stem cells. In the adult, microglia are largely a self-renewing population and are distinct from macrophages and monocytes, which infiltrate the injured and diseased CNS.

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

History

Glia were discovered in 1846 by the pathologist Rudolf Virchow

Rudolf Virchow

Rudolph Carl Virchow was a German doctor, anthropologist, pathologist, prehistorian, biologist and politician, known for his advancement of public health...

in his search for a 'connective tissue' in the brain. (see as reference: http://physrev.physiology.org/content/81/2/871.long)

Numbers

The human brain contains roughly equal numbers of glial cells and neurons, with 84.6 billion glia and 86.1 billion neurons. The ratio differs from one part of the brain to another. The glia/neuron ratio in the cerebral cortex is 3.72 (60.84 billion glia; 16.34 billion neurons) while that of the cerebellum is only 0.23 (16.04 billion glia; 69.03 billion neurons). The ratio in the cerebral cortex gray matter is 1.48 (the white matter part has few neurons). The ratio of the basal ganglia, diencephalon and brainstem combined is 11.35.

Most cerebral cortex glia are oligodendrocytes (75.6%); astrocytes account for 17.3% and microglia

Microglia

(6.5%)

The amount of brain tissue that is made up of glial cells increases with brain size: the nematode

Nematode

The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...

brain contains only a few glia; a fruitfly

Drosophila melanogaster

Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...

's brain is 25% glia; that of a mouse

Mouse

A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...

, 65%; a human

Human

Humans are the only living species in the Homo genus...

, 90%; and an elephant

Elephant

Elephants are large land mammals in two extant genera of the family Elephantidae: Elephas and Loxodonta, with the third genus Mammuthus extinct...

, 97%.

Functions

Microglia

Macroglia

Capacity to divide

Embryonic development

History

Numbers

Further reading