Glial Cells

Overview:

The brain is undoubtedly one of the most important organs of the body, without which the body would not receive any commands about how to function. It is quite evident that the brain seems very complex, with its different parts that are interconnected together to execute various body functions. However, over years of scientific studies and neurobiological research, it has been discovered that even the tiniest parts of this complex organ play a significant role in the overall working of the human body and the numerous accomplishments that coordinating organs and tissues achieve together.

One of these constituents of the brain are glial cells, which act side by side with neurons.

Glial cells, which are primarily composed of microglia, astrocytes, and oligodendrocyte lineage cells, make up a significant portion of the mammalian brain. Initially thought to be merely non-functional glue for nerve cells, years of research have emphasized the importance and also additional functions of glial cells.

In this article, the location, structure, function, and different kinds of glial cells will be discussed alongside their differences with neurons and their relevant clinical complications.

Summary:

• Glial cells are essentially any of several kinds of cells that principally focus on supporting nerve cells
• Glial cells are found in the central as well as the peripheral nervous system, alongside nerve cells
• Glial cells have a fibrous appearance due to thick bundles of cytoplasmic filaments
• The five types of glial cells found in the central nervous system are: Astrocytes, Oligodendrocytes, Microglia, Ependymal cells, and Radial glia
• There are two kinds of glial cells present in the PNS: Schwann cells  and Satellite cells
• Astrocytes perform a variety of functions, including regulation of blood circulation, regulation of blood circulation, forming the blood-brain barrier, axon synchronization, and many others
• Microglia serve as the brain’s devoted immune system, Radial glia are a kind of stem cell, so they generate new cells. Ependymal cells produce cerebrospinal fluid and play a role in the Blood-brain barrier
• Schwann cells supply myelin sheaths for axons, but they are found in the PNS instead of the CNS. Satellite cells seem to regulate the surroundings around nerve cells, keeping chemicals in a state of equilibrium.
• Oligodendrocytes have the primary function of helping in the movement of information along axons
• Neuroglia outnumber nerve cells by a ten to one ratio in the nervous system, and they do not develop synapses and maintain the power to divide throughout their lives
• Demyelination is the loss of the myelin sheath, which causes nerve cells to reduce their capacity to transmit messages and signals properly.
• There are several demyelinating illnesses: Neuromyelitis Optica, Multiple sclerosis, CCPD (Combined central & peripheral demyelination), Guillan-Barré Syndrome

Location:

Glial cells are also known as neuroglia or simply glia and are essentially any of several kinds of cells that principally focus on supporting nerve cells.  The word neuroglia translates as ‘nerve glue’.   Glial cells are found in the central as well as the peripheral nervous system, alongside nerve cells.

Glial cells in the central nervous system are generated by stem cells in the neural tube’s ventricular zone, whereas those found in the peripheral nervous system are manufactured by progenitor cells that are present in the neural crest.

Structure:

In the nerve cord, supportive glial cells establish a visible cortex. The inner area of this cortex is made up of densely packed processes and cell bodies of fibrous glial cells organized in concentric rings around the neuropile’s boundary. Glial cells have a fibrous appearance due to thick bundles of cytoplasmic filaments.

Numerous fibrous glial processes enter the neuropile and expand among the neuronal components. Bigger, oddly shaped cells are the primary supportive glial components of the cortex’s peripheral area, where they surround the stromal sheath and provide extended concentric processes of lamellate processes to the neuronal perikarya.

Glial cells typically have an indistinct cytoplasm with dispersed glycogen granules, but they can also have an excellently developed Golgi apparatus, tightly packed particulate glycogen, and endoplasmic reticulum.

Function:

Emilio Lugaro was an Italian biologist who proposed in the year 1907 that neuroglial cells swap compounds with extracellular fluid and thus maintain control over the neuronal surroundings. Substances such as amino acids, glucose, and ions, all of which impact neuronal activity, have since been observed to be exchanged between the neuroglial cells and the extracellular space. For example, after elevated amounts of neuronal activity, neuroglial cells can pick up and spatially buffer potassium ions, allowing the normal neuronal function to be maintained.

Immunologic technologies, in addition to traditional histological and electron-microscopic strategies, are being used to describe specific neuroglial cell types. Neuroscientists have been able to distinguish four types of glial cells by staining these cells with antibodies that adhere to specific protein constituents of different glial cells:

1. Astrocytes, which are further subdivided into fibrous and protoplasmic types

2. Oligodendrocytes, which are further subdivided into interfascicular and perineuronal types

3. Ependymal cells.

4. microglia 

Based on their location in the nervous system, they are also categorized as the glial cells in the central nervous system and the neuroglia of the peripheral nervous system.

The five types of glial cells found in the central nervous system are:

• Astrocytes
• Oligodendrocytes
• Microglia
• Ependymal cells
• Radial glia

Glial cells are also found in the peripheral nervous system (PNS), which includes the nerves in the limbs that are not connected to the spine. There are two kinds of glial cells present there:

• Schwann cells 
• Satellite cells

The functions of each of these glial cells are briefly explained below.

1. Astrocytes: The astrocyte, also known as astroglia, is the most popular type of glial cell in the central nervous system. The “Astro” part of the name attributed to the idea that they resemble stars, with extrapolations strewn about. A few astrocytes, identified as protoplasmic astrocytes, have thick projections with numerous branches. Others, known as fibrous astrocytes, have lengthy, slender arms with comparatively fewer branches.

The protoplasmic type is commonly discovered among nerve cells in grey matter, whereas the fibrous type is usually found in white matter. Despite their differences, they serve functions that are similar. Some of their functions are described below.

Axon synchronization: This is the process of synchronizing the function of axons, which are long, thread-like pieces of nerve cells that conduct electricity to send signals from one cell to the next.

Energy metabolism and homeostasis in the brain: Astrocytes regulate brain metabolism by stockpiling glucose from the blood and supplying it as energy to nerve cells. It’s one of their most important duties.

Creating the blood-brain barrier (BBB): The Blood-brain barrier acts as a tight security system, only allowing compounds that are intended to be in the brain while managing to keep out chemicals that could be detrimental. This filtering procedure is critical to the health of the brain.

Neurotransmitter regulation: Neurotransmitters are chemical messengers that allow nerve cells to interact with one another. Neurotransmitters persist after the command is given until they are recycled by an astrocyte. Several medications, including antidepressants, focus on this reuptake procedure.

Cleaning it up: Astrocytes also remove what remains after a nerve cell dies, and also surplus potassium ions, which are substances that play an essential part in neurological function.  

Regulating blood circulation: The brain requires a certain level of blood flow to all of its various regions to complete tasks and interpret all the information that it receives properly. An active region receives more than one that is inactive.

2. Microglia: Microglia are small glial cells, as the name implies. They serve as the brain’s devoted immune system, which is required because the Blood-brain barrier separates the brain from the remaining parts of the body. 

Microglia are on the lookout for signs of damage and sickness. When they identify it, they begin charging in and deal with the issue, whether it’s cleaning up dead cells or eliminating a toxic substance or an infectious agent.

Microglia lead to inflammation as part of the process of healing when they react to harm. Further than that, microglia are thought to play an important part in performing many duties, including roles in learning-associated plasticity and directing brain growth, in which they play a significant housekeeping position.

The brains form a plethora of interconnected nerve cells that allow data to be transferred back and forth. In reality, the brain produces far more than we require, which is inefficient. Microglia identify and ‘cut off’ unneeded synapses, to keep them functioning healthily.

3. Radial Glia: Radial glia is thought to be a kind of stem cell, which means they generate new cells. They are the ‘parents’ of nerve cells, astrocytes, and oligodendrocytes in the growing brain (Figure 1).

They also supplied scaffolding for nerve cells that were continuing to develop when a person was just an embryo, all thanks to long fibres that direct and guide young brain cells into position as the brain shapes. Because of their duties as stem cells, particularly as nerve cell originators, they have become the topic of studies into how to rebuild harm inflicted on the brain by injury or illness. They also play a significant role in neuroplasticity later on in life.

4. Oligodendrocytes: Oligodendrocytes are formed by neural stem cells. The word is made up of Greek terms that indicate ‘a cell with many branches’. Their primary function is to aid in the movement of information along axons.

Oligodendrocytes have the appearance of spikey balls. There are white, shiny membranes that enclose the axons of nerve cells on the tips of their spikes. Their function is to form a protective layer.  The myelin sheath is the name given to this protective layer. However, the sheath is not continuous. The “node of Ranvier” is a gap between every membrane that aids in the efficient transmission of electrical signals along neurons.

5. Ependymal cells: Ependymal cells are best known for forming the ependyma, which is a thin membrane that lines the central canal of the spinal cord and the ventricles of the brain. They also produce cerebrospinal fluid and play a role in the Blood-brain barrier. Ependymal cells are incredibly tiny and form the surface by lining up firmly together. They possess cilia inside the ventricles, which resemble small hairs, moving in back and forth motion to circulate cerebrospinal fluid.

6. Schwann Cells: Schwann cells are named after Theodor Schwann, a physiologist who explored them. They act similarly to oligodendrocytes in the sense that they supply myelin sheaths for axons, but they are found in the peripheral nervous system (PNS) instead of the central nervous system (CNS).

Schwann cells form spirals directly all around the axon rather than being a central cell with membranes at the tips of their arms. The nodes of Ranvier are located between them, much like the nodes of Ranvier are located between the membranes of oligodendrocytes, and they similarly aid in nerve conduction.

Schwann cells are indeed immune cells in the Peripheral nervous system. When a nerve cell is injured, it tends to engulf the nerve’s axons and provide a safe route for a new axon to develop.

7. Satellite cells: Satellite cells are so-called because of the way they encircle specific nerve cells, with many satellites establishing a sheath all around the surface of the cells. 

Satellite cells are detected in the peripheral nervous system, whereas astrocytes are spotted in the central nervous system. The primary function of satellite cells seems to regulate the surroundings around nerve cells, keeping chemicals in a state of equilibrium (Figure 3).

Satellite cells feed the nerve cells and soak up heavy metal toxic substances like lead and mercury to stop them from harming the cells. Satellite cells, the same as microglia, react to inflammation and damage. Their involvement in repairing cellular damage, regrettably, is not well comprehended.

They are also thought to aid in the transport of many neurotransmitters and other chemicals, including Glutamate, Capsaicin, GABA, Acetylcholine, Norepinephrine, ATP, and Substance P.

Although many elements of these cells are now well understood, the functions of the various glial cells in the brain under pathological and physiological circumstances remain, at least to some degree, unanswered. They perform a variety of functions in the brain and the nerves that operate all across the body, according to important advancements. As a consequence, research has boomed, and scientists now know a lot about them. However, there is still a lot to learn.

Differences between neurons and glial cells:

Glial cells occur in both invertebrate and vertebrate nervous systems and are differentiated from nerve cells by the absence of axons and the existence of only one type of process. Neuroglia outnumbers nerve cells by a ten to one ratio in the nervous system. 

Furthermore, they do not develop synapses and maintain the power to divide throughout their lives. Even though nerve cells and neuroglia are in proximity, there are no direct junctional specializations between the two types (such as gap junctions).  There are gap junctions between glial cells.

Complications involving Glial Cells:

Glial cells play critical roles in the regulation of Central nervous system homeostasis, axon potential data transfer, and immune system response mediation. Demyelination is the loss of the myelin sheath, which causes nerve cells to reduce their capacity to transmit messages and signals properly.

Demyelination of the peripheral nervous system and the central nervous system can happen as a consequence of inflammatory disease, a viral disease, or a toxic attack.

When demyelination begins, myelin generated by oligodendrocytes is reduced from around axons, and the oligodendrocyte could die of apoptosis. Concurrently, B cells, T cells, and macrophages from the boundary are influx, as is stimulation and propagation of resident microglial cells in the Central nervous system.

Astrocytes play an important role in the development of demyelination. As myelin is reduced from around the axon, astrocytes propagate and upregulate glial fibrillary acidic protein (also abbreviated as GFAP), a characteristic of many Central nervous system diseases. 

There are several demyelinating illnesses. A few of these are:

• NMO   Neuromyelitis Optica
• MS: Multiple sclerosis
• CCPD: Combined central & peripheral demyelination
• Guillan-Barré Syndrome
• ADEM: Acute-hemorrhagic leucoencephalitis
• Peripheral nerve demyelination

Conclusion

Glial cells are also known as neuroglia or simply glia and are essentially any of several kinds of cells that principally focus on supporting nerve cells. Glial cells occur in both invertebrate and vertebrate nervous systems and are differentiated from nerve cells by the absence of axons and the existence of only one type of process. They occur in many forms in the CNS and PNS, such as astrocytes, Microglial cells, radial glial cells, ependymal cells, oligodendrocytes, Schwann cells, and Satellite cells and perform a variety of functions such as axon synchronization, maintaining the blood-brain barrier, acting as the brain’s immune system and forming the myelin sheath. Even though they are expected to support nerve cells, they are very important and their malfunctioning can cause serious problems, such as demyelination, which is the loss of the myelin sheath.

Reference list

• Dellwo, A. (2018) ‘What Are Glial Cells and What Do They Do?’ Verywell Health, 13 March. Available at: https://www.verywellhealth.com/what-are-glial-cells-and-what-do-they-do-4159734 (Accessed: 30 June 2021).
• Encyclopedia Britannica (2021) science | Definition, Disciplines, & Facts, 30 June. Available at: https://www.britannica.com/science/science (Accessed: 30 June 2021).
• MicroscopeMaster (2021) What is the Function and Location of Glial Cells?, Vs Neurons, 30 June. Available at: https://www.microscopemaster.com/what-is-the-function-and-location-of-glial-cells.html (Accessed: 30 June 2021).
• ResearchGate (2021) Figure 6. A radial glial cell and a migrating neuron. Modified from, 30 June. Available at: https://www.researchgate.net/figure/A-radial-glial-cell-and-a-migrating-neuron-Modified-from_fig5_47933701 (Accessed: 30 June 2021).
• https://www.abcam.com/pathways/glia-in-demyelinating-diseases (2021), 30 June (Accessed: 30 June 2021).
• https://www.quora.com/What-is-the-difference-between-satellite-cells-and-Schwann-cells-in-terms-of-structure-and-function (2021), 30 June (Accessed: 30 June 2021).