Types of Glial Cells

Glial cells are the most abundant cell type in your brain. Previously glial cells were thought to be non-functional glue for neurons, however, years of research have highlighted their key role in regulating neuron activity. The main glial cells are microglia, astrocytes and oligodendrocyte cells.

Glia of the nervous system are divided into two main types: white matter glia and grey matter glia. White matter glia support nerve fibers that transmit information between different regions of the brain, while grey matter glial cells are active in processing information. Glial cells make up about half of the brain and spinal cord tissue. Glial cells outnumber neurons in a ratio of about 10:1 and they are responsible for repairing damage to neurons, removing debris after neural injury and helping them communicate with each other. As well as this, glial cells are responsible for producing myelin which is a fatty material that insulates the axons of neurons. This myelin has been shown to have effects on the speed of action potential conduction.

Glia come in different shapes, but their basic function is the same for all types. There are three main shapes that glia take on: star-shaped astrocytes, small round oligodendrocytes and non-neural cells.

Microglial cells are brain cells that function as part of the immune system. Microglia act as the first line of defense against infections and pathogens, secreting inflammatory factors to protect surrounding tissue. They also act as scavengers, clearing cellular debris and infectious agents.  

Microglial cells can be found throughout the entire brain with particularly high concentrations in the meninges and choroid plexus, with sparse expression across cortical layers and scarcely any expression in white matter. Microglial dysfunction has been implicated in a number of neurological diseases including Parkinson's disease, Alzheimer's disease (AD), Multiple Sclerosis (MS), stroke, amyotrophic lateral sclerosis (ALS) and Huntington's disease as well as in acute neurodegenerative conditions such as trauma or epilepsy. Some common markers for microglia are the expression of CD45, CX3CR1, ASC, HS1, TMEM119, CD11b and IBA1/AIF-1.

Astrocytes have many functions including regulation of blood flow, provision of nutrients to surrounding tissue, clearance of neurotransmitters from the extracellular space, regulating neuronal synapses and maintaining extracellular ion concentrations by buffering potassium ions. Astrocytes also express glutamate receptors on their cell membrane to allow for direct communication between neurons and astrocytes in a process called synaptically-driven astrocyte calcium waves or SDACs.

Some common astrocyte markers are glial fibrillar acidic protein (GFAP) and the gap junction protein connexin 30 (Cx30). As well as this, there are markers for mature astrocytes such as aquaporin 4, aldolase C (AldoC), glutamate transporter-1 (Glt1), S100 calcium-binding protein B (S100b) and aldehyde dehydrogenase family 1 member L1 (Aldh1L1).

Oligodendrocytes are a type of glial cell in the central nervous system. Oligodendrocytes have very few branches compared to neurons. Oligodendrocytes are responsible for forming the myelin sheath surrounding axons. Oligodendrocytes also supply neurons with nutrients and oxygen and they remove waste products from neurons. Oligodendrocytes are produced in large numbers by oligodendrocyte stem cells, as well as Oligodendrocyte progenitor cells that originate from Oligodendrocyte precursor cells (OPCs).

Some common markers for Oligodendrocytes are 2’,3’-cyclic nucleotide 3’-phosphohydrolase (CNPase), proteolipid protein (PLP1), myelin basic protein (MBP), Sox10, myelin oligodendrocyte glycoprotein (MOG) and carbonic anhydrase II.

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