Neuroglia, glial cells or, to their family and friends, simply glia are the connective tissue cells of nervous system. The term “glia” comes from the Greek for glue. However, the old axiom that these cells do nothing more than “glue” the neurons together has long been disproven. Let’s learn more about glial cells generally and look at their different types. 

What Do Glial Cells Do?

  • Maintain the ionic environment surrounding neurons.
  • Control the rate of neuron signal propagation.
  • Establish, maintain and repair synapses.
  • Assist (or hamper!) the recovery neurons post damage.

How Are They Different From Neurons?

  • Glia can’t generate electrical impulses.
  • There are far more glia than neurons. In fact, 90% of the cells of the CNS are glia.
  • Even with the long tendrils that extend from glia, they’re generally smaller than neurons.
  • Glia lack axons and dendrites.

Types of Glial Cells

Roughly speaking, there are 6 types of glial cells: astrocytes, microglia, satellite, ependymal, oligodendrocytes and Schwann cells. These are all located in either the Central Nervous System (CNS) or the Peripheral Nervous System (PNS). A seventh type is the Pituicyte of the pituitary gland, which sadly often gets forgotten, it not living in either the CNS or PNS. Macroglia is used to describe cells that live in the CNS but are not neurons or microglia. In our list below, astrocytes and oligodendrocytes would be counted as the macroglia.

Let’s learn about them all here! To help you avoid getting too bogged down in the subtypes, we’ll just give an overview of them for now.

Types Subtypes Location About Functions
Astrocytes (subtypes)

Fibrous

CNS: mainly located in the white matter

Long processes associate with blood vessels

Function is unclear but associated with blood vessels.

Protoplasmic.

CNS

Highly branched, tree-like appearance

Processes finish in “endfeet” which wrap around blood vessels. This is called the glial limiting membrane and it is the outermost wall of the blood brain barrier (BBB). Also play a role in blood flow regulation and the formation and elimination of synapses.

Layer-1

CNS

Similar appearance to fibrous astrocytes

Form the glial limiting membrane.

Interlaminar

CNS: processes extend into the cortical gray matter

Straight and poorly branched processes. May be derived from layer 1 cells

Function unknown.

Varicose projection

CNS: layers 5 and 6 of the cortex

Very long processes with many varicosities along projections

Function unknown.

Oligodendrocytes

Interfascicular

CNS: White matter between myelin sheaths

Similar in shape to astrocytes but with fewer protuberances

Myelinate axons in the CNS

Perineuronal satellite oligodendrocytes

CNS: Grey matter next to neurons

Perivascular oligodendrocytes

Along blood vessels

Function in humans unknown.

Ependymocytes

Can specialise into tanycytes in the  median eminence of the hypothalamus

CNS: line CSF-filled ventricles

Ciliated and columnar in appearance

Form epithelial linings. Tanycytes may transfer chemical signals from the CSF to the brain

Schwann cells

Myelinating

PNS

Roughly looks like a rolled up newspapers surrounding nerve cells.

Facilitate myelin production, neuronal homeostasis and antigen-presentation to T cells

Non-myelinating

Less defined but also wrapped around nerve cells.

Axon maintenance

Microglia

Ramified (develop from Amoeboid)

CNS

Numerous branching processes; are capable of very localized movement and pinocytosis

Specialized macrophages that munch up cell debris and play a role in immunodefense. They also maintain the CNS.

Reactive

Reactive microglia are rod-like. Quiescent until damage is detected.

Pituicyte

Posterior of the pituitary gland

Capable of huge morphological plasticity depending on extracellular chances

Role in storage and release of neurohypophysial hormones, neuronal maintenance and autophagy

Satellite glial cells

PNS

Similar roles to astrocytes

Surround individual sensory and parasympathetic neurons with a complete, unbroken sheath to regulate the microenvironment of neurons

Conclusion

Today, we learnt that glial cells are far more than just scaffolds for neurons and that there are approximately 7 types of them (and far too many subtypes!). For more information on astrocytes and microglia, see our previous articles.


Article by Olwen Reina. Contact Olwen at olwen@tempobioscience.com.
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