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.
|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|
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.