About glia

This is the first post of several I intend to make about glia, a class of neural (but not neuronal) cells that I really don't know enough about yet. Now, having become rather interested in synapses, I've come to see the need to know more about the cell basics.

I started out with the Encyclopedia of the Human Brain. In Volume 3, p. 480, I gained a sense of proportion. Glial numbers exceed neurons by a factor of 10 to 1. They account for 50% of the volume of the entire brain. Volume, not numbers. They are tinier.


p. 406, I learned a bit about four main types:

INTRODUCTION:
- Glia are far more abundant than neurons in the brain
- Glia of the brain and spinal cord are classified into four types:
1. astrocytes
2. oligodendrocytes
3. microglia
4. ependymal cells


Astrocytes
- are starshaped glial cells found in both gray and white matter
- have a role in the mechanical support of neurons
- contribute to metabolic regulation of the micro environment of the brain
- participate in its response to injury

Oligodendrocytes
- are confined mainly to white matter
- are responsible for the myelination of brain axons (as Schwann cells are in the PNS)

Microglia
- are small cells found in gray and white matter
- serve as the phagocytes of the brain
- migrate as necessary to damaged areas where they consume pathogens and neuronal debris

Ependymal cells
- line the ventricles of the brain
- at a specialized structure called the choroid plexus (one of which is found in each ventricle) they form a secretory epithelium that produces the CSF that fills the ventricles and bathes the entire CNS

There is some debate about their origins. For now I'm going to go with neural crest being their parent progenitor, but will bring the different opinions here later. Microglia pose the biggest departure, because they are scavengers, macrophagic in behavior, thought to come possibly from a hemopoietic source. However, I wouldn't put it past neural crest to be quite capable of making a version of brain cell that behaves just like a macrophagic cell that originates with mesoderm.

Further reading:

1. NIH Public Access: Glial cells: Old cells with new twists (2008) (mostly about oligodendrocytes)

2. Neurophilosophy: Nerve glue comes unstuck
- Background on Rudolf Ludwig Karl Virchow (who was responsible for suggesting that glial cells were merely filler, and whose other claim to fame was that washing one's hands to prevent spread of infection was not important.)

3. A Wikipedia link explaining membrane proteins, connexins (small) and connexons (larger, 6 connexins from each cell forming a gap junction between two cells)

4. Neurophilosophy: Starring role in the brain for astrocytes

5. Neurophilosophy: Astrocytes take center stage in brain function

6. Neurophilosophy: Getting a grip on cerebral bloodflow

7. Neurophilosophy: Six iconoclastic discoveries about the brain

(How can you tell I'm a huge Neurophilosophy fan?)

8. Fifty-eight page paper about oligodendroctyes by Nicole Baumann and Danielle Pham-Dihn: Biology of Oligodendrocyte and Myelin in the Mammalian Central Nervous System (2001)

(These two authors also have a chapter on astrocytes in Encyclopedia of the Human Brain)