Microglia Function and Neurodegenerative Diseases

Study Notes

Microglia are the immune defense cells of the CNS and are similar to monocytes, a type of white blood cell.
They are derived from the same bone-marrow tissue that gives rise to monocytes.
In the resting state, microglia are wispy cells with many long branches that radiate outward.
They release low levels of growth factors, such as nerve growth factor, which help neurons and other glial cells survive and thrive.
Activated microglia release destructive chemicals to assault against their target, which can damage neurons they are meant to protect, contributing to neurodegenerative diseases.
Microglia are the only CNS cell type that can be infected by HIV, leading to AIDS-related dementia.

Ependymal cells line the internal, fluid-filled cavities of the CNS and help form cerebrospinal fluid.
They have cilia that beat to contribute to the flow of cerebrospinal fluid through the ventricles.
Ependymal cells serve as neural stem cells with the potential to form new neurons, especially after injury.
They can form other glial cells, and unlike neurons, they do not lose the ability to undergo cell division.

The brain is not supplied by the lymphatic system, but instead uses the glymphatic system for waste clearance.
Astrocytes facilitate a cleansing fluid exchange within spaces between microscopic brain blood vessels and long astrocyte projections.
This "brainwashing" increases during sleep.

Oligodendrocytes form the insulative myelin sheaths around axons in the CNS.
Each oligodendrocyte has several elongated projections that wrap around a section of an interneuronal axon to form a patch of myelin.

Glial cells help support neurons both physically and metabolically.
They maintain the composition of the specialized extracellular environment surrounding neurons within optimal limits.
They actively modulate synaptic function and are considered crucial to learning and memory.
There are four major types of glial cells in the CNS: astrocytes, oligodendrocytes, microglia, and ependymal cells.

Astrocytes are the most abundant glial cells and fill several critical functions.
They hold neurons together in proper spatial relationships, serving as the "glue" of the CNS.
They guide neurons to their proper final destination during fetal brain development.
They induce the formation of the blood-brain barrier, a highly selective and protective barricade between the blood and brain.
They help transfer nutrients from the blood to the neurons and form neural scars to repair brain injuries.