Microglia Function and Neurodegenerative Diseases

Microglia

• 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

• 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.

Protection and Nourishment of the Brain

• 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

• 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

• 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

• 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.