| Type of glial cell | Origin | Appearance | Functions |
|---|---|---|---|
| Astrocytes | Neuroectoderm | Round vesicular nuclei Contain glial fibrils, composed of glial fibrillary acidic protein | • Repair • Structural & metabolic support • Blood-brain barrier • Remove excess neurotransmitters, related with Hyperammonemia |
| Oligodendrocytes | Neuroectoderm | Small nuclei surrounded by a pale halo Fewer processes than astrocytes | • Myelinate axons in the CNS, including CN II • Main glial cells in the cerebral white matter |
| Microglia | Primitive yolk sac macrophages | Small elongated nuclei Many short branching processes | Special macrophages in CNS, i.e. phagocytic |
| Ependymal cells | Neuroepithelial cells (neuroectoderm) | Simple columnar glial cells | Form the epithelium lining the ventricles and central canal of the spinal cord |
Mnemonic
Glial cells guard the axons of the nerve cells as COPS: CNS axons are myelinated by Oligodendrocytes; PNS axons are insulated by Schwann cells. Oligodendrocytes → multiple sclerosis Schwann cells → Guillain-Barré syndrome
CNS Glial Cells
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Astrocytes
- Function: Most abundant glial cell in the CNS. They provide structural and metabolic support, form the blood-brain barrier (BBB), regulate the extracellular environment by taking up K+ and neurotransmitters (e.g., glutamate), and contribute to scar formation (gliosis) after injury.
- Key Features: Star-shaped cells. Marker: Glial Fibrillary Acidic Protein (GFAP). Communicate via gap junctions.
- Clinical Correlation: Glioblastoma Multiforme (GBM) is the most common and aggressive primary brain tumor in adults, composed of astrocytes. It can cross the corpus callosum, creating a “butterfly glioma” appearance on MRI. Reactive gliosis is the brain’s response to traumatic injury.
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Oligodendrocytes
- Function: Myelinate axons in the CNS. A single oligodendrocyte can myelinate multiple axons (up to 30-50).
- Key Features: Derived from the neuroectoderm. On histology, they often have a “fried egg” appearance with a round nucleus and clear cytoplasm.
- Clinical Correlations:
- Damaged in demyelinating diseases like Multiple Sclerosis (MS).
- Also affected in progressive multifocal leukoencephalopathy (PML) and leukodystrophies.
- Oligodendrogliomas are slow-growing tumors, often found in the frontal lobes.
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Microglia
- Function: Act as the resident macrophages of the CNS, responsible for phagocytosis of debris, damaged cells, and pathogens. They are involved in immune surveillance and synaptic pruning.
- Key Features: Originate from the mesoderm (yolk sac). When activated by injury or inflammation, they change shape and proliferate.
- Clinical Correlation: Microglia are activated in response to CNS injury and infection. They play a role in neurodegenerative diseases like Alzheimer’s.
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Ependymal Cells
- Function: Line the ventricles of the brain and the central canal of the spinal cord. Their cilia help to circulate cerebrospinal fluid (CSF), and they participate in its production.
- Key Features: Ciliated simple columnar or cuboidal epithelial-like cells. Form a somewhat permeable barrier between CSF and the brain parenchyma.
- Clinical Correlation: Ependymomas are glial tumors that most commonly arise in the 4th ventricle in children.
PNS Glial Cells
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Schwann Cells
- Function: Myelinate axons in the PNS. One Schwann cell myelinates only one segment of a single axon. They also aid in nerve regeneration after injury.
- Key Features: Derived from the neural crest.
- Clinical Correlations:
- Destroyed in Guillain-Barré syndrome, an acute inflammatory demyelinating polyneuropathy.
- Schwannomas are benign tumors of Schwann cells. A common example is a vestibular schwannoma (acoustic neuroma) at the cerebellopontine angle, which is S-100 positive.
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Satellite Cells
- Function: Surround the cell bodies of neurons in sensory and autonomic ganglia, providing support and regulating the microenvironment. Their function is analogous to astrocytes in the CNS.
- Key Features: Small cells closely associated with neuron cell bodies in the PNS.