Medical School · Year 1 · Histology · includes a quiz and discussion video

Lecture 10: Nervous Tissue - Glial Cells

Unit 1.2: Histology and Basic Tissues

Learning Objectives

By the end of this lecture, students will be able to:

1. Identify the different types of glial cells in the CNS (astrocytes, oligodendrocytes, microglia, ependymal cells) and their functions
2. Describe the structure and function of Schwann cells and satellite cells in the PNS
3. Explain the process of myelination in both CNS and PNS
4. Describe the structure and function of the blood-brain barrier
5. Explain the role of glial cells in pathological conditions
6. Distinguish different glial cell types in histological sections

Introduction to Glial Cells

The term "glia" derives from the Greek word for glue, reflecting the early misconception that these cells merely filled space between neurons. We now understand that glial cells, also called neuroglia, are essential partners to neurons in virtually every aspect of nervous system function. Glial cells outnumber neurons in the nervous system, though estimates of the ratio vary by region from roughly equal to perhaps ten-fold excess. Unlike neurons, glial cells do not generate action potentials, yet they are indispensable for neural function.

Glial cells perform diverse and critical functions in the nervous system. They provide structural scaffolding that organizes neural tissue and supports neuronal architecture. They deliver metabolic support to neurons, supplying energy substrates and removing waste products. Specialized glial cells produce the myelin sheaths that enable rapid signal conduction. Glial cells maintain ionic and chemical homeostasis in the extracellular environment, regulate synaptic transmission, and provide immune surveillance. During development, glial cells guide migrating neurons and growing axons to their targets.

The nervous system contains several distinct types of glial cells, divided between the central and peripheral nervous systems. In the CNS, astrocytes provide metabolic support and homeostatic regulation; oligodendrocytes produce myelin; microglia serve as immune cells; and ependymal cells line the ventricular system. In the PNS, Schwann cells provide myelination and axon support, while satellite cells surround neuronal cell bodies in ganglia.

<image>Panel A: Central diagram of brain and spinal cord in blue as CNS with peripheral nerves extending outward in green as PNS showing overall nervous system organization. Panel B: CNS glial cells showing large purple star-shaped astrocyte with end-feet contacting red blood vessel, smaller blue-gray oligodendrocyte wrapping multiple axons, small brown ramified microglia with fine surveying processes, and pink cuboidal ependymal cells with cilia lining ventricle. Panel C: PNS glial cells showing gold Schwann cell wrapping single axon segment with visible myelin layers and light purple satellite cells surrounding large pseudounipolar neuron in ganglion. Panel D: Gray central neuron shown for scale with all glial types in relative positions with each cell type labeled with name and primary function at 20 micrometer scale.</image>

CNS Glial Cells

Astrocytes

Astrocytes are the largest and most numerous glial cells in the central nervous system, deriving their name from their characteristic star-shaped appearance. These versatile cells perform more functions than any other glial type, and their importance to neural function has become increasingly appreciated.

Two main types of astrocytes exist, distinguished by their location and morphology. Protoplasmic astrocytes reside in gray matter and possess numerous short, highly branched processes that give them a bushy appearance. These cells intimately associate with neurons, synapses, and blood vessels in regions of dense neuronal activity. Fibrous astrocytes are found in white matter and have fewer, longer, and straighter processes. Their morphology reflects the more linear organization of myelinated fiber tracts they inhabit.

All astrocytes share certain structural features. Their cytoplasm contains abundant intermediate filaments composed of glial fibrillary acidic protein, commonly known as GFAP, which serves as the definitive immunohistochemical marker for astrocytes. A distinctive feature of astrocytes is their end-feet, specialized expansions at the terminations of their processes. Vascular end-feet completely ensheath the capillaries of the brain, while other processes contact neurons at synapses and nodes of Ranvier.

The functions of astrocytes are remarkably diverse. They provide structural support, creating a scaffold that maintains tissue architecture. Their vascular end-feet are essential components of the blood-brain barrier, inducing and maintaining the specialized properties of brain capillaries. Astrocytes provide metabolic support to neurons through the lactate shuttle, converting glucose to lactate and shuttling it to neurons as an energy substrate. They maintain ionic homeostasis by buffering extracellular potassium that accumulates during neuronal activity. They clear neurotransmitters, particularly glutamate, from the synaptic cleft, preventing excitotoxicity. When the nervous system is injured, astrocytes proliferate and form glial scars through a process called reactive gliosis, walling off damaged areas but also potentially impeding axon regeneration. Current research has revealed that astrocytes participate in synaptic function as part of the "tripartite synapse," responding to and modulating synaptic transmission.

<image>Panel A: Protoplasmic astrocyte with large irregular nucleus and pale cytoplasm with numerous purple branching processes extending in all directions and darker purple expanded end-foot contacting red blood vessel with endothelial cells, tight junctions, and basement membrane. Panel B: Process extending toward synapse forming tripartite synapse with glutamate transporters removing neurotransmitter plus fibrous astrocyte comparison inset with longer straighter processes and brown GFAP immunostain corner panel. Panel C: Potassium ion K-plus symbols showing buffering by astrocyte plus lactate shuttle diagram with glucose entering from blood vessel, conversion to lactate, and transfer to neuron. Panel D: All structures and functions labeled at 15 micrometer scale showing astrocyte roles in metabolic support, ionic homeostasis, and synaptic function.</image>

Oligodendrocytes

Oligodendrocytes are the myelinating cells of the central nervous system, responsible for producing the myelin sheaths that insulate axons and enable rapid saltatory conduction. Their name, meaning "cells with few branches," reflects their simpler morphology compared to astrocytes, though this somewhat understates their functional complexity.

Oligodendrocytes are found predominantly in white matter, where they extend processes to myelinate the axons of fiber tracts. However, they also exist in gray matter as perineuronal satellites, closely associated with neuronal cell bodies. The oligodendrocyte cell body is smaller than that of astrocytes and contains a distinctive round nucleus that often shows a characteristic perinuclear halo in histological sections, an artifact of fixation and processing that nonetheless aids identification.

The defining capability of oligodendrocytes is their ability to myelinate multiple axons simultaneously. A single oligodendrocyte can extend up to fifty separate processes, each of which wraps around a different axon segment to form one internode of myelin. This is a fundamental difference from the peripheral nervous system, where each Schwann cell myelinates only one segment of one axon.

The myelination process involves the oligodendrocyte process contacting an axon segment, then wrapping around it multiple times in a spiral fashion. As wrapping proceeds, the cytoplasm is squeezed out from between membrane layers, producing the compact myelin sheath consisting of tightly apposed lipid bilayers. This myelin is rich in lipid, giving white matter its characteristic appearance. The major proteins of CNS myelin include myelin basic protein (MBP) and proteolipid protein (PLP), which are essential for myelin compaction and stability.

Between adjacent oligodendrocyte processes along an axon lie the nodes of Ranvier, gaps where the axon membrane is exposed. The remarkable efficiency of saltatory conduction, with action potentials jumping from node to node, depends on this precise arrangement of myelinated internodes and exposed nodes.

<image>Panel A: Central blue-gray oligodendrocyte cell body with round nucleus and indicated perinuclear halo artifact with multiple blue processes extending outward each connecting to different gold-tan axon. Panel B: Detailed inset of single process wrapping around axon in cross-section showing multiple concentric blue membrane layers with cytoplasm squeezed out to form dark blue compact myelin. Panel C: Longitudinal view of myelinated axon showing internode myelin segment, node of Ranvier gap with exposed axon membrane, and next internode from different oligodendrocyte process. Panel D: Comparison diagram showing single oligodendrocyte serving multiple axon segments versus Schwann cell serving only one with MBP and PLP myelin proteins labeled at internode length approximately 1 mm and node width 1-2 micrometers.</image>

Microglia

Microglia are unique among CNS glial cells in their origin and function. While all other glia arise from neuroectoderm, microglia derive from bone marrow-derived monocytes that colonize the brain during embryonic development. These cells serve as the resident immune cells of the central nervous system, performing continuous surveillance and responding to injury or infection.

The morphology of microglia changes dramatically depending on their functional state. In healthy tissue, resting or surveying microglia have small, elongated cell bodies with numerous fine, branching processes. These ramified processes are in constant motion, extending and retracting as they sample the local environment for signs of damage, pathogens, or abnormal cellular debris. This surveillance function allows rapid detection of problems anywhere in the neural parenchyma.

When activated by injury, infection, or disease, microglia transform from their ramified resting state to an amoeboid activated form. The elaborate processes retract, the cell body enlarges and becomes rounded, and the cell acquires pronounced phagocytic capability. Activated microglia migrate toward sites of damage, engulf debris and pathogens, and release cytokines and other inflammatory mediators. They can also present antigens to T lymphocytes, linking the innate immune response of the CNS to adaptive immunity.

Beyond their immune functions, microglia participate in normal neural development and plasticity through synaptic pruning. They selectively eliminate unnecessary or weak synapses, sculpting neural circuits during development and potentially during learning in the adult brain. This process involves complement proteins that tag synapses for elimination.

Microglia can be identified in tissue sections using markers such as CD68, Iba1, and CD11b, which distinguish them from other glial types and allow assessment of their activation state.

<image>Panel A: Resting surveying microglia with small elongated brown cell body and multiple fine ramified processes extending in all directions sampling environment with arrows indicating dynamic process tip movement. Panel B: Transformation sequence showing cell body enlarging and processes retracting with arrow leading to activated amoeboid microglia as large rounded dark brown body with short stubby processes and visible phagocytic vacuoles containing irregular debris particles. Panel C: Function icons around activated cell showing phagocytosis engulfing bacteria, cytokine release as small emanating dots, and T-cell interaction antigen presentation diagram. Panel D: Synaptic pruning function showing microglia process contacting and engulfing complement-tagged synapse plus Iba1 brown immunohistochemistry corner panels of ramified and activated microglia at 10 micrometer scale.</image>

Ependymal Cells

Ependymal cells form a specialized epithelial lining of the ventricular system of the brain and the central canal of the spinal cord. These cells create the interface between the brain parenchyma and the cerebrospinal fluid that fills these cavities.

The basic ependymal cell, or ependymocyte, is a cuboidal to columnar epithelial cell arranged in a simple single-layered epithelium. The apical surface facing the ventricle bears cilia whose coordinated beating helps circulate cerebrospinal fluid through the ventricular system. Microvilli interspersed among the cilia increase the surface area for exchange between CSF and brain tissue. Adjacent ependymal cells are connected by gap junctions that allow communication and some degree of coupling, but notably, they lack the tight junctions that characterize most blood-tissue barriers. This means the ependymal layer provides only a limited barrier between CSF and brain.

Specialized ependymal cells exist in certain locations. Tanycytes are elongated ependymal cells found particularly along the floor of the third ventricle. Unlike typical ependymocytes, tanycytes extend long processes deep into the hypothalamus, where they contact neurons and blood vessels. These cells are thought to sense the composition of CSF and relay information to hypothalamic nuclei that regulate feeding, metabolism, and hormone release.

The choroid plexus represents a critically important modification of the ependyma. In specific locations in each ventricle, the ependymal lining becomes associated with a core of highly vascularized connective tissue, creating a cauliflower-like structure that protrudes into the ventricle. The ependymal cells covering the choroid plexus are called choroidal epithelial cells and are specialized for CSF production. Unlike the fenestrated capillaries they cover, choroidal epithelial cells possess tight junctions, forming the blood-CSF barrier. These cells actively secrete CSF at a rate of approximately 500 milliliters per day in adults, completely replacing the CSF volume several times daily.

<image>Panel A: Ependymal lining of ventricle showing simple pink cuboidal-columnar cells with hair-like cilia and microvilli on apical surface facing light blue ventricular space with basal surface contacting brain parenchyma and gap junctions between cells without tight junctions. Panel B: Tanycyte as elongated ependymal cell with single cilium and long basal process extending toward hypothalamic neuron and blood vessel for sensing CSF composition. Panel C: Choroid plexus detail showing cauliflower-like ventricle projection with tan connective tissue core containing red fenestrated capillaries with pores covered by darker pink cuboidal choroidal epithelial cells with zigzag tight junctions. Panel D: Fluid movement arrows from blood through epithelium into CSF plus gross choroid plexus inset in brain ventricle and CSF circulation direction flow arrows with all structures labeled.</image>

PNS Glial Cells

Schwann Cells

Schwann cells are the principal glial cells of the peripheral nervous system, analogous in many ways to oligodendrocytes but with important differences that reflect the distinct requirements of peripheral nerves. Like other PNS glia, Schwann cells derive from the neural crest during embryonic development.

Schwann cells exist in two functionally distinct forms: myelinating Schwann cells that wrap large-diameter axons in myelin, and non-myelinating Schwann cells that support smaller axons without producing myelin.

Myelinating Schwann cells associate with large axons, typically those greater than one micrometer in diameter. Unlike oligodendrocytes, which myelinate segments of many different axons, each myelinating Schwann cell wraps only one segment (one internode) of one axon. The Schwann cell spirals around the axon many times, its cytoplasm being progressively squeezed out to form compact myelin. The nucleus and bulk of the cytoplasm become displaced to the outer surface of the myelin sheath, where they form a visible bulge. Nodes of Ranvier occur at the junctions between adjacent Schwann cells, where the axon membrane is exposed for saltatory conduction.

A distinctive feature of Schwann cells, not shared by oligodendrocytes, is the presence of a basal lamina that surrounds each Schwann cell and its associated axon. This basal lamina has crucial importance for nerve regeneration: after peripheral nerve injury, Schwann cells proliferate and align within their basal lamina tubes, forming bands of Büngner that guide regenerating axons back to their targets.

Non-myelinating Schwann cells envelop multiple small-diameter axons without forming myelin. In this arrangement, called a Remak bundle, the Schwann cell cytoplasm invaginates to create separate troughs for each axon, but no spiral wrapping or myelin production occurs. Small unmyelinated axons, including many sensory fibers and postganglionic autonomic fibers, travel in these bundles.

The proteins of PNS myelin differ somewhat from those in the CNS. While myelin basic protein is shared, the major transmembrane protein of peripheral myelin is protein zero (P0), and peripheral myelin protein 22 (PMP22) is also important. Mutations in these proteins cause inherited peripheral neuropathies.

<image>Panel A: Myelinating Schwann cell showing large gold axon greater than 1 micrometer diameter surrounded by spiral blue-purple compact myelin layers with dark purple nucleus displaced to outer surface as visible bulge and thin brown basal lamina surrounding entire unit. Panel B: Cross-section showing concentric myelin layers plus longitudinal view of internode with node of Ranvier between two Schwann cells each providing one segment. Panel C: Non-myelinating Schwann cell Remak bundle showing single light purple cell with multiple small gold axon circles less than 1 micrometer embedded in cytoplasmic invaginations without myelin in separate channels. Panel D: Comparison table of myelinating with one axon per cell and myelin for large axons versus non-myelinating with multiple axons per cell and no myelin for small axons plus Bands of Bungner injury inset showing Schwann cells aligned in basal lamina tubes guiding regenerating axon at 5 micrometer scale.</image>

Satellite Cells

Satellite cells are the glial cells associated with neuronal cell bodies in peripheral ganglia, including dorsal root ganglia that contain sensory neuron cell bodies and autonomic ganglia that contain postganglionic autonomic neuron cell bodies. These cells should not be confused with the satellite cells of skeletal muscle, which are myogenic progenitor cells with an entirely different function.

In peripheral ganglia, satellite cells form a complete capsule around each neuronal cell body, creating a microenvironment that separates the neuron from surrounding connective tissue. Satellite cells are typically flattened or squamous in shape, with their nuclei often visible as a ring around the much larger neuronal cell body in histological sections.

The functions of satellite cells parallel many of those performed by astrocytes in the CNS. They provide structural support, surrounding and protecting the neuronal soma. They regulate the microenvironment around the cell body, controlling ionic composition and exchanging metabolites with the neuron. They provide a degree of electrical insulation. When neurons are damaged or during pathological states, satellite cells can proliferate and undergo reactive changes similar to the gliosis seen with astrocytes.

In dorsal root ganglia, the relationship between satellite cells and the pseudounipolar sensory neurons they surround is particularly important. Any signal exchange between the cell body and its central and peripheral processes must occur across the satellite cell envelope, and these cells are increasingly recognized as active participants in sensory processing and pain states.

<image>Panel A: Dorsal root ganglion section showing large light purple pseudounipolar neuron cell bodies at 40-80 micrometers each completely surrounded by ring of darker purple flattened satellite cells with visible nuclei as distinct neuron-satellite modules. Panel B: High magnification inset of single neuron with large pale nucleus and prominent nucleolus surrounded by satellite cell envelope with 2-3 visible satellite cell nuclei forming complete shell at 20 micrometer scale. Panel C: H&E appearance panel showing typical histology with pink neuronal cytoplasm and small dark satellite nuclei at periphery plus autonomic ganglion diagram with smaller neurons similarly surrounded by satellite cells. Panel D: Functions listed including structural support, metabolic exchange with glucose-lactate arrows, ionic regulation with K-plus buffering, and electrical insulation at 50 micrometer overview scale.</image>

Myelination

The Purpose of Myelin

Myelination is one of the most important adaptations of the vertebrate nervous system. The myelin sheath is a specialized extension of glial cell membrane that wraps around axons in multiple layers, creating an insulating structure that dramatically increases the speed of action potential conduction while simultaneously reducing energy expenditure.

In unmyelinated axons, action potentials propagate by continuous conduction, with voltage-gated sodium channels opening sequentially along the entire length of the axon. This process is relatively slow, typically around 1 meter per second for small unmyelinated fibers, and energetically expensive because sodium must be pumped back out along the entire axonal membrane.

Myelinated axons conduct via saltatory conduction, from the Latin word "saltare" meaning to jump. The myelin sheath provides such effective electrical insulation that the depolarizing current from one node of Ranvier can flow through the axoplasm to depolarize the next node, approximately one millimeter away, without significant current loss through the internodal membrane. Action potentials effectively jump from node to node, achieving conduction velocities up to 100 meters per second or more in large myelinated fibers. Because voltage-gated channels and the sodium-potassium ATPase operate only at the nodes, energy expenditure is dramatically reduced.

Comparison of CNS and PNS Myelination

While the principle of myelination is the same in both central and peripheral nervous systems, important differences exist in the cells involved and the organization of myelin.

In the CNS, oligodendrocytes produce myelin. Each oligodendrocyte can myelinate segments of up to fifty different axons, with separate processes extending to different axon segments. CNS myelin lacks a basal lamina around the myelinating cells. The major proteins are myelin basic protein and proteolipid protein. When CNS myelin is damaged, as in multiple sclerosis, regeneration is extremely limited.

In the PNS, Schwann cells produce myelin. Each myelinating Schwann cell wraps only one segment of one axon. The entire Schwann cell-axon unit is surrounded by a basal lamina. The major proteins include myelin basic protein, protein zero, and peripheral myelin protein 22. The PNS has much greater capacity for remyelination after injury, with Schwann cells proliferating, clearing debris, and remyelinating regenerated axons.

Structure of the Node of Ranvier

The node of Ranvier is the short gap, typically one to two micrometers wide, between adjacent myelin segments where the axon membrane is exposed to the extracellular space. This specialized region is critical for saltatory conduction and has a highly organized molecular architecture.

The nodal axon membrane contains an extremely high density of voltage-gated sodium channels, approximately 1000 times the density found in unmyelinated axons. This concentration of channels ensures robust action potential regeneration at each node. The channels are anchored by scaffold proteins linked to the cytoskeleton.

Flanking the node are the paranodal regions, where the terminal loops of the myelin sheath form septate-like junctions with the axon membrane. These junctions create a barrier that prevents lateral diffusion of nodal proteins. Beyond the paranodes are the juxtaparanodal regions, which contain clusters of voltage-gated potassium channels that help repolarize the membrane and prevent repetitive firing.

<image>Panel A: Central node of Ranvier as 1-2 micrometer gap with exposed gold axon membrane containing densely packed red voltage-gated sodium channel rectangles with paranodal regions on either side where blue myelin terminal loop semicircles form junctions with axon membrane. Panel B: Juxtaparanodal regions containing blue potassium channel rectangles with compact myelin internode extending outward on each side plus cross-section inset showing multiple myelin layers wrapped around axon. Panel C: Current flow diagram showing inward sodium current at active node, local circuit current flowing through axoplasm to next node, and outward capacitive current at next node triggering new action potential. Panel D: Speed comparison of unmyelinated conduction at 1 m/s versus myelinated conduction at 100 m/s with Schwann cell or oligodendrocyte nucleus indicated at outer surface at 2 micrometer node region scale.</image>

The Blood-Brain Barrier

The blood-brain barrier is a selective permeability barrier that separates the circulating blood from the brain extracellular fluid. This barrier maintains the precise ionic and chemical composition required for proper neural function, protects the brain from circulating toxins and pathogens, and prevents free exchange of molecules between blood and brain.

The structural basis of the blood-brain barrier involves multiple cell types working together. The primary barrier is formed by the endothelial cells of brain capillaries, which differ fundamentally from endothelial cells elsewhere in the body. Brain endothelial cells are connected by continuous tight junctions that prevent paracellular diffusion of molecules between cells. They also have very few pinocytic vesicles, limiting transcellular transport. Unlike the fenestrated or discontinuous endothelium of many peripheral capillaries, brain endothelium forms a continuous, sealed layer.

The endothelium rests on a basement membrane that provides structural support. Embedded within this basement membrane are pericytes, contractile cells that can regulate capillary diameter and contribute to barrier function. Importantly, astrocyte end-feet cover more than 99 percent of the capillary surface. While astrocyte end-feet do not themselves form a barrier, they are essential for inducing and maintaining the barrier properties of brain endothelium.

The blood-brain barrier is highly selective about what it allows to cross. Small lipophilic molecules such as oxygen, carbon dioxide, and ethanol diffuse freely across the endothelial membrane. Essential hydrophilic molecules that cannot cross by diffusion are transported by specific carrier systems: glucose enters via the GLUT1 transporter, amino acids via various amino acid transporters. Large molecules such as proteins, most ions, and most drugs cannot cross the intact barrier.

The blood-brain barrier has important clinical implications. It presents a major challenge for drug delivery to the brain, as most therapeutic molecules cannot cross effectively. In various pathological conditions, including multiple sclerosis, stroke, and brain tumors, the barrier breaks down, allowing entry of immune cells, plasma proteins, and other molecules that can cause or exacerbate neural damage. Certain specialized regions of the brain, called circumventricular organs, deliberately lack a blood-brain barrier to allow neurons there to directly monitor blood composition for hormones, osmolarity, and other factors.

<image>Panel A: Brain capillary cross-section showing red circular lumen surrounded by pink endothelial layer with prominent zigzag tight junctions with no-entry symbols, thin brown basement membrane, tan pericyte embedded within, and purple astrocyte end-feet completely covering outer surface connecting to cell body. Panel B: Comparison inset of peripheral capillary with fenestrations and gaps contrasting with continuous sealed brain endothelium. Panel C: Right panel showing molecules that cross including small lipophilic O2 and CO2 with arrows and carrier-mediated glucose via GLUT1 and amino acids versus blocked proteins, polar drugs, and bacteria marked with X. Panel D: Disease breakdown panel showing MS lesion with lymphocyte infiltration, stroke with plasma protein leakage, and circumventricular organs lacking barrier diagram with all structures labeled with leader lines.</image>

Histological Identification of Glial Cells

Different staining techniques are used to visualize glial cells in tissue sections, each revealing different aspects of their structure and identity.

Standard hematoxylin and eosin staining provides a general overview but can make glial cells difficult to identify definitively. Astrocyte nuclei appear relatively pale and oval, larger than oligodendrocyte nuclei. Oligodendrocytes show smaller, round, often darker nuclei, frequently with a perinuclear halo artifact in paraffin sections. Microglia have small, elongated, dark nuclei that can be difficult to distinguish from other small cells. Ependymal cells are identified by their location lining the ventricles and their epithelial arrangement.

Immunohistochemistry provides specific identification of glial cell types. GFAP staining specifically labels astrocytes, demonstrating their cell bodies and processes. This is particularly useful for identifying reactive astrocytes in pathological conditions. Markers such as CD68 and Iba1 specifically label microglia, allowing assessment of their density and activation state. Myelin stains such as Luxol Fast Blue highlight myelinated fiber tracts, staining myelin blue while leaving gray matter pale. This is useful for assessing myelination patterns and detecting demyelination in disease.

Silver impregnation methods, though less commonly used today, can demonstrate the full morphology of glial cells including their fine processes. The presence and distribution of glial cells can provide important diagnostic information about the health of neural tissue.

<image>Panel A: H&E stained cerebral cortex showing astrocyte with pale oval nucleus and oligodendrocyte with smaller rounder nucleus and indicated perinuclear halo artifact plus GFAP immunostain of astrocyte cell body and radiating brown-stained processes with unstained background neurons. Panel B: Iba1 immunostain showing ramified microglia with brown staining and fine processes compared to activated amoeboid morphology microglia demonstrating state-dependent identification. Panel C: Luxol Fast Blue myelin stain showing white matter tracts stained deep blue with pale-pink gray matter plus H&E stained ependymal lining of ventricle with cuboidal cells and visible apical surface cilia. Panel D: Schwann cells in peripheral nerve cross-section showing myelin rings around axons with each panel labeled for stain type, cell type, and key identifying features at appropriate magnification scale bars.</image>

Clinical Correlations

Multiple Sclerosis

Multiple sclerosis is an autoimmune inflammatory disease affecting the central nervous system in which the immune system attacks oligodendrocytes and myelin. The disease results in formation of demyelinating plaques, areas where myelin is destroyed and replaced by glial scarring. Within plaques, axons may be preserved initially but become increasingly damaged over time.

The clinical manifestations of multiple sclerosis depend on the location of plaques and include visual disturbances, weakness, sensory abnormalities, and cognitive impairment. The disease may follow a relapsing-remitting course, with periods of symptom flares and recovery, or a progressive course with steady decline. Because oligodendrocytes have limited capacity for regeneration in the adult CNS, remyelination is incomplete and disability accumulates over time.

Guillain-Barré Syndrome

Guillain-Barré syndrome is an acute inflammatory demyelinating polyneuropathy affecting the peripheral nervous system. In this condition, autoantibodies attack Schwann cells and peripheral myelin, often following a viral infection or vaccination. The result is rapid-onset ascending paralysis that can involve respiratory muscles, requiring intensive care support.

Unlike multiple sclerosis, Guillain-Barré syndrome generally has a good prognosis. Because the peripheral nervous system has much greater regenerative capacity than the CNS, and because Schwann cells can proliferate and remyelinate axons, most patients recover substantially over months, though some residual deficits may persist.

Gliomas

Glial cells can give rise to tumors called gliomas, classified according to their presumed cell of origin. Astrocytomas arise from astrocytes and range from low-grade tumors with relatively favorable prognosis to glioblastoma, the most aggressive and common primary brain tumor. Glioblastoma is characterized by rapid growth, necrosis, and florid vascular proliferation. Despite intensive treatment with surgery, radiation, and chemotherapy, median survival remains approximately 15 months.

Oligodendrogliomas arise from oligodendrocytes and typically have a more favorable prognosis than astrocytomas of similar grade. They often show distinctive molecular markers including 1p/19q codeletion. Ependymomas arise from ependymal cells and most commonly occur in the posterior fossa in children.

Alexander Disease

Alexander disease is a rare genetic disorder caused by mutations in the GFAP gene, which encodes the intermediate filament protein of astrocytes. The abnormal GFAP protein aggregates in astrocytes, forming characteristic Rosenthal fibers, eosinophilic cytoplasmic inclusions visible on histological examination. The disease causes progressive leukodystrophy, affecting white matter throughout the brain and causing developmental delay, seizures, and progressive neurological deterioration.

Charcot-Marie-Tooth Disease

Charcot-Marie-Tooth disease represents a group of inherited peripheral neuropathies, many of which affect Schwann cells and myelination. The most common form, CMT1A, results from duplication of the PMP22 gene encoding peripheral myelin protein 22. Mutations in the gene for protein zero (P0) cause CMT1B. These disorders result in demyelination followed by attempts at remyelination, producing characteristic "onion bulb" formations on nerve biopsy where Schwann cells and their basal laminae form concentric layers around axons. Patients experience progressive distal weakness and sensory loss, typically beginning in the feet and legs.

Summary of Glial Cell Types

The nervous system contains six major glial cell types, each with distinct functions. In the CNS, astrocytes are star-shaped cells that provide structural support, contribute to the blood-brain barrier, supply metabolic support to neurons, maintain ionic and neurotransmitter homeostasis, and form glial scars after injury; they are marked by GFAP expression. Oligodendrocytes are the myelinating cells of the CNS, each able to myelinate segments of up to 50 different axons; their marker proteins include MBP and CNPase. Microglia are the immune cells of the CNS, derived from bone marrow monocytes, capable of surveillance in ramified form and phagocytosis in activated form; they express CD68 and Iba1. Ependymal cells line the ventricular system, bearing cilia to circulate CSF; specialized choroidal epithelial cells produce CSF at the choroid plexus.

In the PNS, Schwann cells are the myelinating cells of peripheral nerves, each cell wrapping one segment of one axon, and are essential for peripheral nerve regeneration; they express S100 and p75 markers. Satellite cells surround neuronal cell bodies in peripheral ganglia, providing support and regulating the perikaryal microenvironment; some express GFAP.

Summary

Glial cells are essential partners to neurons, performing functions without which the nervous system could not operate. Astrocytes provide metabolic support, maintain ionic homeostasis, form part of the blood-brain barrier, and respond to injury with reactive gliosis. Oligodendrocytes myelinate CNS axons, with each cell wrapping multiple axon segments, while Schwann cells myelinate PNS axons, with each cell wrapping one segment of one axon. Microglia serve as the immune cells of the CNS, surveying for damage and pathogens and responding with phagocytosis and inflammation. Ependymal cells line the ventricles, circulate CSF, and at the choroid plexus produce CSF.

The blood-brain barrier, formed by specialized brain endothelium with essential support from astrocyte end-feet, restricts entry to the CNS and maintains the optimal environment for neural function. Demyelinating diseases result from damage to oligodendrocytes in the CNS (multiple sclerosis) or Schwann cells in the PNS (Guillain-Barré syndrome), with different prognoses reflecting the different regenerative capacities of these systems.

Key Terms

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