Astrocytes

What Are They?

Astrocytes (from Greek astro- = star, kyotos = cell) are star-shaped glial cells and by far the most abundant cell type in the CNS - they constitute nearly half of all brain cells. They originate from progenitor cells in the embryonic neural tube (neuroectoderm). The defining molecular marker is GFAP (glial fibrillary acidic protein), an intermediate filament protein found in all astrocytic processes.

GFAP-stained astrocyte (A) with perivascular feet (PF) enveloping capillaries - Anti-GFAP immunoperoxidase, ×500

Anti-GFAP stained fibrous astrocyte (A) with perivascular feet (PF). - Junqueira's Basic Histology, p. 435

Types

Type	Location	Processes
Protoplasmic	Gray matter	Numerous short, branching processes; closely associated with synapses
Fibrous (fibrillary)	White matter	Long, thin, relatively few processes; contact axons and nodes of Ranvier
Bergmann glia	Cerebellum	Specialized type, processes run parallel to Purkinje cell dendrites
Müller cells	Retina	Span entire retinal width

Terminal processes of a single astrocyte can occupy a volume much larger than its cell body and associate with up to 2 million synaptic sites - Junqueira's Basic Histology, p. 435.

Key Functions

1. Blood-Brain Barrier (BBB) Support

Astrocytic perivascular end-feet completely envelop CNS capillaries, forming a critical component of the BBB. They regulate the passage of molecules and ions from blood into brain tissue. - Robbins Pathologic Basis of Disease, p. ~3373

2. K⁺ Spatial Buffering

When neurons fire action potentials, K⁺ floods the extracellular space. Astrocytes have high densities of K⁺ channels and act as spatial buffers - absorbing K⁺ near active synapses and releasing it at distant blood vessel contacts. This maintains ionic homeostasis needed for normal neuronal excitability. - Principles of Neural Science (Kandel), p. 201

3. Neurotransmitter Regulation

Glutamate: High-affinity transporters on astrocytes rapidly clear glutamate from the synaptic cleft. Inside the astrocyte, glutamate → glutamine (via glutamine synthetase) → transferred back to neurons as a precursor. Failure of this system → excitotoxicity (neuronal death).
Monoamines: Astrocytes degrade dopamine, norepinephrine, epinephrine, and serotonin.
GABA: Astrocytes can also release GABA, reducing neuronal excitability.
Principles of Neural Science (Kandel), p. 201

4. Metabolic Support - The Lactate Shuttle

Astrocytes store virtually all the brain's glycogen. They break glucose/glycogen down to lactate, which is transferred via monocarboxylate transporters (MCT1/MCT2) to neurons for aerobic metabolism. This is especially important during periods of intense neuronal activity.

Astrocyte-neuron lactate shuttle showing glucose uptake via GLUT1, conversion to lactate, and transfer to neurons via MCT1/MCT2

Astrocyte-to-neuron lactate shuttle - Medical Physiology (Boron & Boulpaep), p. 438

5. Synaptic Regulation - The Tripartite Synapse

Astrocytes are often described as forming a tripartite synapse with the pre- and postsynaptic neuron. They:

Sense neurotransmitter release via their own receptor expression
Modulate synapse formation and elimination (especially during development)
Respond to glutamate binding with a rise in intracellular Ca²⁺, which propagates as a Ca²⁺ wave through neighboring astrocytes via gap junctions over hundreds of micrometers
Neuroscience: Exploring the Brain (5e), p. ~2625

6. Trophic and Structural Support

Astrocytes secrete trophic factors that promote neuronal survival and synaptogenesis. They also communicate with each other through gap junctions, forming a large syncytium-like network. - Medical Physiology

Reactions to Injury - Gliosis

Any CNS injury triggers reactive gliosis - the most important histopathologic marker of CNS damage. Astrocytes undergo:

Hypertrophy and hyperplasia
Nuclear enlargement, vesicular chromatin, prominent nucleoli
Bright pink cytoplasm due to increased GFAP expression
Formation of gemistocytic (reactive) astrocytes

Special pathological forms include:

Alzheimer type II astrocyte: seen in hyperammonemia (chronic liver disease, urea cycle disorders); large pale nucleus with intranuclear glycogen droplet
Rosenthal fibers: thick eosinophilic inclusions in processes; seen in long-standing gliosis and pilocytic astrocytoma; contain αB-crystallin, HSP27, and ubiquitin
Corpora amylacea (polyglucosan bodies): round PAS-positive lamellated structures in subpial/perivascular zones; increase with age
Robbins Pathologic Basis of Disease, pp. 3373-3382

Clinical Relevance Summary

Condition	Astrocyte Involvement
TBI / Hypoxia	Cellular swelling, reactive gliosis
Liver failure	Alzheimer type II astrocytes (hyperammonemia)
Pilocytic astrocytoma	Rosenthal fibers
Alexander disease	GFAP gene mutation → massive Rosenthal fiber accumulation
Excitotoxicity (stroke)	Failure of glutamate reuptake
Head injury edema	Astrocyte K⁺/water accumulation → cerebral edema

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