Tell me about cell injury mechanisms

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Cell Injury Mechanisms

Cell injury occurs when cells are exposed to stresses that exceed their adaptive capacity. The outcome ranges from reversible injury (full recovery if the stimulus is removed) to irreversible injury and cell death - by either necrosis or apoptosis.

Progression from healthy cell to cell death

1. Causes of Cell Injury

Before examining mechanisms, it helps to know the major categories of injurious stimuli:

Category	Examples
Hypoxia / ischemia	Arterial blockage, respiratory failure, severe anemia
Physical agents	Mechanical trauma, extremes of temperature, radiation, electric shock
Chemical agents / drugs	Glucose/salt in hypertonic concentrations, arsenic, cyanide, ethanol, CCl4
Infectious agents	Viruses, bacteria, fungi, parasites (via toxins and immune responses)
Immunologic reactions	Autoimmunity, allergic reactions, chronic inflammation
Genetic abnormalities	Chromosomal defects, single-gene mutations (e.g., sickle cell anemia)
Nutritional imbalances	Protein-calorie deficiency, vitamin deficiencies, obesity

Robbins & Kumar Basic Pathology, p. 10-11

2. General Principles

Before individual pathways are discussed, several principles apply to all forms of cell injury:

Dose- and time-dependence: Low doses of a toxin or brief ischemia cause reversible injury; larger doses or prolonged ischemia lead to irreversible injury and necrosis.
Cell-type specificity: Skeletal muscle survives complete ischemia for 2-3 hours; cardiac muscle dies after only 20-30 minutes. Neurons are even more sensitive.
Multiple overlapping pathways: Any single injurious stimulus often triggers several biochemical mechanisms simultaneously, which is why targeting a single pathway rarely prevents cell death completely.
Genetic variation matters: Individuals with polymorphisms in cytochrome P-450 genes, for example, metabolize the same toxin at different rates, yielding different outcomes.

Robbins & Kumar Basic Pathology, p. 11-12; Robbins, Cotran & Kumar Pathologic Basis of Disease, p. 62

3. Key Biochemical Mechanisms

3a. Mitochondrial Dysfunction and ATP Depletion

Mitochondria are the primary targets in most forms of injury. They are damaged by hypoxia, reactive oxygen species (ROS), and elevated cytosolic Ca2+, making them sensitive to virtually all injurious stimuli.

Consequences of mitochondrial damage:

ATP depletion: Failure of oxidative phosphorylation reduces ATP, which is required for virtually every enzymatic and biosynthetic activity in the cell.
- Na+/K+-ATPase pump fails → intracellular Na+ accumulates → osmotic water influx → cell swelling and ER dilation
- Anaerobic glycolysis increases → lactic acid accumulates → intracellular pH drops → enzymatic activity impaired
- Ribosomes detach from rough ER → protein synthesis falls
- Lipid deposition occurs (fatty change) due to defective lipoprotein assembly
Mitochondrial permeability transition pore (MPTP): Opens under injury conditions → loss of mitochondrial membrane potential → failure of oxidative phosphorylation. MPTP opening is a key step in both necrosis and the intrinsic (mitochondrial) apoptosis pathway.
Release of pro-apoptotic proteins: Cytochrome c, Smac/DIABLO, and apoptosis-inducing factor (AIF) leak into the cytoplasm when the outer mitochondrial membrane is permeabilized, activating caspases and triggering apoptosis.
Robbins & Kumar Basic Pathology, p. 10; Pathologic Basis of Disease, p. 62-63

3b. Oxidative Stress and Reactive Oxygen Species (ROS)

Oxidative stress refers to the accumulation of ROS - superoxide (O2•-), hydrogen peroxide (H2O2), and the hydroxyl radical (•OH) - which overwhelm the cell's antioxidant defenses.

Sources of ROS:

Normal mitochondrial respiration (low-level "leak")
Ischemia-reperfusion injury (major source - see below)
Activated phagocytes (respiratory burst)
Ionizing radiation
Reactions with redox-active metals (Fe2+ + H2O2 → •OH, Fenton reaction)
Metabolism of drugs/chemicals (e.g., CCl4)

Cellular damage caused by ROS:

Lipid peroxidation of membrane phospholipids → membrane disruption
Protein oxidation → cross-linking, fragmentation, enzymatic inactivation
DNA damage → strand breaks, base modifications

Antioxidant defenses include: Superoxide dismutase (SOD), catalase, glutathione peroxidase, and vitamins E and C. When these are overwhelmed, injury escalates.

Robbins & Kumar Basic Pathology, p. 11-12

3c. Membrane Damage

Plasma membrane integrity is critical for cell viability. Damage to membranes occurs through:

Direct injury: Toxins (e.g., complement, bacterial toxins) form pores or disrupt bilayer structure
Phospholipase activation: Elevated cytosolic Ca2+ activates phospholipases that degrade membrane phospholipids
Lipid peroxidation by ROS: Oxidative attack on polyunsaturated fatty acids
Cytoskeletal damage: Protease activation detaches the plasma membrane from the cytoskeleton, making cells fragile and prone to rupture under mechanical stress
Lysosomal membrane rupture: Leakage of acid hydrolases into the cytoplasm causes digestion of cellular components, contributing to irreversible damage and necrosis

Membrane damage to mitochondria (as above) has additional consequences for the apoptotic pathway.

Robbins & Kumar Basic Pathology, p. 13

3d. Disturbance in Calcium Homeostasis

Normally, free cytosolic Ca2+ is kept at very low levels (~0.1 μmol), while extracellular Ca2+ is ~1.3 mmol. Most intracellular Ca2+ is sequestered in mitochondria and the ER.

Ischemia and toxins raise cytosolic Ca2+ by:

Releasing it from intracellular stores (ER and mitochondria) early on
Increasing influx across the dysfunctional plasma membrane later

Downstream effects of elevated cytosolic Ca2+:

Protease activation → cytoskeletal and membrane protein degradation
Phospholipase activation → membrane phospholipid breakdown
ATPase activation → accelerated ATP depletion
Endonuclease activation → chromatin fragmentation
MPTP opening → mitochondrial dysfunction and apoptosis
Robbins & Kumar Basic Pathology, p. 13; Pathologic Basis of Disease, p. 64

3e. Endoplasmic Reticulum (ER) Stress and the Unfolded Protein Response

The ER is responsible for folding newly synthesized proteins. When misfolded proteins accumulate (due to mutations, toxins, ischemia, or viral infection), the cell mounts an Unfolded Protein Response (UPR):

Mild ER stress (adaptive UPR): Sensors like IRE1 detect misfolded proteins → ↑ chaperone synthesis, ↓ protein translation, ↑ misfolded protein degradation → reduces the load of misfolded proteins
Severe ER stress (terminal UPR): If the load cannot be cleared → activation of BH3 proteins and caspases → apoptosis via the intrinsic (mitochondrial) pathway

ER stress and the unfolded protein response

Diseases linked to misfolded proteins include cystic fibrosis (CFTR), familial hypercholesterolemia (LDL receptor), Tay-Sachs disease, and many neurodegenerative diseases (Alzheimer's, Parkinson's).

Robbins & Kumar Basic Pathology, p. 13-14

3f. DNA Damage

DNA damage from radiation, chemotherapeutic drugs, ROS, or other mutagens activates the p53 tumor suppressor pathway:

If damage is repairable → cell cycle arrest to allow time for repair
If damage is too great → p53 activates pro-apoptotic genes (e.g., BAX) → apoptosis via the intrinsic pathway

This serves as a safeguard against propagation of cells with mutations that could lead to cancer.

Robbins & Kumar Basic Pathology, p. 14

4. Reversible vs. Irreversible Injury

Feature	Reversible Injury	Irreversible Injury
ATP depletion	Moderate	Severe
Cell swelling	Present	Severe
Membrane integrity	Intact (plasma)	Disrupted
Ribosomes	Detached	Degraded
Mitochondria	Swollen but intact	MPTP open, vacuolated
Lysosomes	Intact	Ruptured
Outcome	Full recovery if stimulus removed	Necrosis or apoptosis

The point of no return from reversible to irreversible injury corresponds to: severe mitochondrial dysfunction with MPTP opening, profound membrane damage, and lysosomal rupture.

5. Clinicopathologic Examples

Hypoxia / Ischemia

The sequence: reduced oxidative phosphorylation → ATP depletion → Na+ pump failure → cell swelling → ribosome detachment → protein synthesis failure → fatty change. If ischemia continues: membrane damage → influx of Ca2+ → enzyme activation → irreversible injury and necrosis.

Ischemia-Reperfusion Injury

Paradoxically, restoring blood flow to ischemic tissue can worsen injury. On reperfusion:

Burst of ROS generation (from xanthine oxidase, neutrophils, mitochondria) damages membranes and proteins
Calcium overload into reversibly injured cells pushes them to irreversibility
Complement activation and neutrophil recruitment amplify tissue damage
This mechanism is relevant in myocardial infarction, stroke, and organ transplantation
Robbins & Kumar Basic Pathology, p. 15

Chemical/Toxic Injury

Direct toxins (e.g., HgCl2): bind to membrane proteins and sulfhydryl groups → ↑ membrane permeability → loss of ion homeostasis
Indirect toxins (e.g., CCl4): converted by cytochrome P-450 in the ER to CCl3• free radical → lipid peroxidation → membrane destruction → liver cell necrosis

6. Cell Death Pathways Summary

Feature	Necrosis	Apoptosis
Trigger	Severe injury (ischemia, toxins)	DNA damage, loss of growth signals, immune attack
Morphology	Cell swelling, membrane rupture, cytoplasmic release	Cell shrinkage, chromatin condensation, apoptotic bodies
Inflammation	Yes (pro-inflammatory)	No (non-inflammatory; phagocytosed quickly)
Pathway	Uncontrolled, passive	Programmed (intrinsic or extrinsic caspase cascade)

Other forms of cell death include necroptosis (programmed necrosis, RIPK3-mediated), pyroptosis (inflammatory caspase-1/11-mediated, with cytokine release), ferroptosis (iron-dependent lipid peroxidation), and autophagy-associated cell death (when the stress exceeds the recycling capacity).

Sources:

Robbins & Kumar Basic Pathology (10th ed.), Chapter 1 - Cellular Responses to Stress and Toxic Insults
Robbins, Cotran & Kumar Pathologic Basis of Disease (10th ed.), Chapter 2 - Cellular Responses to Stress and Toxic Insults

Tell me in short

Cell Injury Mechanisms - Quick Summary

Six core mechanisms cause most cell injury:

ATP Depletion (Mitochondrial dysfunction) - Loss of energy → Na+/K+ pump fails → cell swells → proteins stop synthesizing → eventually irreversible damage
Oxidative Stress (ROS) - Free radicals (O2•-, H2O2, •OH) damage membrane lipids, proteins, and DNA; major player in ischemia-reperfusion injury
Calcium Overload - Elevated cytosolic Ca2+ activates destructive enzymes: proteases, phospholipases, endonucleases → membrane breakdown + DNA fragmentation
Membrane Damage - From phospholipases, ROS, toxins, or complement → loss of ionic gradients → cell rupture (necrosis)
ER Stress / Misfolded Proteins - Overwhelms the unfolded protein response → apoptosis via the intrinsic (mitochondrial) pathway
DNA Damage - Activates p53 → either cell cycle arrest (for repair) or apoptosis if damage is irreparable

The outcome depends on severity:

Mild/brief injury → Reversible (cell recovers)
Severe/prolonged → Irreversible → Necrosis (inflammatory, passive) or Apoptosis (programmed, non-inflammatory)

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