Explain Cell injury-causes, mechanism & toxic injuries with special reference to Physical, Chemical, & ionizing radiation
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Cell Injury: Causes, Mechanisms, and Toxic Injuries (Physical, Chemical & Ionizing Radiation)
Based on Robbins & Kumar Basic Pathology and Robbins, Cotran & Kumar Pathologic Basis of Disease
1. Overview
Cell injury is the basis of all disease. Injurious stimuli damage cells through diverse biochemical mechanisms, but they all tend to induce a stereotypic sequence of morphologic and structural alterations. There is always a time lag between the stress and the morphologic changes - biochemical changes may appear in minutes to hours, while light microscopic changes may take hours to days.
The two fundamental outcomes of cell injury are:
- Reversible injury - the cell can recover if the noxious stimulus is removed
- Irreversible injury (cell death) - the cell passes the "point of no return" and dies by necrosis or apoptosis
2. Causes of Cell Injury
| Category | Examples |
|---|---|
| Hypoxia & Ischemia | Arterial blockage, cardiorespiratory failure, anemia, CO poisoning |
| Physical agents | Mechanical trauma, extremes of temperature, radiation, electric shock, pressure changes |
| Chemical agents & toxins | Glucose/salt in hypertonic concentrations, arsenic, cyanide, mercury, CCl4, alcohol, drugs, environmental pollutants |
| Infectious agents | Viruses, bacteria, fungi, parasites - through toxins & immune responses |
| Immunologic reactions | Autoimmune diseases, allergy, chronic immune responses to microbes |
| Genetic abnormalities | Chromosomal defects (Down syndrome), point mutations (sickle cell anemia), inborn errors of metabolism |
| Nutritional imbalances | Protein-calorie deficiency; vitamin deficiencies; obesity leading to type 2 DM and atherosclerosis |
Robbins & Kumar Basic Pathology, Chapter 1 - Causes of Cell Injury
3. Sequence of Events: Reversible vs. Irreversible Injury
Reversible Cell Injury
Defined as a derangement of function and morphology that cells can recover from. Two consistent features:
- Cell swelling (hydropic change) - results from influx of water due to failure of the ATP-dependent Na⁺/K⁺-ATPase pump. Sodium accumulates inside the cell, causing osmotically driven water entry → cell swelling and ER dilation.
- Fatty change - occurs in organs involved in lipid metabolism (especially liver). Toxic injury disrupts metabolic pathways → rapid accumulation of triglyceride-filled lipid vacuoles.
Irreversible Cell Injury
When the injurious stimulus is too severe or prolonged, cells pass the point of no return. Two critical events mark this transition:
- Inability to reverse mitochondrial dysfunction even after restoration of oxygen
- Profound disturbances in membrane function, including permeabilization of plasma and lysosomal membranes
Morphologic features of necrosis:
- Cytoplasmic changes: Increased eosinophilia (denatured proteins), glassy homogeneous appearance (glycogen loss), vacuolated "moth-eaten" cytoplasm, myelin figures
- Nuclear changes (3 patterns):
- Pyknosis - nuclear shrinkage with increased basophilia; DNA condenses into a dark shrunken mass
- Karyorrhexis - fragmentation of the pyknotic nucleus
- Karyolysis - fading of basophilia due to DNase digestion; complete dissolution in 1-2 days
4. Mechanisms of Cell Injury
The four principal sites of damage are illustrated below:
4a. Mitochondrial Damage - ATP Depletion
Mitochondria are the most sensitive targets in cell injury. Their damage produces:
ATP Depletion (to 5-10% of normal) causes widespread cellular effects:
- Failure of Na⁺/K⁺-ATPase → Na⁺ influx + K⁺ efflux → water accumulation → cell swelling
- Anaerobic glycolysis is activated → glycogen stores depleted → lactic acid accumulation → pH falls → chromatin clumping
- Ribosomal detachment from ER → reduced protein synthesis
- Protein misfolding (low pH, lack of energy for chaperones)
- Mitochondrial permeability transition pore (MPTP) opens → loss of membrane potential → failure of oxidative phosphorylation → necrosis
Increased ROS generation from damaged mitochondria damages lipids, proteins, and nucleic acids.
4b. Membrane Damage
Membranes are damaged by:
- Phospholipid loss - decreased synthesis due to ATP depletion or direct damage
- Lipid peroxidation by ROS - reactive oxygen species attack polyunsaturated fatty acids in membranes
- Cytoskeletal damage - Ca²⁺-activated proteases cleave cytoskeletal anchors, leading to membrane detachment and rupture
- Lysosomal membrane damage - releases lysosomal enzymes (DNases, RNases, proteases) into the cytoplasm → autodigestion of organelles
Plasma membrane damage: Impaired transport functions; leakage of cellular contents (LDH, transaminases, creatine kinase - useful as serum biomarkers of cell death).
4c. Oxidative Stress (Reactive Oxygen Species)
Key ROS in cell injury:
- Superoxide anion (O₂•⁻) - generated by reduction of O₂; converted by superoxide dismutase (SOD) to H₂O₂
- Hydrogen peroxide (H₂O₂) - converted to hydroxyl radical (OH•) by iron via Fenton reaction
- Hydroxyl radical (OH•) - most reactive and damaging ROS
Mechanisms of ROS damage:
- Lipid peroxidation of cell and organelle membranes (autocatalytic chain reaction)
- Oxidation of proteins - sulfhydryl cross-links, polypeptide fragmentation → enzyme inactivation
- DNA damage - single/double-strand breaks, base modifications → mutations
Cellular antioxidant defenses:
- Superoxide dismutase (converts O₂•⁻ → H₂O₂)
- Catalase (breaks down H₂O₂ → H₂O + O₂)
- Glutathione peroxidase (converts H₂O₂ and organic peroxides)
- Vitamins E, A, C; metal-binding proteins (transferrin, ceruloplasmin) - reduce free metal ion availability
4d. Intracellular Calcium Overload
- Normal cytoplasmic Ca²⁺ is very low (~0.1 μM); ischemia and toxins cause influx from extracellular space and release from ER/mitochondria
- Elevated Ca²⁺ activates:
- Phospholipases → membrane damage
- Proteases → cytoskeletal and membrane protein breakdown
- Endonucleases → DNA and chromatin fragmentation
- ATPases → ATP depletion
- Ca²⁺ overload also promotes MPTP opening → further ATP depletion and ROS generation
4e. DNA Damage and p53 Activation
- Damage to nuclear DNA (by radiation, ROS, chemotherapeutic drugs, or spontaneous deamination during aging) activates p53
- p53 arrests cells in G1 phase to allow DNA repair; if repair fails, p53 triggers apoptosis via the mitochondrial pathway
- Cells with TP53 mutations survive with abnormal genomes → malignant transformation
4f. Endoplasmic Reticulum (ER) Stress
- Accumulation of misfolded proteins in the ER activates the Unfolded Protein Response (UPR)
- If stress is not resolved, apoptosis is triggered via the mitochondrial (intrinsic) pathway
- Causes: ischemia, mutations, toxins, viral infections, aging
5. Toxic (Chemical) Injury - Special Reference
"Chemical injury remains a frequent problem in clinical medicine and is an important limitation to drug therapy. Because many drugs are metabolized in the liver, this organ is a major target of drug toxicity."
- Robbins, Cotran & Kumar Pathologic Basis of Disease
Chemicals induce cell injury by two general mechanisms:
5a. Direct Toxicity
Some chemicals injure cells directly by combining with critical molecular components:
| Toxin | Mechanism | Target |
|---|---|---|
| Mercuric chloride (HgCl₂) | Binds sulfhydryl (-SH) groups of membrane proteins → increased permeability + inhibition of ion transport | GI tract, kidney |
| Cyanide | Poisons mitochondrial cytochrome oxidase → blocks oxidative phosphorylation | All cells, especially brain/heart |
| CO (carbon monoxide) | Binds hemoglobin with 200× affinity vs O₂ → functional anemia + direct cytochrome damage | Blood, all tissues |
| Antineoplastic drugs / antibiotics | Direct cytotoxic effects on cellular machinery | Rapidly dividing cells |
5b. Conversion to Toxic Metabolites (Indirect Toxicity)
Most toxic chemicals are biologically inactive in their native form. They are converted to reactive toxic metabolites by cytochrome P-450 mixed-function oxidases in the smooth ER of the liver (and other organs).
The toxic metabolites cause membrane damage mainly by:
- Free radical formation → lipid peroxidation
- Direct covalent binding to membrane proteins and lipids
Key examples:
Carbon tetrachloride (CCl₄):
- Previously used in dry cleaning industry
- Converted by cytochrome P-450 to the highly reactive free radical •CCl₃ (trichloromethyl radical)
- •CCl₃ + O₂ → •OOCCl₃ (trichloromethyl peroxy radical)
- Attacks polyunsaturated fatty acids → autocatalytic lipid peroxidation cascade
- Damages: ER membranes (fatty liver), mitochondria, plasma membranes
- Clinical: acute hepatic necrosis
Acetaminophen (Paracetamol):
- Converted in the liver by CYP2E1 to the highly reactive metabolite NAPQI (N-acetyl-p-benzoquinone imine)
- Normally detoxified by conjugation with glutathione
- Overdose depletes glutathione → NAPQI binds covalently to hepatocyte proteins → hepatocellular necrosis
- Clinical: acute liver failure (zone 3/centrilobular necrosis)
Alcohol (Ethanol):
- Converted by alcohol dehydrogenase and CYP2E1 → acetaldehyde + ROS
- Acetaldehyde forms adducts with proteins → cell injury
- CYP2E1 induction → increased ROS production → oxidative stress
- Clinical: fatty liver → alcoholic hepatitis → cirrhosis
6. Physical Injury - Special Reference
Physical agents capable of causing cell injury include:
6a. Mechanical Trauma
- Direct disruption of cell membranes and organelles
- Massive force → tearing of vessels, nerve injury, fractures
6b. Extremes of Temperature
Burns (heat injury):
- Mild heat (41-45°C): increased metabolic activity, reversible denaturation of proteins
- Severe heat (>50°C): Protein denaturation, coagulative necrosis, vascular damage
- Widespread burns cause systemic fluid loss, inflammatory cytokine release, risk of infection and sepsis
Cold (hypothermia):
- Indirect damage: Vasoconstriction → ischemia → hypoxic injury
- Direct damage: Ice crystal formation inside and outside cells → mechanical membrane disruption; concentration of electrolytes in unfrozen water → osmotic injury
- Frostbite: vascular damage + ice crystal formation → gangrene
6c. Sudden Changes in Atmospheric Pressure
- High pressure (dysbarism): Nitrogen dissolves in blood under high pressure; rapid decompression → nitrogen bubble formation in blood and tissues ("decompression sickness" / "the bends") → vascular occlusion, ischemia, joint pain, neurological deficits
- Low pressure (altitude): Reduced O₂ partial pressure → hypoxia
6d. Electric Shock
- Passage of electrical current through tissues generates heat (joule heating) → coagulative necrosis along the current path
- Can cause cardiac arrhythmias (ventricular fibrillation) by depolarizing myocardial cells
7. Ionizing Radiation - Special Reference
Types of Ionizing Radiation
| Type | Nature | Penetration | Damage per Unit |
|---|---|---|---|
| X-rays / Gamma rays | Electromagnetic waves (very high frequency) | Deep, long course | Less damage per unit tissue |
| Alpha particles | 2 protons + 2 neutrons | Restricted area | Heavy damage in restricted area |
| Beta particles | Electrons/positrons | Intermediate | Intermediate |
| High-energy neutrons | Neutral particles | Deep | Very high LET |
~50% of ionizing radiation received by the US population is human-made (mainly medical devices, CT scans, radioisotopes).
Radiation Units
| Unit | Definition |
|---|---|
| Curie (Ci) | Disintegrations per second of a radionuclide; 1 Ci = 3.7 × 10¹⁰ disintegrations/sec |
| Gray (Gy) | Energy absorbed per unit mass; 1 Gy = 10⁴ erg/g tissue; cGy = 100 Rad (R) |
| Sievert (Sv) | Equivalent dose accounting for biologic effects; Sv = Gy × relative biologic effectiveness |
Determinants of Biologic Effects
- Rate of delivery (dose rate): Fractionated doses allow repair between exposures - normal cells recover faster than tumor cells (exploited in cancer radiotherapy)
- Field size: Smaller fields are tolerated at higher doses; large field irradiation with smaller doses can be lethal
- Rate of cell division: Dividing cells are MORE vulnerable; nondividing cells (neurons, muscle) can survive DNA damage that would be lethal in dividing cells
- Oxygen levels: Hypoxic tissues (e.g., center of tumors) are LESS sensitive - oxygen enhances free radical production by radiolysis of water
Mechanisms of Cell Injury by Ionizing Radiation
Two major mechanisms:
1. Indirect effect (predominant - ~70% of damage):
- Ionizing radiation causes radiolysis of water → free radicals (H•, OH•, O₂•⁻)
- OH• is the most damaging - attacks DNA bases, causes strand breaks, and DNA-protein crosslinks
- Enhanced at high oxygen tension (oxygen fixation hypothesis)
2. Direct effect:
- Radiation energy is directly absorbed by DNA or other macromolecules
- Causes base damage, single-strand breaks (SSBs), double-strand breaks (DSBs)
- DSBs are the most serious - repair is error-prone via non-homologous end joining
DNA Damage and Consequences:
- Simple defects may be corrected by enzyme repair systems
- ATM (ataxia-telangiectasia mutated) acts as the sensor for DNA damage
- p53 acts as the effector → cell cycle arrest → allows DNA repair
- If irreparable → p53 triggers apoptosis via the mitochondrial pathway
- If checkpoints are impaired (e.g., TP53 mutations) → cells with abnormal genomes survive → carcinogenesis
Consequences of Radiation Injury
Short-term effects:
- Cell death in rapidly dividing tissues: bone marrow, GI epithelium, gonads
- Hematopoietic and lymphoid systems: Most sensitive; radiation directly destroys lymphocytes in blood, lymph nodes, spleen, thymus; high doses → bone marrow aplasia → pancytopenia → hemorrhage + infection
- GI tract: Mucosal cell loss → ulceration, malabsorption, diarrhea, infection
- Gonads: Spermatogonia and oogonia are highly radiosensitive → sterility
Long-term effects:
- Fibrosis: Endothelial cell damage → vascular narrowing/occlusion → ischemia → parenchymal cell death replaced by connective tissue (weeks to months post-irradiation); inflammatory cytokines and chemokines activate fibroblasts
- Carcinogenesis: Mutations accumulate over years/decades → cancer (leukemia, thyroid cancer, solid tumors)
- Mutagenesis & Teratogenesis: Germ cell mutations; fetal exposure → teratogenic effects
Tissues ranked by radiosensitivity (most to least sensitive):
- Gonads
- Hematopoietic / lymphoid tissue
- GI tract epithelium
- Skin, oral/respiratory mucosa
- Kidney, liver, lung
- Bone, cartilage, nerve, muscle (least sensitive)
8. Summary: Comparison of Toxic Injury Types
| Feature | Chemical/Toxic | Physical | Ionizing Radiation |
|---|---|---|---|
| Primary target | Membranes, enzymes, DNA | Membranes, organelles, vasculature | DNA (primary), membranes |
| Key mechanism | Direct toxicity OR metabolic conversion → free radicals | Mechanical disruption, thermal denaturation, ice crystal formation | Radiolysis → free radicals; direct DNA strand breaks |
| Key mediator | Cytochrome P-450 metabolites, free radicals | Temperature, pressure, electrical current | OH• radical (indirect); direct ionization |
| Reversibility | Dose-dependent; low doses often reversible | Depends on extent; severe burns irreversible | Low dose: reversible; high dose → necrosis, apoptosis |
| Long-term risks | Organ-specific toxicity (liver, kidney) | Scarring, ischemic atrophy | Fibrosis, mutagenesis, carcinogenesis, teratogenesis |
Sources:
- Robbins & Kumar Basic Pathology (Robbins Pathology), Chapter 1 - Cell Injury, Cell Death, and Adaptations; Chapter 7 - Environmental and Nutritional Diseases
- Robbins, Cotran & Kumar Pathologic Basis of Disease, Chapter 2 - Cellular Responses to Stress and Toxic Insults; Chapter 9 - Environmental and Nutritional Diseases
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