Mechanisms of Cell Injury

Slide 1 · Mechanisms of Cell Injury

From biochemical derangement to cellular dysfunction and death

Insult

Biochemical derangement

Cellular outcome

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Relate the intensity and duration of an insult to reversible or irreversible injury.

Identify the key cellular targets and the consequences of damaging each.

Trace ATP, membrane, oxidative, DNA, and calcium disturbances to cell death.

Integrate mechanisms in ischemic and chemical injury.

Slide 3 · Cell injury is a network of linked failures

Injurious stimulus

Mitochondria and ATP

Cellular membranes

DNA and protein quality

ROS and Ca²⁺ imbalance

The same insult can damage several essential systems at once.

Slide 4 · Principle 1: The nature of the insult matters

An insult is not defined by its name alone: Mechanism and intensity influence the response.

Low-dose chemical exposure may permit recovery; A greater dose may cause death.

A brief reduction in perfusion may be reversible; Prolonged ischemia may not.

Dose and duration must be interpreted together.

Slide 5 · Severity changes the injury outcome

Mild perturbation

Functional impairment

Recovery if insult ceases

Mechanistic sequence — schematic

Low-intensity injury may remain reversible.

Slide 6 · Prolonged injury crosses a threshold

Persistent insult

Escalating organellar injury

Irreversible damage

Cell death

Mechanistic sequence — schematic

A gradual deterioration can culminate in necrosis or apoptosis.

Slide 7 · Brief and prolonged ischemia

Brief ischemia

ATP falls before cell structure is destroyed

Swelling and dysfunction may be reversible

Restored perfusion can rescue viable cells

Prolonged ischemia

Energy deficit deepens

Membrane and mitochondrial damage become severe

Irreversibly injured cells undergo death

Slide 8 · Principle 2: The cell determines its vulnerability

Cell type and differentiated function influence injury thresholds.

Nutritional and hormonal state modify resilience.

Metabolic demand and ability to adapt are critical.

Equivalent exposures need not produce equivalent outcomes.

Slide 9 · Metabolic demand shapes ischemic injury

Skeletal muscle

Activity can be reduced during hypoperfusion

Energy demand may decline with rest

Functional state influences survival

Cardiac myocyte

Continuous contractile demand limits rest

ATP depletion rapidly compromises function

Energy-dependent integrity is essential

Vulnerability partly reflects whether demand can be reduced.

Slide 10 · Individual toxicity may differ

Same chemical exposure

Different metabolic enzyme activity

Different toxic-metabolite burden

Different degree of injury

Mechanistic sequence — schematic

The source uses carbon tetrachloride to illustrate metabolic variability.

Slide 11 · Genetic variation can change toxic responses

Metabolic enzymes may convert a parent chemical to toxic products.

Enzyme polymorphisms may alter the extent of activation.

Identical exposure does not guarantee identical cellular injury.

This is susceptibility variation, not proof that every difference is genetic.

Slide 12 · Principle 3: Cell injury is mechanistically interconnected

A single stimulus can injure multiple organelles simultaneously.

Mitochondrial failure can amplify ionic, oxidative, and membrane disturbances.

Secondary biochemical derangements can sustain injury after the initiating event.

A single dominant molecular cause often cannot be assigned.

Slide 13 · Four essential cellular targets

CELL INJURY

Mitochondria: Energy and death signaling

Membranes: Compartment integrity

ER: Synthesis and proteostasis

Nucleus: Genome surveillance

Distinct abnormalities generate overlapping consequences.

Slide 14 · Target injury predicts a characteristic consequence

Cellular target

Primary impairment

Dominant consequence

Mitochondria

Oxidative phosphorylation

ATP depletion / death signaling

Plasma membrane

Selective permeability

Ion imbalance and leakage

ER / protein machinery

Synthesis and folding

Proteostatic stress

Nucleus

DNA integrity

Arrest, repair, or apoptosis

Slide 15 · Why ATP is central to viability

ATP supports active ion transport and osmotic stability.

Protein synthesis and lipid turnover require energy.

Metabolic pathways and cellular housekeeping depend on ATP.

Failure of ATP-dependent functions explains many early injury changes.

Slide 16 · Two routes to ATP generation

Oxidative phosphorylation

Mitochondrial electron transport uses oxygen

Dominant ATP source in many mammalian cells

Sensitive to ischemia and respiratory toxins

Glycolysis

Uses glucose or glycogen-derived substrate

Can generate ATP without oxygen

Lower total ATP yield than respiration

Slide 17 · Mitochondria are exposed to diverse insults

Mitochondrial injury

Reduced oxygen supply

Raised cytosolic Ca²⁺

Reactive oxygen species

Selected chemicals / toxins

Slide 18 · Permeability transition drives energy failure

Mitochondrial dysfunction

Permeability transition pore opens

Membrane potential dissipates

Oxidative phosphorylation fails

Mechanistic sequence — schematic

Progressive ATP depletion favors necrotic cell death.

Slide 19 · ATP depletion undermines the sodium pump

ATP concentration falls

Na⁺/K⁺-ATPase slows

Intracellular Na⁺ rises

Water enters the cell

Mechanistic sequence — schematic

Cell swelling is a characteristic change of reversible injury.

Slide 20 · Why cells swell during early injury

Active extrusion of sodium requires ATP.

Reduced pump activity promotes intracellular sodium accumulation.

The net gain of intracellular solute draws water osmotically.

Cell and endoplasmic reticulum swelling can follow.

Slide 21 · Ischemia redirects energy metabolism

Oxygen delivery falls

Oxidative ATP production decreases

Glycogenolysis and glycolysis increase

Glycogen reserves diminish

Mechanistic sequence — schematic

Slide 22 · Anaerobic metabolism promotes acidosis

Glycolytic compensation

Lactic acid accumulates

Intracellular pH falls

Cytosolic enzyme activity declines

Mechanistic sequence — schematic

Slide 23 · Prolonged ATP loss suppresses translation

Sustained energy deficit

Ribosomes detach from rough ER

Polysomes dissociate

Protein synthesis decreases

Mechanistic sequence — schematic

Slide 24 · Protein synthesis and proteostasis fail together

Rough-ER disruption diminishes new protein synthesis.

Misfolded proteins can accumulate during cellular stress.

Severe, unresolved protein stress can become a cell-death signal.

This pathway connects an organellar disturbance to the injury response.

Slide 25 · ATP depletion progresses toward necrosis

Persistent energetic failure

Severe ion and metabolic imbalance

Mitochondrial / lysosomal membrane injury

Necrosis

Mechanistic sequence — schematic

Slide 26 · Mitochondria can promote two death routes

Necrosis-linked route

Permeability transition dissipates membrane potential

Oxidative ATP formation fails

Advanced energy collapse promotes necrosis

Apoptosis-linked route

BAX and BAK form membrane channels

Pro-apoptotic mitochondrial proteins escape

Intrinsic apoptotic pathway is activated

The initiating mitochondrial event determines the downstream program.

Slide 27 · Mitochondria sit at a biochemical crossroads

Damaged mitochondrion

Less ATP

More ROS from impaired respiration

Calcium-associated dysfunction

Release of apoptotic proteins

Mitochondrial injury is not synonymous with necrosis only.

Slide 28 · Membrane damage is a shared injury endpoint

Most forms of severe injury disrupt selective membrane permeability.

Plasma, mitochondrial, and lysosomal membranes may all be affected.

Loss of barrier function impairs ionic and biochemical compartmentation.

Apoptosis is an important exception to early overt membrane disruption.

Slide 29 · Four mechanisms weaken cellular membranes

Membrane injury

ROS-mediated lipid peroxidation

Reduced phospholipid synthesis

Increased phospholipid breakdown

Cytoskeletal tether disruption

Slide 30 · Oxidants destabilize lipid bilayers

Excess reactive oxygen species

Membrane lipid oxidation

Peroxide chain reactions

Loss of membrane integrity

Mechanistic sequence — schematic

Slide 31 · Energy failure impedes membrane renewal

Hypoxia or mitochondrial failure

ATP-dependent synthesis declines

Fewer new phospholipids

Membranes become vulnerable

Mechanistic sequence — schematic

Slide 32 · Calcium accelerates phospholipid breakdown

Cytosolic Ca²⁺ rises

Phospholipases are activated

Membrane phospholipids degrade

Permeability abnormalities increase

Mechanistic sequence — schematic

Slide 33 · Lipid breakdown products amplify injury

Free fatty acids and lysophospholipids accumulate after phospholipid degradation.

These molecules can have detergent-like effects.

Insertion into membrane bilayers alters permeability and function.

Membrane lipid injury therefore generates additional damaging products.

Slide 34 · Protease activity disconnects the cell surface

Intracellular Ca²⁺ increases

Proteases cleave cytoskeletal elements

Membrane anchoring weakens

Swelling favors rupture

Mechanistic sequence — schematic

Slide 35 · Membrane compartment determines the clinical signal

Membrane

Consequent failure

Illustrative result

Plasma membrane

Barrier loss

Cell contents may leak

Mitochondrial membranes

Bioenergetic disruption

ATP collapse / death signals

Lysosomal membrane

Enzyme compartment disruption

Self-digestion contributes to necrosis

Slide 36 · Plasma membrane loss is a major step toward necrosis

Severe membrane permeability defects abolish ion gradients.

Cellular constituents escape from damaged cells.

The loss of a continuous plasma membrane distinguishes advanced necrotic injury.

This barrier failure complements mitochondrial energy collapse.

Slide 37 · Lysosomal membrane disruption worsens necrosis

Lysosomes compartmentalize degradative enzymes.

Severe injury can disrupt lysosomal membrane integrity.

Released enzymes participate in degradation of cellular constituents.

The effect converges with plasma and mitochondrial injury.

Slide 38 · Nuclear DNA can be injured in several ways

DNA DAMAGE

Ionizing radiation

Cancer chemotherapy

Reactive oxygen species

Spontaneous changes with aging

Slide 39 · DNA surveillance can impose a protective arrest

DNA lesions accumulate

P53-dependent signaling

G1 cell-cycle arrest

DNA repair attempted

Mechanistic sequence — schematic

Slide 40 · DNA repair success versus failure

Damage can be repaired

P53-mediated cell-cycle arrest allows time

Repair mechanisms correct lesions

Cell may resume cycling

Damage remains extensive

Unrepaired lesions persist

P53 activates mitochondrial apoptosis

Potentially abnormal cells are removed

Slide 41 · The p53 response limits propagation of damaged DNA

DNA damage activates cell-cycle checkpoints and repair.

Irreparable injury can trigger mitochondrial apoptosis.

Failure to arrest or eliminate damaged cells permits genetic instability.

Defective p53 signaling is linked to many cancers.

Slide 42 · Oxidative stress is a balance failure

Low ROS levels are generated during normal cell metabolism.

Intracellular scavenging usually limits their accumulation.

Increased formation or impaired removal produces oxidative stress.

Excess oxidants damage lipids, proteins, and nucleic acids.

Slide 43 · Physiologic ROS versus oxidative stress

Controlled ROS

Low concentrations can support signaling

Antioxidant defenses maintain equilibrium

Production and removal remain balanced

Excess ROS

Scavenging capacity is exceeded

Macromolecules undergo oxidative modification

Cell injury or death can follow

Slide 44 · Why free radicals react so readily

Free radicals have an unpaired electron in an outer orbital.

They can alter adjacent proteins, lipids, carbohydrates, or DNA.

Some reactions yield new radical species and propagate damage.

Hydrogen peroxide is a reactive oxidant but not itself a free radical.

Slide 45 · Cellular sources of damaging oxidants

ROS / reactive species

Electron transport and oxidases

Radiation-mediated water reactions

Activated phagocytes

Reactive chemical metabolism

Slide 46 · Mitochondrial respiration can form ROS

Electrons move through transport chain

Some oxygen reduction is incomplete

Reactive intermediates appear

Scavengers usually limit accumulation

Mechanistic sequence — schematic

Slide 47 · Inflammatory cells deliberately produce ROS

Microbial encounter

NADPH oxidase activation

Rapid oxidant production

Microbe killing and potential tissue injury

Mechanistic sequence — schematic

A defense mechanism can contribute to collateral injury.

Slide 48 · Radiation can generate hydroxyl radicals

Ionizing radiation

Water radiolysis

Highly reactive radical generation

Macromolecular injury

Mechanistic sequence — schematic

Slide 49 · Chemical bioactivation yields reactive radicals

Carbon tetrachloride

Cytochrome P450 metabolism

Trichloromethyl radical generation

Membrane lipid peroxidation

Mechanistic sequence — schematic

Slide 50 · Transition metals catalyze oxidant formation

Redox-active Fe²⁺ / Cu

Hydrogen peroxide available

Highly reactive species form

Oxidative injury increases

Mechanistic sequence — schematic

Slide 51 · Selected reactive species have distinct chemistry

Species

Source or route

Important property

Superoxide (O₂•⁻)

Electron leakage; phagocyte oxidase

Converted by SOD

Hydrogen peroxide (H₂O₂)

Superoxide dismutation; oxidases

Can diffuse and form stronger oxidants

Hydroxyl radical (•OH)

Fenton reaction; radiolysis

Extremely reactive

Peroxynitrite (ONOO⁻)

Superoxide plus nitric oxide

Damages multiple macromolecules

Slide 52 · Antioxidants prevent runaway oxidation

Cells use small-molecule antioxidants to scavenge radicals.

Vitamin E and vitamin A are lipid-soluble examples.

Ascorbic acid and glutathione act in aqueous cellular environments.

Antioxidant action complements enzymatic ROS disposal.

Slide 53 · Metal-binding proteins limit radical chemistry

Free iron or copper

Binding to transport or storage proteins

Less catalytic free metal

Reduced oxidant generation

Mechanistic sequence — schematic

Examples include ferritin, transferrin, lactoferrin, and ceruloplasmin.

Slide 54 · Superoxide dismutase limits superoxide

Superoxide (O₂•⁻)

Superoxide dismutase

Hydrogen peroxide + oxygen

Further peroxide removal required

Mechanistic sequence — schematic

Slide 55 · Catalase removes hydrogen peroxide

Hydrogen peroxide

Catalase in peroxisomes

Water + oxygen

Lower peroxide burden

Mechanistic sequence — schematic

Slide 56 · Glutathione peroxidase consumes reducing power

Hydrogen peroxide + reduced glutathione

Glutathione peroxidase

Water + oxidized glutathione

Peroxide detoxification

Mechanistic sequence — schematic

Slide 57 · Reduced and oxidized glutathione report redox state

Reduced glutathione is denoted GSH; Oxidized glutathione is GSSG.

Glutathione peroxidase uses GSH to detoxify peroxide.

The cellular GSSG:GSH balance reflects oxidative state.

Loss of reducing capacity limits oxidant defense.

Slide 58 · Lipid peroxidation can propagate itself

Radical attacks unsaturated lipid

Lipid radical / peroxide forms

Adjacent membrane lipids react

Chain injury spreads

Mechanistic sequence — schematic

Slide 59 · Protein oxidation changes activity and structure

Oxidation can alter amino acid side chains or the protein backbone.

Covalent cross-links can distort protein conformation.

Enzyme active sites and structural-protein function may be lost.

Misfolded or damaged proteins may undergo increased degradation.

Slide 60 · Oxidative DNA injury has multiple manifestations

DNA oxidation

Single-strand breaks

Double-strand breaks

DNA cross-linking

Abnormal adduct formation

Slide 61 · ROS attack three classes of macromolecules

Target

Representative lesion

Cellular consequence

Membrane lipids

Lipid peroxidation

Loss of permeability control

Proteins

Oxidation / cross-links

Dysfunction and degradation

DNA

Breaks / adducts

Damage response or mutation

Slide 62 · Calcium gradients must remain tightly controlled

Cytosolic free Ca²⁺ is normally very low compared with extracellular Ca²⁺.

The ER and mitochondria sequester much of the intracellular calcium.

Calcium acts as a useful second messenger at controlled concentrations.

Sustained cytosolic calcium elevation promotes injury; Its precise in vivo contribution remains incompletely established.

Slide 63 · Two sources produce cytosolic Ca²⁺ overload

Ischemia or toxic insult

Release from intracellular stores

Increased influx across plasma membrane

Cytosolic Ca²⁺ rises

Mechanistic sequence — schematic

Slide 64 · Calcium excess damages mitochondria

Cytosolic Ca²⁺ overload

Mitochondrial Ca²⁺ accumulates

Permeability transition increases

ATP generation fails

Mechanistic sequence — schematic

Slide 65 · Excess calcium activates injurious enzymes

High cytosolic Ca²⁺

Phospholipases → lipid damage

Proteases → cytoskeletal breakdown

Endonucleases → DNA fragmentation

ATPases → faster ATP depletion

Slide 66 · Energy and calcium injury reinforce one another

ATP-dependent ion control fails

Cytosolic Ca²⁺ increases

Mitochondrial function worsens

ATP depletion deepens

Mechanistic sequence — schematic

This sequence is a reinforcing loop, not a single isolated lesion.

Slide 67 · Multiple lesions converge on limited outcomes

Trigger

Intermediate injury

Possible outcome

Energy failure

Ion pump loss; cell swelling

Reversible injury → necrosis

Mitochondrial protein release

Intrinsic apoptotic signaling

Apoptosis

DNA injury / misfolded proteins

Stress sensing and failed repair

Apoptosis

ROS / elevated Ca²⁺

Membrane and organelle damage

Severe dysfunction / death

Slide 68 · Hypoxia and ischemia are not equivalent

Hypoxia

Oxygen supply is reduced

Blood flow may remain intact

Glycolytic substrates can still arrive

Ischemia

Tissue perfusion is reduced

Both oxygen and substrate delivery suffer

Waste metabolites are less effectively removed

Ischemia often produces more rapid and severe damage.

Slide 69 · The ischemic injury sequence is mechanistically coherent

Reduced perfusion

Oxidative ATP synthesis fails

Pump failure and acidosis

Advanced membrane disruption

Mechanistic sequence — schematic

Early stages may reverse; Severe persistent injury proceeds to necrosis.

Slide 70 · Reperfusion can add a second wave of damage

Reperfused tissue

ROS / reactive nitrogen species

Additional Ca²⁺ overload

Leukocyte-mediated inflammation

Complement activation

Slide 71 · Two general pathways of chemical injury

Direct-acting toxicants

Bind or inhibit essential cell constituents

Mercury disrupts membrane protein function

Cyanide inhibits mitochondrial respiration

Metabolic activation

Parent chemical converted to a reactive metabolite

Cytochrome P450 frequently participates

Radicals and covalent binding damage cells

Slide 72 · Cyanide produces an energy catastrophe

Cyanide exposure

Cytochrome oxidase inhibited

Oxidative phosphorylation stops

Cellular ATP falls

Mechanistic sequence — schematic

Slide 73 · Carbon tetrachloride damages liver-cell membranes

Carbon tetrachloride exposure

Hepatic cytochrome P450 activity

Reactive trichloromethyl radical

Lipid peroxidation and cell injury

Mechanistic sequence — schematic

Slide 74 · Case 1: Abrupt loss of perfusion

A tissue region experiences arterial occlusion. Early cells are swollen but have not yet lost plasma-membrane integrity.

Questions for interpretation

1. Which bioenergetic defect occurs first?

2. What explains the swelling?

3. Why can rapid reperfusion restore viability?

Slide 75 · Case 1: Connect the findings to mechanisms

Oxygen / perfusion decreases

Mitochondrial ATP output declines

Na⁺/K⁺ pump function falls

Na⁺ and water accumulate

Mechanistic sequence — schematic

Preserved membrane integrity supports the possibility of reversibility.

Slide 76 · Case 2: Toxic-metabolite injury

Two persons have similar carbon tetrachloride exposure but show different degrees of hepatocellular injury.

Questions for interpretation

1. Which enzymatic system activates the toxin?

2. What kind of molecular intermediate is formed?

3. Why might the outcomes differ?

Slide 77 · Case 2: Trace toxic activation

Cytochrome P450 exposure-dependent metabolism

Reactive radical generation

Lipid peroxidation

Membrane damage

Mechanistic sequence — schematic

Interindividual variation in metabolic activation can modify injury severity.

Slide 78 · A unified framework for cell injury

Primary defect

Detectable cellular change

Implication

ATP depletion

Swelling; synthesis failure

May be reversible initially

Membrane injury

Permeability loss; leakage

Favors necrotic injury

DNA / proteostasis stress

Cell-cycle arrest / stress programs

Repair or apoptosis

ROS and Ca²⁺ overload

Amplified organelle injury

Multiple modes of cell death

Slide 79 · Synthesis: Three principles to retain

Insult identity, severity, and duration jointly influence injury.

Cell type, energy needs, and adaptability shape susceptibility.

Several reinforcing mechanisms can operate simultaneously.

Injury to mitochondria, membranes, ER, and DNA leads to overlapping dysfunction.

Slide 80 · References and source boundaries

Robbins, Cotran & Kumar: Pathologic Basis of Disease. 11th ed. (2026). Chapter 2.

Primary section: “Mechanisms of Cell Injury,” pp. 45–46.

Mechanistic context: mitochondrial and membrane damage; DNA damage; ROS; calcium homeostasis, pp. 46–50.

Selected clinical integration: hypoxia / ischemia, reperfusion, and chemical injury, pp. 50–51.