1. 2 concept of injury and cellular response to injury cells are constantly exposed to a variety of...

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Cell Injury, Adaptation,Free Radicals

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CONCEPT OF INJURY AND CELLULAR RESPONSE TO INJURY

Cells are constantly exposed to a variety of stresses. When too severe, INJURY results.

Injury alters the preceding normal steady state of the cell.

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Overview of Cell Injury and Cell Death– Reversible cell injury ( nonlethal hit)– Irreversible injury and cell death ( lethal hit)

Mechanisms of Cell InjuryFree radical injuryNecrosisApoptosis

Objectives

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Cell Injury

What hurts cells? Causes of Cell Injury/Lesions

1. oxygen deprivation (anoxia, hypoxia) 2. physical agents 3. chemical agents 4. infections agents 5. immunologic reactions 6. genetic defects7. Nutritional imbalances8. Aging

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Brain – massive haemorrhagic focus (ischemia) in the cortex

This is a lesion caused by

oxygen deprivation

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Abscess of the brain (bacterial)

This is a lesion caused by

infectious agent

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This is a lesion caused by

chemical agent

Hepatic necrosis (patient poisoned by carbon tetrachloride)

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Pulmonary caseous necrosis (coccidioidomycosis)

This is a lesion caused by

infectious agent

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Gangrenous necrosis of fingers secondary to freezing

This is a lesion caused by

physical agent

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The “boutonnière” (buttonhole) deformity

This is a lesion caused by

intrinsic factors(auto aggression)

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Liver: macronudular cirrhosis (HBV)

This is a lesion caused by

infectious agent

(chemical:alcohol, genetic:a1-AT

deficiency)

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This is a lesion caused by HBVinfectious agent

(chemical:alcohol, genetic:a1AT deficiency)

Liver: cirrhosis

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MECHANISMS OF CELL INJURY

General principles:- The cellular response to injurious stimuli

depends on 1. type of injury

2. Its duration3. Severity

-The consequences depend onthe type, status, adaptability, and genetic

makeup of the injured cell.

-The structural and biochemical components of a cell are so integrally connected that multiple secondary effects rapidly occur

-Cellular function is lost far before cell death occurs

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1. EXCESS or DEFICIENCY OF OXYGEN2. PHYSICAL AGENTS3. CHEMICAL AGENTS4. INFECTION5. IMUNOLOGICAL REACTIONS6. GENETIC DERANGEMENTS7. NUTRITIONAL IMBALANCE

Etiologic agents

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CONCEPT OF INJURY AND CELLULAR RESPONSE TO INJURY

• one of two things can happen to the cell:

1. It can survive in a damaged state and adapt to the injury (REVERSIBLE INJURY) or

2. It can die (IRREVERSIBLE INJURY) or cell death.

• Injury of a CHRONIC nature: the cell may be able to adapt to it, resulting in a variety of cellular changes known as

ADAPTATIONS

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Environment – ECM – other cells

Signals/injury

No change Adjustment Adaptation

1. Atrophy 2. Metaplasia 3. Hypertrophy 4. Hyperplasia 5. Dysplasia

No adaptation No adjustment Change

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Principle of signaling

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Principle of signaling

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Cellular adaptations include:

1. Atrophy - shrinkage of cells

2. Hypertrophy - increase in the size of cells which results in enlargement of the organs

3. Hyperplasia - increased number of cells in an organ or tissue

4. Metaplasia - transformation or replacement of one adult cell type with another

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Atrophy Hypoplasia:

• Developmental failure• Atrophy of organ• Failure in morphogenesis

Reversible

Decrease in size of cell (-s) previously of normal size

Physiologic

• Morphogenetic (apoptosis)• Thymus• Ductus arteriosus• Uterus• Bones

Pathologic • Decreased function• Loss of innervation• Pressure (“bed soars”)• Malnutrition/cahexia/cancer-

TNF• Loss of endocrine stimulation• Aging

Branchial cleftsNotochordMullerian ductsWolffian ducts

Net results: tissue /organ smaller than normal

Signals/injury

Atrophy

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Atrophy - testis

Normal

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Small intestine

Normal Atrophy

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Alzheimer disease – brain atrophy

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Hypertrophy – cell or organ

Reversible

Increase in size of cell (-s) in response to increased functional demand (-s) and/or in response to H/GF stimulation

Physiologic

• Cardiac muscle• Athletes muscle• Uterine muscle• Prostatic tissue (elderly)

Pathologic

• Cardiac muscle• Thyroid• Arterial smooth muscle• Cushing syndrome

Net effect: increase in size/volume/weight of tissue / organ

Signals/injury

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Left ventricular hypertrophy

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Hyperplasia – cell or organ

Reversible

Increase in number of cell (-s) in response to increased functional demand (-s) and/or in response to H/GF stimulation

Physiologic

• Lactating breast• Uterine muscle• Prostatic tissue (elderly)

Pathologic • Thyroid• Arterial smooth muscle• Breast , fibrocystic disease • Focal nodular hyperplasia (liver)

Net effect :increase in size/volume/weight of tissue / organ

Signals/injury

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Polipoid endometrium

Polypoid hyperplasia of endometrium

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Cystic endometrial hyperplasia

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Endometrial carcinoma and endometrial hyperplasia

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Signals/injury

Reversible

But not always

Metaplasia

Substitution of mature (differentiated) cell for another mature cell

Physiologic (metaplastic tissue/organs)

• cervical canal

Pathologic (metaplastic tissue/organs)

• Gastric/duodenal metaplasia• Squamous metaplasia-cervix• Ciliated to squamous• Osseous metaplasia• Barret’s oesophagus• Myeloid metaplasia

Net effect: another cell/tissue - protective – changes in function

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Metaplasia

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Metaplasia

Ciliated

Squamous

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Metaplasia

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Overview of Cell Injury and Cell Death– Reversible cell injury (nonlethal hit)– Irreversible injury and cell death ( lethal hit)

Mechanisms of Cell InjuryFree radical injuryNecrosisApoptosis

Objectives

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Cellular Responses to Injury

Nature and Severity of Injurious Stimulus

Cellular Response

Altered physiologic stimuli: Cellular adaptations:

Increased demand, increased trophic stimulation (e.g. growth factors, hormones)

Hyperplasia, hypertrophy

Decreased nutrients, stimulation Atrophy

Chronic irritation (chemical or physical)

Metaplasia

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Reduced oxygen supply; chemical injury; microbial

infectionCell injury:

Acute and self-limited Acute reversible injury

Progessive and severe (including DNA damage)

Irreversible injury → cell deathNecrosisApoptosis

Mild chronic injury Subcellular alterations in various organelles

Metabolic alterations, genetic or acquired

Intracellular accumulations; calcifications

Prolonged life span with cumulative sublethal injury Cellular aging

Nature and Severity of Injurious Stimulus

Cellular Response

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• The most important targets of injurious stimuli are:

(1) aerobic respiration involving mitochondrial oxidative phosphorylation and production of ATP

Result: 1- ATP depletoion 2- Mitochondrial damage

3- loss of Calcium homeostasis 4- Generation of reactive oxygen species

(2) the integrity of cell membranes, on which the ionic and osmotic homeostasis of the cell and its organelles depends

(3) the cytoskeleton (4) the integrity of the genetic apparatus of the cell (5) protein synthesis

MECHANISMS OF CELL INJURY

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• The most important targets of injurious stimuli are:

(1) aerobic respiration involving mitochondrial oxidative phosphorylation and production of ATP

Result: 1- ATP depletoion 2- Mitochondrial damage

3- loss of Calcium homeostasis 4- Generation of reactive oxygen

species (2) the integrity of cell membranes, on which the ionic and

osmotic homeostasis of the cell and its organelles depends (3) the cytoskeleton (4) the integrity of the genetic apparatus of the cell (5) protein synthesis

MECHANISMS OF CELL INJURY

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(1) DEPLETION OF ATP• ATP depletion and decreased ATP synthesis are

frequently associated with both hypoxic and chemical (toxic) injury

• Depletion of ATP to <5% to 10% of normal levels has widespread effects on many critical cellular systems:

– Plasma membrane energy-dependent sodium pump is reduced, resulting in cell swelling

– increased rate of anaerobic glycolysis, glycogen stores are rapidly depleted. Glycolysis results in the accumulation of lactic acid. This reduces the intracellular pH, resulting in decreased activity of many cellular enzymes.

– Failure of the Ca2+ pump leads to influx of Ca2+ – In cells deprived of oxygen or glucose, unfolded

protein formed, that may lead to cell injury and even death.

MECHANISMS OF CELL INJURY

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MITOCHONDRIAL DAMAGE– Mitochondria are important targets for

virtually all types of injurious stimuli, including hypoxia and toxins.

– Cell injury is frequently accompanied by morphologic changes in mitochondria.

MECHANISMS OF CELL INJURY

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GENERAL MECHANISMS OF INJURY

Result in apoptosis

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INFLUX OF INTRACELLULAR CALCIUM AND LOSS OF CALCIUM HOMEOSTASIS

• Calcium ions are important mediators of cell injury.• Cytosolic free calcium is maintained at extremely low

concentrations (<0.1 μmol) compared with extracellular levels of 1.3 mmol, and most intracellular calcium is sequestered in mitochondria and endoplasmic reticulum.

• Such gradients are modulated by membrane-associated, energy-dependent Ca2+, Mg2+-ATPases.

• Ischemia and certain toxins cause an early increase in cytosolic calcium concentration, owing to the net influx of Ca2+ across the plasma membrane and the release of Ca2+ from mitochondria and endoplasmic reticulum

GENERAL MECHANISMS OF INJURY

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GENERAL MECHANISMS OF INJURY

Failure of intracellular calcium homeostasis

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FREE RADICAL MEDIATION OF CELL INJURY Generation of reactive oxygen species

Important mechanism of cell damage.Free radical are chemical species with a

single unpaired electron in an outer orbital.

This state is unstable and react with organic and inorganic chemical.

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Cell injury by oxygen radicals 

1. Superoxide2. Hydrogen peroxide3. Hydroxy radical 

MECHANISMS OF INJURY BY FREE RADICALS

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Causes of Cell Injury/LesionsSeveral injurious processes produce injury by free radical.

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Cell injury by oxygen radicals 

1. Superoxide2. Hydrogen peroxide3. Hydroxy radical  What happen when the cell is injured by free radicals?

4. Lipid peroxidation 5. Protein damage6. DNA damage 

MECHANISMS OF INJURY BY FREE RADICALS

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GENERAL MECHANISMS OF INJURY –FREE RADICALS

Normal mechanism to protect against free radical injury

1. Enzyme A. Superoxide dismutase. 2O2

- + 2H ---> H2O2 + O2

B. Glutathione peroxidase.

H2O2 + 2 GSH ---> 2H2O + GSSG

C. Catalase. 2H2O2 ----> O2 + 2 H2O

2. Antioxidant:• vit. E, vit. C• Sulfhydryl containing compounds e.g. cysteine • Proteins e.g., transferrin and albumin

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• The most important targets of injurious stimuli are:

(1) aerobic respiration involving mitochondrial oxidative phosphorylation and production of ATP

(2) the integrity of cell membranes, on which the ionic and osmotic homeostasis of the cell and its organelles depends

(3) the cytoskeleton (4) the integrity of the genetic apparatus of the cell (5) protein synthesis

MECHANISMS OF CELL INJURY

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Integrity of Cell Membranes

Mechanisms of membrane damage in cell injury:

Decreased O2 and increased cytosolic Ca2+ are typically seen in ischemia but may accompany other forms of cell injury.

Production of reactive oxygen species Lysis of enzymes Activation of complement system Lysis by viruses

MECHANISMS OF CELL INJURY

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GENERAL MECHANISMS OF INJURY

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Effect of plasma membrane damage

1. Loss of structural integrity2. Loss of function

MECHANISMS OF CELL INJURY

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• The most important targets of injurious stimuli are:

(1) aerobic respiration involving mitochondrial oxidative phosphorylation and production of ATP

(2) the integrity of cell membranes, on which the ionic and osmotic homeostasis of the cell and its organelles depends

(3) the cytoskeleton (4) the integrity of the genetic apparatus of the cell (5) protein synthesis

MECHANISMS OF CELL INJURY

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Cytoskeletal abnormalities

• Cytoskeletal filaments serve as anchors connecting the plasma membrane to the cell interior.

• Activation of proteases by increased cytosolic calcium may cause damage to elements of the cytoskeleton.

• This damage results, particularly in myocardial cells, in detachment of the cell membrane from the cytoskeleton, rendering it susceptible to stretching and rupture.

MECHANISMS OF CELL INJURY

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• The most important targets of injurious stimuli are:

(1) aerobic respiration involving mitochondrial oxidative phosphorylation and production of ATP

(2) the integrity of cell membranes, on which the ionic and osmotic homeostasis of the cell and its organelles depends

(3) the cytoskeleton (4) the integrity of the genetic apparatus of the cell (5) protein synthesis

MECHANISMS OF CELL INJURY

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(4) the integrity of the genetic apparatus of the cell

• Caused by:1.Ionizing radiation

2.Viruses

3.Mutagenic chemicals

MECHANISMS OF CELL INJURY

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(4) the integrity of the genetic apparatus of the cell

• Effect of DNA abnormalities:

1. Failure of synthesis of proteins and enzyme

2. Failure of mitosis

3. Progression to cancer

MECHANISMS OF CELL INJURY

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• The most important targets of injurious stimuli are:

(1) aerobic respiration involving mitochondrial oxidative phosphorylation and production of ATP

(2) the integrity of cell membranes, on which the ionic and osmotic homeostasis of the cell and its organelles depends

(3) the cytoskeleton (4) the integrity of the genetic apparatus of the cell (5) protein synthesis

MECHANISMS OF CELL INJURY

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• Oxygen is required for oxidative phosphorylation.• Defective ATP production occur in:

a] Hypoglycaemia.

b] Hypoxia due to :

1. Respiratory obstruction or disease.

2. Ischemia.

3. Anaemia.

4. Alteration of hemoglobin.

c] Enzyme inhibition by cyanide.

d] Uncoupling of oxidative phosphorylation.• First cells affected are those with highest demand

of oxygen.

ISCHEMIC AND HYPOXIC INJURY

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MECHANISMS OF INJURY: Ischemia

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• Effect of defective energy production:• Morphologic changes in reversible injury: A] Intracellular accumulation of water and

electrolytes due to failure of energy-dependent sodium pump

(cloudy swelling or hydropic changes)

HYPOXIC INJURY(Reversible)

B] Changes in organelles, swollen due to loss of osmotic regulation. C] Switch to anaerobic metabolism with production of lactic acid, reduction in intracellular pH and detachment of ribosomes from RER. D] Clumping of nuclear chromatin. These changes are reversible if oxygenation is restored.

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Vacuolar (hydropic) change in cells lining the proximal tubules of the kidney

Reversible changes

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Hydropic vacuoles in the endoplasmic reticulum of hepatocyte

Reversible changes

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Vacuoles

Hydropic vacuoles in the endoplasmic reticulum of hepatocyte

M

Nucleus

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• Morphologic changes in irreversible injury:

1. Severe vacuolization of the mitochondria, with accumulation of calcium-rich densities.

2. Extensive damage to plasma membranes.

3. Massive calcium influx activate phospholipase, proteases, ATPase and endonucleases with break down of cell component.

4. Leak of proteins, ribonucleic acid and metabolite.

5. Breakdown of lysosomes with autolysis.

6. Nuclear changes: Pyknosis, karyolysis, karyorrhexis.

HYPOXIC IRREVERSIBLE INJURY

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IRREVERSIBLE CELL INJURY- NECROSIS

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- Dead cell are either collapsed and form a

whorled phospholipid masses or degraded

into fatty acid with calcification.

- Cellular enzymes are released into circula-

tion. This provides important clinical

parameter of cell death.

HYPOXIC INJURY

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Myocardial infarct This is a lesion caused by

oxygen deprivation

Cell Pathology

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Following ischemic heart injury, the following sequence is observed: - rapid biochemical and ultrastructural responses- light microscopic evidence of reversible injury after several minutes - ultrastructural evidence of irreversible injury in 20-60 minutes - unequivocal light microscopic evidence of cell death after 11-12

hours

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How Ionizing Radiation Kills Cells 

• Proliferating Cells - by DNA damage. Leads to apoptosis. 

• Nonproliferating cells- by lipid peroxidation.

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How Viruses Kill Cells 

• Directly Cytopathic Viruses – e.g. Poliovirus • Indirectly cytopathic Viruses -  e.g. hepatitis B Summary of Cytopathic Viruses 

• Direct cytopathic viruses insert their proteins into the plasma membranes, disrupting the cells permeability (membrane damage)

• Indirect cytopathic viruses also in insert their proteins into the plasma membrane, but to create an antigenic target for cytotoxic T lymphocytes. 

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How Chemicals Kill Cells :

Group I : interact directly with cellular contents to cause damage (mercury, lead and iron (toxic heavy metals) Group II: whose metabolite is toxic e.g. hepatotoxins: (Carbon tetrachloride(CCl4), acetominophen, bromobenzene)Group III: bind cytochrome P450 (the mixed function oxygenase involved in drug metabolism)   Summary of Liver Necrosis by Cytotoxic Chemicals 

The metabolism of hepatotoxic chemicals by mixed function oxidation (cytochrome P450) leads to irreversible cell injury. This is caused by membrane damage to the cell as a result of lipid peroxidation.

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Reversible and irreversible cell

injury : time and

exposure factors