Cell Injury

Cell Injury

Wednesday, 09/02/2011

Walayat Shah, PhD

Xi’an Jiaotong Uinversity, China Assistant Professor

IBMS-KMU Walayats.ibms@kmu.edu.pk

Key Concepts

• Normal cells have a fairly narrow range of function or steady state: Homeostasis

• Excess physiologic or pathologic stress may force the cell to a new steady state:

Adaptation

• Too much stress exceeds the cell’s adaptive capacity: Injury

Key Concepts (cont’d)

• Cell injury can be reversible or irreversible

• Reversibility depends on the type, severity and duration of injury

• Irreversible injury results in cell death

Causes of Cell Injury

• Hypoxia---oxygen deprivation

– Ischemia----inadequate flow of blood to a part of the body

– Hypoxemia---inadequate oxygenation of arterial blood---pneumonia

– Reduced oxygen carrying capacity of blood---^anemia

• Free radical damage---^O₂^{-, H}₂^O2, OH^-, NO etc

• Physical agents---trauma, excessive heat or cold, radiations etc.

• Chemicals agents^---drugs, toxins, CO, environmental pollutants etc.

• Infections---viral, bacterial, fungal, protozoan, etc.

• Immunologic reactions---anaphylaxis, autoimmune diseases

• Genetic abnormalities---Down’s syndrome, sickle cell anemia etc.

• Nutritional imbalance---protein-calorie insufficiency, vitamin deficiency, type 2 diabetes, atherosclerosis, cancer etc.

Cell Injury – General Mechanisms

• Four very interrelated cell systems are particularly vulnerable to injury:

– Aerobic respiration (ATP synthesis) – Membranes (cellular and organellar) – Protein synthesis (enzymes, structural

proteins, etc)

– Genetic apparatus (DNA, RNA)

Cell Injury – General Biochemical Mechanisms

• Oxygen deprivation- hypoxia or ischemia

• Oxygen-derived free radicals

• ATP depletion

• Loss of calcium homeostasis

• Defects in plasma membrane permeability

• Mitochondrial damage---MPT, Cytochrome c leakage

Hypoxic, Ischemic & Ischemic/Reperfusion injury

↑ Ca⁺²

Membrane Damage& MPT

↓O₂

Accumulation of inorganic phosphates &

lactic acid

Sources & Consequences of increased cytosolic Calcium

Free Radicals-Induced Cell Injury

• Free radicals have an unpaired electron in their outer orbit

• Generated by:

– Oxidation of endogenous constituents-^normal

physiologic processes in mitochondria and cytoplasm

– Absorption of radiant energy-UV light, X-Ray

– Oxidation of exogenous compounds-CCl₄

Examples of Free Radical Injury

• Chemicals (e.g., CCl₄, acetaminophen)

• Inflammation / Microbial killing

• Irradiation (e.g., UV rays  skin cancer)

• Oxygen (e.g., exposure to very high oxygen tension on ventilator)

• Reperfusion injury

• Cellular Aging

Cell injury

Mechanism of Free Radical Injury

• Lipid peroxidation  Damage to cellular and organellar membranes due to vulnerability of double bonds of polyunsaturated lipids

• Protein cross-linking  ↑Protein degradation or loss of enzymatic activity due to oxidative modification of amino acids and proteins, polypeptide fragmentation

• DNA fragmentation Reactions of free radicals with thymine producing single-strand breaks

Reactive oxygen species

Free Radical Generation

P450 & b5 oxidases

Peroxisome

Exogenous or endogenous antioxidants e.g. vitamin E, A, C & β-carotene Lysosomes

Myeloperoxidase NO synthase

Chemical Injury

• Two general mechanisms

• Direct combining with critical molecular component or cellular organelle e.g. mercuric chloride + sulfhydryl groups of membrane proteins –inhibit ATPase- dependent transport & increase membrane permeability. other examples include antineoplastic chemotherapeutic agents &

antibiotics.

• Indirect through reactive toxic metabolites-

e.g. CCl4 converted to CCl₃^.by P450 typically in liver→phospholipid peroxidation→breakdown of SER →↓enzyme & plasma proteins

→↓ lipoprotein secretion →fatty liver → mitochondrial injury,→↓ ATP synthesis → defective transport→ cell swelling → Ca ion influx →cell death

Types of Injury

• Reversible injury

– Mild injury

– Cell is restored to its normal state if the injurious stimulus is over or

– Become adapted to the new state if the stimulus continues

• Irreversible injury

– Severe injury

– There is no way back & cell dies

Reversible Injury

• Mitochondrial oxidative phosphorylation is disrupted first  Decreased ATP 

– Decreased Na/K  gain of intracellular Na  cell swelling

– Decreased ATP-dependent Ca pumps  increased cytoplasmic Ca concentration – Altered metabolism depletion of glycogen – Lactic acid accumulation  decreased pH – Detachment of ribosomes from RER 

decreased protein synthesis

• End result is cytoskeletal disruption with

Irreversible Injury

• Mitochondrial swelling & formation of large amorphous Ca-rich densities in matrix

• Lysosomal membrane damage  leakage of proteolytic enzymes into cytoplasm

• Mechanisms include:

– Irreversible mitochondrial dysfunction  markedly decreased ATP

– Severe impairment of cellular and organellar membranes

Ischemic injury

Mechanisms of injury

Mechanisms of Membrane Damage in Irreversible Injury

Damaged mitochondria

Clinical Correlation

• Injured membranes are leaky

• Enzymes and other proteins that escape through the leaky membranes make their way to the bloodstream, where they can be measured in the serum

Morphology of Cell Injury – Key Concept

• Morphologic changes follow functional changes

Reversible Injury -- Morphology

• Light microscopic changes

– Cell swelling (hydropic change)  Clear Vacuoles

– Fatty change Lipid vacuoles e.g. hepatocytes & Myocardial cells

• Ultrastructural changes

– Alterations of cell membrane blebs formation &

loosening of intercellular attachments

– Mitochondrial Changes  swelling & appearance of small amorphous deposits in mitochondria

– Swelling of RER & detachment of ribosomes

Irreversible Injury -- Morphology

• Light microscopic changes

– Increased cytoplasmic eosinophilia (loss of RNA, which is more basophilic)

– Myelin figures

• Ultrastructural changes

– Breaks in cellular and organellar membranes – Rupture of lysosomes and autolysis

– Larger amorphous densities in mitochondria – Nuclear changes

Irreversible Injury – Nuclear Changes

• Pyknosis

– Nuclear shrinkage and increased basophilia

• Karyorrhexis

– Fragmentation of the pyknotic nucleus

• Karyolysis

– Fading of basophilia of chromatin

Reversible and irreversible injury

Karyolysis & karyorrhexis --

micro