Cellular Injury Lecture Notes PDF

CELLULAR INJURY CELL INJURY Cell injury is defined as the effect of a variety of stresses due to etiologic agents a cell encounters resulting in changes in its internal and external environment.  STRESS= ETIOLOGICAL AGENTS LIKE BACTERIA STRESS Effects...

CELLULAR INJURY CELL INJURY Cell injury is defined as the effect of a variety of stresses due to etiologic agents a cell encounters resulting in changes in its internal and external environment.  STRESS= ETIOLOGICAL AGENTS LIKE BACTERIA STRESS Effects Affected cell CHANGES IN INTERNAL AND STRESS EXTERNAL ENVIRONMENT OF CELL STRESS  CELLULAR RESPONSES TO CELL INJURY. STRESS ALTERED FUNCTIONAL SEVERE STRESS AND DEMAND PERSISTANT STRESS MILD TO MODERATE STRESS ADAPTATIONS REVERSIBLE CELL INJURY IRREVERSIBLE CELL -ATROPHY -DEGENERATIONS INJURY -HYPERTROPHY -SUBCELLULAR ALTERATIONS -CELL DEATH -HYPERPLASIA -INTRACELLULAR ACCUMULATIONS -DYSPLASIA ETIOLOGY OF CELL INJURY The cells may be broadly injured by two major ways: A. Genetic causes B. Acquired causes  ETIOLOGY OF CELL INJURY ACQUIRED GENETIC CAUSES CAUSES 1. Hypoxia and ischemia 2. Physical agents 3. Chemical agents and drugs 4. Microbial agents 5. Immunologic agents 6. Nutritional derangements 7. Ageing 8. Psychogenic diseases 9. Iatrogenic factors 10. Idiopathic diseases. HYPOXIA AND ISCHEMIA Hypoxia is the most common cause of cell injury TWO REASONS Reduced blood supply Due to defects in oxygen to cells due to carrying RBC’S or due to interruption in blood or HEART DISEASES, flow leading to LUNG DISEASES, due to ischemia increased demand from tissues. INTERRUPTED BLOOD FLOW HEART DISEASES ABNORMAL RBC’S LUNG DISEASES 1-PHYSICAL AGENTS Physical agents in causation of disease are: Mechanical Trauma (e.g. Road Accidents) Thermal Trauma (e.g. By Heat And Cold) Electricity Radiation (e.g. Ultraviolet And Ionizing) Rapid Changes in atmospheric pressure  COLD TRAUMA HEAT TRAUMA RADIATION TRAUMA ELECTRICITY TRAUMA 2-CHEMICALS AND DRUGS - Chemical poisons such as cyanide, arsenic, mercury. - Strong acids and alkalis - Environmental pollutants - Insecticides and pesticides - Oxygen at high concentrations - Hypertonic glucose and salt - Social agents such as alcohol and narcotic drugs - Therapeutic administration of drugs. 3-MICROBIAL AGENTS Injuries by microbes include Infections caused by bacteria, rickettsiae, viruses, fungi, protozoa, metazoa, and other parasites. 4-PSYCHOGENIC DISEASES There are no specific biochemical or morphologic changes in common acquired mental diseases due to mental stress, strain, anxiety, overwork and frustration e.g. depression, schizophrenia. However, problems of drug addiction, alcoholism, and smoking result in various organic diseases such as liver damage, chronic bronchitis, lung cancer, peptic ulcer, hypertension, ischemic heart disease etc. 5-IMMUNOLOGIC AGENTS Immunity is a ‘double-edged sword’: it protects the host against various injurious agents but it may also turn lethal and cause cell injury e.g. Hypersensitivity reactions; Anaphylactic reactions; and Autoimmune diseases. 6-AGEING Cellular ageing or senescence leads to impaired ability of the cells to undergo replication and repair, and ultimately lead to cell death culminating in death of the individual. PATHOGENESIS OF CELL INJURY Objective of lecture Types of Cell- injuries. Pathogenesis of Cell-injury Pathogenesis in Reversible and Irreversible Hypoxic Cell-injury in particular Outcomes/Events in Reversible and Irreversible Hypoxic Cell injury.  Injury to the normal cell by one or more of the above listed etiologic agents may result in a state of CELL REPAIR AND HEAL REVERSIBLE CELL INJURY CELL DEATH IRREVERSIBLE CELL INJURY OUTCOME OF THE CELL-INJURY DEPENDS ON: TYPE, DURATION, OF INJURIOUS AGENT SEVERITY TYPE, STATUS , OF TARGET CELL ADAPTABILITY  BASED ON TYPE, DURATION, SEVERITY: Small Dose of Chemical/Physical Agent Large Dose of Chemical/Physical Agent REVERSIBLE CELL INJURY IRREVERSIBLE CELL INJURY Short Duration of Ischemia Long Duration of Ischemia REVERSIBLE CELL INJURY IRREVERSIBLE CELL INJURY BASED ON TYPE, STATUS, ADAPTABILITY OF TARGET CELL: IF A CELL IS HIGHLY SUSCEPTABLE TO HYPOXIA EARLY CELL INJURY EXAMPLE: Skeletal muscle can withstand hypoxic injury for long-time, While cardiac muscle suffers Irreversible cell injury if exposed to Hypoxia for > 20 mins Skeletal Muscle Cardiac Muscle PATHOGENESIS OF ISCHAEMIC AND HYPOXIC INJURY  Ischemia and hypoxia are the most common forms of cell injury. It leads to - REVERSIBLE CELL INJURY - IRREVERSIBLE CELL INJURY REVERSIBLE CELL INJURY IN HYPOXIA  If hypoxia is of short duration -> the effects may be reversible on rapid restoration of circulation. Example: In coronary artery occlusion, the myocardial contractility, metabolism and ultra structure are reversed if the circulation is quickly restored CORONARY ARTERY OCCLUSION EVENTS IN REVERSIBLE CELL INJURY ARE: 1. DECREASED GENERATION OF CELLULAR ATP: 2. INTRACELLULAR LACTIC ACIDOSIS: 3. DAMAGE TO PLASMA MEMBRANE PUMPS: 4. REDUCED PROTEIN SYNTHESIS: 1. DECREASED GENERATION OF CELLULAR ATP: All living cells require continuous supply of oxygen to produce ATP which is essentially required for a variety of cellular functions. ATP is generated from: - AEROBIC PATHWAY - ANEROBIC PATHWAY  ATP in human cell is derived from 2 sources AEROBIC ANEROBIC RESPIRATION PATHWAY Anaerobic Requires Oxygen Glycolytic oxidation ATP is generated from Takes place in the Glucose/Glycogen mitochondria. in the absence of Oxygen WITH INTERRUPTED BLOOD WITH DEFECTS IN OXYGEN SUPPLY CARRYING RBC’S OR HEART OR LUNG DISEASES BOTH OXYGEN AND GLUCOSE SUPPLY IS COMPROMISED ONLY OXYGEN COMPROMISED COMPLETE ATP DEPLETION ATP IS GENERATED BY ANEROBIC PATHWAY SEVERE CELL INJURY CELL INJURY IS LESS Cells which depend only on Aerobic Respiration for ATP generation MYOCARDIUM CELLS Hence these tissues suffer PROXIMAL TUBULAR more severely and CELLS KIDNEYS rapidly due to hypoxia NEURONS OF CNS  2. INTRACELLULAR LACTIC ACIDOSIS: Low oxygen supply to the cell Mitochondria fails (No Oxygen No ATP generation) Switches to Anaerobic Glycolytic Pathway for the requirement of ATP. Accumulation of Lactic Acid Decreased Intracellular pH. Clumping of Nuclear Chromatin. CHROMATIN CLUMPING 3. DAMAGE TO PLASMA MEMBRANE PUMPS  DECREASED A.T.P (ATP)-Dependent Sodium pump Decreased Production of PHOSPHOLIPIDS FAILURE Repair of Cell membrane is stopped Increased accumulation of Na in cell Membrane damage Swelling of Cell Calcium influx into cell (HYGROPIC SWELLING) (Particularly mitochondria) Swelling of Mitochondria (Amorphous densities) HYDROPIC SWELLING Swelling of Mitochondria 4. REDUCED PROTEIN SYNTHESIS: Endoplasmic Golgi apparatus Reticulum  Membranes of endoplasmic Reticulum and Golgi apparatus swell up. Ribosomes are detached from granular (rough) endoplasmic reticulum Reduced protein synthesis Other changes: Loss of Microvilli and Intramembranous particles Focal projections of the cytoplasm (blebs). Myelin figures may be seen lying in the cytoplasm  Note: Up to this point, withdrawal of acute stress that resulted in reversible cell injury can restore the cell to normal state. IRREVERSIBLE CELL INJURY IN HYPOXIA/ISCHEMIA  Persistence of ischemia (i.e., reduced blood flow to a tissue/organ) or hypoxia Continuous ↓ in ATP, ↓ in Proteins, ↓ intracellular pH, and leakage of lysosomal enzymes into the plasma. Irreversible damage to the structure and function of the cell Even after reperfusion with oxygen i.e., CELL DEATH CHANGES IN IRREVERSIBLE CELL INJURY ARE: 1. CALCIUM INFLUX 2. ACTIVATED PHOSPHOLIPASES 3. INTRACELLULAR PROTEASES 4. ACTIVATED ENDONUCLEASES 5. LYSOSOMAL HYDROLYTIC ENZYMES 1. CALCIUM INFLUX Due to continued hypoxia - Large cytosolic influx of calcium ions occurs (Esp. after reperfusion of irreversibly injured cell) Vacuoles Formation in the Mitochondria Deposition of amorphous calcium salts in the mitochondrial matrix VACUOLES 2. ACTIVATED PHOSPHOLIPASES: Increased cytosolic influx of calcium into the cell Activation of endogenous phospholipases Damage to phospholipids layer of Cell membrane Damage to membrane function Phospholipid phospholipids bilayer 3. INTRACELLULAR PROTEASES Activated Intracellular Proteases (released by Lysosomes) Digestion of cytoskeleton of cell Cytoskeleton Damage 4. ACTIVATED ENDONUCLEASES Activated lysosomal enzymes such as Proteases and Endonucleases. Damage to DNA/Nuclear proteins Three types of nuclear damage can happen: 1- PYKNOSIS: 2- KARYORRHEXIS: 3- KARYOLYSIS: Condensation and Nuclear Fragmentation Dissolution of Nucleus clumping of nucleus Into small bits 5. LYSOSOMAL HYDROLYTIC ENZYMES Lysosomal Membrane Damage Escape of lysosomal hydrolytic enzymes into cell medium Activation of hydrolytic enzymes by low pH and Low oxygen Enzymatic digestion of cellular components CELL DEATH DEAD CELLS PHAGOCYTOSED BY MACROPHAGES  Hydrolytic enzymes Lysosomes Digestion of cell organelles Lysosome enzymes: Hydrolase, RNAase, DNAase, Protease, Glycosidase, Phosphatase, Lipase, Amylase. COMMON ENZYME MARKERS IN SERUM FOR DIFFERENT FORMS OF CELL DEATH: ENZYME DISEASE Aspartate aminotransferase Viral Hepatitis, Alcoholic Liver Disease (AST, SGOT) Acute Myocardial Infarction Alanine aminotransferase More specific for diffuse liver cell damage (ALT, SGPT) than AST e.g. Viral Hepatitis Creatine Kinase-MB Acute Myocardial Infarction, Myocarditis (CK-MB) Lipase More specific for acute pancreatitis Amylase Acute Pancreatitis Sialadenitis Lactic dehydrogenase Acute Myocardial Infarction (LDH) Myocarditis Skeletal muscle injury Cardiac troponin (CTn) Specific for Acute Myocardial Infarction  Objectives of our lecture - Learn the Morphological forms seen in Reversible Cell Injury and Irreversible cell Injury MORPHOLOGY OF REVERSIBLE CELL INJURY Morphologic Forms Of Reversible Cell Injury: 1. Hydropic change 2. Hyaline change 3. Mucoid change 4. Fatty change 1. HYDROPIC CHANGE/DEGENERATION Accumulation of water within the cytoplasm of the cell. Synonym= CLOUDY SWELLING , VACUOLAR DEGENERATION (Due to cytoplasmic vacuolation). It is entirely reversible. Commonest form in cell injury Earliest form in cell injury Etiology: - Hypoxia - Microbes - Chemicals agents - Physical agents - Etc…  PATHOGENESIS Cloudy swelling results from impaired regulation of sodium and potassium at the level of cell membrane. MORPHOLOGIC FEATURES: Grossly, the affected organ such as kidney, liver, pancreas, or heart muscle is enlarged due to swelling. The cut surface bulges outwards and is slightly opaque. MICROSCOPICALLY: The features of Hydropic swelling of kidney are as under i) The tubular epithelial cells are swollen ii) cytoplasm contains small clear vacuoles iii) Small cytoplasmic blebs may be seen. 2. HYALINE CHANGE/DEGENERATION The word ‘hyaline’ or ‘hyalin’ means glassy. Hyalinisation is a common descriptive histologic term. It means glassy, homogeneous, eosinophilic appearance of proteinaceous material in haematoxylin and eosin-stained sections. It does not refer to any specific substance.  Hyaline change is seen in heterogeneous pathologic conditions and may be intracellular or extracellular. INTRACELLULAR HYALINE Intracellular hyaline is mainly seen in epithelial cells. A few examples are as follows: 1. Hyaline droplets in the proximal tubular epithelial cells due to excessive reabsorption of plasma proteins in proteinuria. Hyaline droplets in proximal tubular epithelial cells 2. Hyaline degeneration of rectus abdominalis muscle called Zenker’s degeneration, occurring in typhoid fever. Zenker’s degeneration 3. Mallory’s hyaline/Mallory bodies- seen in the hepatocytes in alcoholic liver cell injury. 4. Russell’s bodies representing excessive immunoglobulin in the rough endoplasmic reticulum of the plasma cells RUSSELL’S BODIES EXTRACELLULAR HYALINE Extracellular hyaline commonly seen in connective tissues. A few examples of extracellular hyaline change are as under: 1. Hyaline degeneration in leiomyomas of the uterus 2. Hyalinised old scar of fibrocollagenous tissues. 3. Hyaline arteriolosclerosis in renal vessels in hypertension and diabetes mellitus. 3. MUCOID CHANGE/ DEGENERATION Mucoid means mucus-like Mucin (constituent of mucus) is a glycoprotein normally produced by epithelial cells of MUCUS MEMBRANES and MUCUS GLANDS and by some CONNECTIVE TISSUE (CT) cells. The mucin produced by CT cells is called MYXOID. MUCUS GLANDS MUCUS What is Mucoid Degeneration? Excessive mucus production by the - Epithelial cells of mucous membranes and mucous glands (EPITHELIAL MUCIN) - Connective tissues (CONNECTIVE TISSUE MUCIN) EPITHELIAL MUCIN Examples of functional excess of epithelial Mucin: 1. Catarrhal inflammation of mucous membrane (e.g. of respiratory tract, alimentary tract). 2. Cystic fibrosis of the pancreas/Lungs. 3. Mucin-secreting tumors (e.g. of ovary, stomach, large bowel etc) RESPIRATORY TRACT CATARRAL INFLAMMATION - EXCESS MUCUS PRODUCTION CYSTIC FIBROSIS OF THE LUNGS. CONNECTIVE TISSUE MUCIN Connective tissue mucin or myxoid change are as under: 1. Mucoid or myxoid change in some tumours like fibroadenoma. 2. Myxomatous change in the dermis in Myxoedema. MYXOEDEMA 4. FATTY DEGENERATION (STEATOSIS) Intracellular accumulation of neutral fat within parenchymal cells. Fatty change is particularly common in the liver but may occur in other non-fatty tissues as well e.g. in the heart, skeletal muscle, kidneys (lipoid nephrosis) and other organs. FATTY LIVER MORPHOLOGY OF IRREVERSIBLE CELL INJURY (CELL DEATH)  Cell death is a state of irreversible injury. PROCESSES INVOLVED IN CELL DEATH LOCALLY CHANGES FOLLOWING 1- AUTOLYSIS LOCAL CHANGE 2- NECROSIS 1- GANGRENE 3- APOPTOSIS 2- CALCIFICATION AUTOLYSIS (self-digestion) Disintegration of the cell by its own hydrolytic enzymes liberated from Lysosomes PACE OF AUTOLYSIS RAPID INTERMEDIATE SLOW in tissues rich in in tissues like the in fibrous tissue. hydrolytic HEART, LIVER AND enzymes- KIDNEY PANCREAS, GASTRIC MUCOSA  NECROSIS Necrosis is defined as a localized area of death of tissue followed later by degradation/breakdown of tissue by hydrolytic enzymes liberated from dead cells; it is invariably accompanied by inflammatory reaction.  DEAD TISSUE NECROSIS DEGRADATION /BREAKDOWN OF TISSUE by INFLAMMATION HYDROLYTIC ENZYME ETIOLOGICAL AGENTS: - Hypoxia, - Chemical - Physical agents - Microbial agents - Immunological etc… 5 TYPES OF NECROSIS: Coagulative Liquefaction (Colliquative) Based on Etiology and Caseous Morphologic Fat appearance Fibrinoid Necrosis 1. COAGULATIVE NECROSIS Most common type of necrosis caused by irreversible focal injury. Cause: - From sudden cessation of blood flow (ischemic necrosis) - Less often from bacterial and chemical agents. The organs commonly affected are the heart, kidney, and spleen.  APPEARANCE GROSSLY/MACROSCOPICALLY EARLY STAGE LATE STAGE - Pale - Yellowish INFARCT - Firm - Softer - Slightly swollen - Shrunken.  MICROSCOPICALLY Hallmark of coagulative necrosis = ‘TOMB STONES’ i.e. outlines of the cells are retained and the cell type can still be recognized but their cytoplasmic and nuclear details are lost. OTHER FINDINGS MICROSCOPICALLY: Nuclear changes of pyknosis, karyorrhexis and karyolysis Cell digestion and liquefaction fail to occur. Eventually, the necrosed focus is infiltrated by inflammatory cells and the dead cells are phagocytosed leaving granular debris and fragments of cells 2. LIQUEFACTION (COLLIQUATIVE) NECROSIS Due to ischemic injury and bacterial or fungal infections Liquefaction is due to Hydrolytic enzymes. The common examples are infarct brain and abscess cavity.  Grossly, the affected area is soft with liquefied centre containing necrotic debris. Later, a cyst wall is formed.  Microscopically, the cystic space contains necrotic cell debris and macrophages filled with phagocytosed material. The cyst wall is formed by - Proliferating capillaries, - Inflammatory cells, and - Gliosis (proliferating glial cells) in the case of brain and Abscess cavity - Proliferating fibroblasts 3. CASEOUS NECROSIS Caseous (caseous= cheese-like) necrosis is found in the centre of foci of tuberculous infections. It combines features of both coagulative and liquefactive necrosis. 4. FAT NECROSIS Fat necrosis is a special form of cell death occurring at mainly fat-rich anatomic locations in the body. Examples are: 1. Traumatic fat necrosis of the breast, 2. Mesenteric fat necrosis due to acute pancreatitis. TRAUMATIC FAT NECROSIS OF BREAST MESENTERIC FAT NECROSIS  In fat necrosis: HYDROLYSIS AND RUPTURE OF ADIPOCYTES Release of neutral fat CHANGES INTO GLYCEROL & FREE FATTY ACIDS. Free fatty acids complex with Calcium to form CALCIUM SOAPS (SAPONIFICATION)  Fat necrosis appears as yellowish-white and firm Deposits. Formation of calcium soaps imparts the necrosed foci FIRMER AND CHALKY WHITE SAPONIFICATION APPEARANCE. 5. FIBRINOID NECROSIS Fibrinoid necrosis is characterized by deposition of fibrin-like material which has the staining properties of fibrin such as phosphotungistic acid haematoxylin (PTAH) stain. It is encountered in various examples of immunologic tissue injury. E.g. - Immune Complex Vasculitis - Autoimmune Diseases - Arthus Reaction Etc APOPTOSIS  Apoptosis is a form of ‘coordinated and internally programmed cell death’. When the cell is not needed, pathway of cell death is activated (‘cell suicide’). Unlike necrosis, apoptosis is not accompanied by any inflammation and collateral tissue damage. APOPTOSIS IN BIOLOGIC PROCESSES 1. Physiological apoptosis 2. Pathological apoptosis Physiologic Processes: 1. Physiologic involution of cells in hormone- dependent tissues e.g. endometrial shedding, regression of lactating breast after withdrawal of breast-feeding. 2. Normal cell destruction followed by replacement proliferation such as in intestinal epithelium. 3. Involution of the thymus in early age. Pathologic Processes: 1. Cell death in tumors exposed to chemotherapeutic agents. 2. Progressive depletion of CD4+T cells in the pathogenesis of AIDS. 3. Pathologic atrophy of organs and tissues on withdrawal of stimuli e.g. prostatic atrophy after orchiectomy. 5. Cell death in response to low dose of injurious agent involved in causation of necrosis e.g. radiation, hypoxia and mild thermal injury. 6. In degenerative diseases of CNS e.g. in Alzheimer’s disease, Parkinson’s disease, and chronic infective dementias. 7. Heart diseases e.g. in acute myocardial infarction (20% necrosis and 80% apoptosis). MOLECULAR MECHANISMS OF APOPTOSIS Several physiologic and pathologic processes activate apoptosis in a variety of ways. Molecular events involved in apoptosis: 1- INITIATORS OF APOPTOSIS 2- INTIAL STEPS IN APOPTOSIS 3- FINAL PHASE OF APOPTOSIS 4- PHAGOCYTOSIS 1. Initiators of apoptosis: All cells have inbuilt effector mechanisms for cell survival and signals of cell death. It is the loss of this balance that determines survival or death of a cell. A cell may be initiated to programmed cell death by: Withdrawal of normal cell Agents of cell injury survival signals: OR e.g. absence of certain hormones, growth e.g. Heat, Radiation, Hypoxia, factors, cytokines. Toxins, Free Radicals. 2. Initial steps in apoptosis After the cell has been initiated into self-destruct mode, Cell death signaling mechanisms gets activated from I. Intrinsic (mitochondrial) pathway II. Extrinsic (cell death receptor initiated) pathway. However, finally mediators of cell death are activated caspases. Caspases are a series of proteolytic or protein-splitting enzymes which act on nuclear proteins and organelles containing protein components. Intrinsic (mitochondrial) pathway Increased mitochondrial permeability activates this pathway Cytochrome- c (protein) released from mitochondria into the cytoplasm of the cell Cytochrome- c triggers the cell into apoptosis. activates Caspases-9 APOPTOSIS  The major mechanism of regulation of this mitochondrial protein (Cytochrome- c) is by pro-apoptotic and anti-apoptotic members of Bcl proteins.  BCL-2 is localized to the outer membrane of mitochondria, where it plays an important role in promoting cellular survival and inhibiting the actions of pro-apoptotic proteins. BCL-2 proteins (Anti-apoptotic proteins) Bad Bim BH3-only proteins (Pro- apoptotic proteins) Extrinsic (cell death receptor initiated) pathway It works by Activation of death receptors on the cell membrane. Cell Death Receptors are: - Type 1 Tumour Necrosis Factor Receptor (TNF- R1) and - a related transmembrane protein called Fas (CD95) and its ligand (FasL). (Fas belongs to TNF Receptor family)  TNF-α (Death Ligand) FasL Extracellular (Ligand) Fas Cell membrane TNFR1 Receptor Receptor ADAPTER Intracellular Fas-associated protein Death domain (FADD) ADAPTER Tumor Necrosis Factor Receptor Type 1-associated Death APOPTOSIS Activation of Domain (TRADD) Caspases 8 and 10 3. Final phase of apoptosis The activated caspases have proteolytic action Proteolytic actions lead to 1. Nucleus damage 2. Chromatin clumping CELL DEATH 3. Cytoskeleton damage 4. Disruption of endoplasmic reticulum 5. Mitochondrial damage 6. Disturbed cell membrane. 4. Phagocytosis Thrombospondin molecules Dead phagocyte Apoptotic Cell Phosphatidylserine molecule Phagocytosed dead cell