Cell Injury And Cell Death

Introduction • Pathology – • scientific study (logos) of suffering (pathos).

Subdivisions of clinical Pathology: • • • • • • • • •

Histopathology Cytopathology Haematology Microbiology Immunology Chemical Pathology Genetics Toxicology Forensic Pathology

Learning Pathology: • General Pathology – Common changes in all tissues. • E.g.. Inflammation, cancer, ageing.

• Systemic Pathology – Specific changes in organs. • E.g.. Goiter, pneumonia, breast cancer.

What is Disease ? • dis + ease (not at ease…)

Disease process • • • •

1) etiology 2) pathogenesis 3) morphologic changes 4) clinical significance- c/f ,functional changes, course & prognosis of disease.

One agent  One disease - Malaria Several agents  One disease Diabetes

One agent  Several diseases Smoking

What is Diagnosis? • The formal name(s) used to describe a patient’s disease. • Based on the symptoms & Signs and the results of Pathology tests • Needs Knowledge of different diseases, their characteristics. • Important for management & knowing prognosis.

Scope of Pathology • • • • • • •

Clinical Pathology Experimental Pathology Molecular Pathology Forensic Pathology Chemical / Microbiology Immunopathology Genetics & Disease.

When we know “Where, Why, When & What, then we can answer the all important question How to solve ?

Cell Injury

Causes of cell injury • • • • • • •

Oxygen deprivation. Physical agents Chemical agents & drugs Infectious agents Immunologic reactions Genetic derangements Nutritional imbalances

Cellular responses to injury • Cellular adaptations – atrophy, hypertrophy, hyperplasia & metaplasia • Cell injury – reversible & irreversible • Cell death – necrosis & apoptosis • Intracellular accumulations • Pathologic calcifications • Cell aging

Mechanisms of cell injury • ATP depletion• 1) oxidative phosphorylation of ADPmajor pathway • 2) glycolytic pathway- minor pathway • O2 & O2 derived free radicals- partially reduced reactive O2 forms – byproduct of mitochondrial respiration..

Contd… • Loss of calcium homeostasis • increase in intracellular CaDefects in membrane permeability. • Irreversible mitochondrial damagemitochondrial permeability transition & release of cytochrome c

Reversible cell injury • Depletion of ATP will• Reduce activity of plasma memb energy -dependent Na pump – Cell swelling. • anaerobic glycolysis- acc of lactic acid • Decreased intracellular pH • Structural disruption of protein synthetic aaparatus

Reversible changes • • • • • •

Cell swelling Mitochondria, ER swollen loss of microvilli “blebs” at cellular surface “Myelin figures” in the cytoplasm If O2 is restored all these changes are reversible.

Irreversible cell injury • Inability to reverse mitochondrial dysfunction marked ATP depletion • Extreme disturbances in memb. function. • Cytoskeletal abnormalities-actn of proteases. • Detachment of cell memb from cytoskt –rupture.

Contd… • Injury to lysosomal memb. – leakage of their enzymes. • Reactive O2 species- highly toxic to cell memb& other constituents. • Lipid breakdown products- unesterified FFA accumulate due to phospholipase action-damage cell memb • Rupture of cell membrane

Ischaemia - reperfusion injury • • • •

Generation of oxygen free radicals Increased Ca concentration in Mitochondria Mitochondrial permeability transition Inflammation

Free radical – induced cell injury

• Free radicals(FR) – chemical species that have a single unpaired electron in an outer orbit. • Reactions with adjacent molecules. • Initiate autocatalytic reactions. • Propagation

Sources of free radicals • • • • •

Radiation Enzymatic metabolism of exogenous chemicals or drugs Reductn-oxidation reactions occuring in normal metabolism Transition metals donate or accept free electrons Nitric oxide(NO) –converted to highly reactive peroxynitrite anion.

Effects of free radicals • Lipid peroxidation of membranes • Oxidative modificatn of proteins-enhances degradatn of critical enzymes • Lesions in DNA

Inactivation of free radicals • Antioxidants- Vit A,E&C & glutathione in cyto. • Binding of ions to storage & transport proteins(transferrin,ferritin etc) • Enzymes(catalase, superoxide dismutases glutathione peroxidase)

Morphology of Reversible cell injury • 1) cellular swelling – occurs in imbalance of ionic/fluid homeostasis, • Hydropic change/Vacuolar degeneration. • 2)fatty change- occurs in hypoxic,toxic or metabolic injury • mainly seen in hepatocyte &myocardial cell

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Cell Death • Necrosis • Apoptosis

Necrosis Morphological changes which follow pathological cell Death in living tissue, Caused by progressive degradative action of enzymes on the lethally injured cell • Always surrounded by inflammation • • • •

Morphology • • • • •

Cytoplasmic changes – Increased eosinophilia – glassy homogenous app of cell . Moth –eaten app of cyto Finally calcification of dead cells

Contd… • Nuclear changes• 1) karyolysis – fading of basophilia of chromatin. • 2) pyknosis – nuclear shrinkage • 3) karyorrhexis – fragmentation of pyknotic nucleus

Types of necrosis

1) coagulative necrosis • Denaturation of cytoplasmic proteins. • Preservation of basic outline of coagulated eosinophilic cells with absent nucleus. • eg. myocardial infarction • Characteristic of hypoxic death of cells in all tissues except brain

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2) Caseous necrosis • Foci of tuberculous infectn • (caseous- white & cheesy) • Amorphous granular debris enclosed in inflammatory border kn as granulomatous reaction • Here tissue architecture is completely destroyed (unlike coagulation necrosis)

3)Liquefactive necrosis • Focal bacterial infections (abscesses) & hypoxic death of Brain • Digestion of dead cells by hydrolytic enzymes,transformation of tissue in a liquid viscous mass • If acute inflammation- material is creamy yellow- presence of dead white cells- PUS

4) Fat necrosis Seen in focal areas of fat destruction EnzymaticAcute pancreatitis-– abdominal emergency. Identified by surgeon as fat saponification (chalky white areas) • Traumatic• Breast tissue or subcutaneous tissue • shadowy outlines of necrotic fat cells with basophilic calcium deposits surrounded by inflammatory reaction. • • • •

Gangrene

• Ischemic necrosis - with superadded infection by putrefactive microorganisms • Common organisms • anaerobic bacteria( clostridia, streptococci, bacteriods etc)

Types of gangrene • Dry gangrene• in atherosclerotic , senile or gangrene due to burger’s disease (TAO) • Wet gangrene• in DM, gangrene of bowel asso with volvulus, intussuceptn or strangulated hernia. • Black disclouration- due to comb of H2S released by bacterial action on proteins with Hb iron.

Wet gangrene

Apoptosis • a form of cell death designed to eliminate unwanted host cells through activation of a coordinated, internally programmed series of events controlled by a set of gene products

Physiologic apoptosis • During embryogenesis • Hormone –dependent involution in adult • Cell deletion in proliferating population

Pathologic apoptosis • cell death in tumors • cell injury in certain viral diseases. • death of immune cells –after cytokine depletn & deletn of autoreactive T –cells in thymus • Cell death by injurious stimuli in low doses • Atrophy of parenchymal organs after duct obstruction

Morphology in apoptosis • 1) Cell shrinkage- small cell with dense cyto, tightly packed organelles • 2)chromatin condensation- peripheral aggregatn of chromatin into dense masses, • 3) Formation of cytoplasmic blebs & apoptotic bodies • 4) Phagocytosis of apoptotic cells or bodies by adjacent healthy cells

Contd… • Plasma membranes remain intact during apoptosis until the last stages. • No inflammation • H&E-Single cells or cluster of cells – app round or oval mass of intensely eosinophilic cytoplasm with dense nuclear chromatin fragments.

Biochemical features • Death trigerring signals- can be lack of growth factor or a ligand-receptor interaction or a specific injurious agent. • Control phase – • direct control by adapter proteins • Bcl-2 family of proteins in mitochondria. • Execution phase – • protein cleavage-by family of cystiene proteasescaspases • Phagocytosis of apoptotic bodies

Contd… • Phagocytic recognition- apoptotic cells express -• phosphatidylserine • thrombospondin

I hear, I forget I see, I remember I do, I understand…