Irreversible Cell Injury (Cell Death)

IRREVERSIBLE CELL INJURY (CELL DEATH)\nDr. Peter Simoonga\n\n# LEARNING OBJECTIVES\n- By the end of this lecture, you should be able to:\n 1. Define the common forms of cell death (apoptosis, necrosis).\n 2. Describe the different morphological patterns of necrosis.\n 3. Describe the 2 main mechanisms of apoptosis.\n 4. Differentiate the various forms of calcification.\n 5. Classify and describe the various forms of gangrene.\n\n# CELL DEATH\n## Morphological Patterns of Irreversible Cell Injury\n- Autolysis or self-digestion\n- Necrosis\n- Apoptosis\n- Gangrene\n- Calcification\n\n# AUTO-lysis or Self-Digestion\n- Defined as the disintegration of the cell by its own hydrolytic enzymes that are liberated from lysosomes.\n - Rapid process in tissues rich in hydrolytic enzymes, e.g., pancreas and gastric mucosa.\n- Morphological features of autolysis include:\n - Homogeneous and eosinophilic cytoplasm\n - Loss of cellular details\n - Remains of the cell appear as debris\n\n# CONCEPT OF CELL DEATH\n- There is no singular biochemical event that equates with cell death.\n- Necrosis: defined as "cell murder".\n- Apoptosis: defined as "programmed cell death or cell suicide".\n- Cell death occurs due to irreversible injury caused by two essential features:\n 1. Inability of the cell to reverse mitochondrial dysfunction\n 2. Disturbance in cell membrane function\n- Additional factors contributing to cell death include:\n - Depletion of proteins\n - Leakage of lysosomal enzymes into cytoplasm\n - Reduced intracellular pH\n - Further reduction in ATP levels\n\n# NECROSIS\n- Necrosis is defined as focal death along with the degradation of tissue by hydrolytic enzymes liberated by cells, and it is accompanied by inflammation.\n- Two critical features of necrosis include:\n 1. Cell digestion by lytic enzymes\n 2. Denaturation of proteins\n- Various causes leading to necrosis include hypoxia, chemical and physical agents, microbial agents, and immunological injury.\n\n## Morphologic Changes in Necrosis\n### Cytoplasmic Features\n- Homogenous and intensely eosinophilic\n- Possible presence of vacuolation or dystrophic calcification\n\n### Nuclear Features\n- Pyknosis: condensation of nuclear chromatin\n- Karyolysis: dissolution of chromatin\n- Karyorrhexis: fragmentation into clumps\n\n# TYPES OF NECROSIS\n- The Type of necrosis depends on the enzymatic degradation patterns of cells and the extracellular matrix as well as the type of necrotic debris and bacterial products present (if any).\n- Morphologically, there are five types of necrosis:\n 1. Coagulative\n 2. Liquefactive\n 3. Caseous\n 4. Fat\n 5. Fibrinoid\n\n## COAGULATIVE NECROSIS\n- Most common type of necrosis, primarily caused by irreversible focal injury from sudden cessation of blood flow (ischemia) or occasionally from bacterial and chemical agents.\n - Affected organs often include the heart, kidney, and spleen.\n- Early stages show foci of coagulative necrosis as pale, firm, and slightly swollen, which later progress to a yellowish, softer, and shrunken appearance.\n- Hallmark of Coagulative Necrosis: Normal cells convert into “tombstones” with loss of cytoplasm and nuclear details.\n- Necrosed cells are swollen and exhibit eosinophilic staining compared to normal cells.\n\n## LIQUEFACTION (COLLIQUATIVE) NECROSIS\n- Caused by ischemic injury and bacterial or fungal infections, resulting from degradation of tissue by powerful hydrolytic enzymes.\n- Common examples include brain infarcts and abscess cavities.\n- The tissue is fully digested, transforming it into a liquid viscous mass, often with acute inflammation yielding creamy yellow pus.\n\n### Morphology of Liquefaction Necrosis\n#### Gross Features\n- Affected area is soft with liquefied centers containing necrotic debris.\n- Later, a cyst wall may form.\n\n#### Microscopic Features\n- Cystic spaces filled with necrotic cell debris and macrophages containing phagocytosed material.\n- Cyst walls formed by proliferating capillaries, inflammatory cells, and gliosis in case of the brain, or by fibroblasts in abscess cases.\n\n## CASEOUS NECROSIS\n- Characterized by foci found in tuberculous infections, combining features of coagulative and liquefactive necrosis.\n- Term "caseous" (cheese-like) comes from the friable yellow-white appearance of the necrotic area.\n\n### Morphology of Caseous Necrosis\n#### Gross Features\n- Foci resemble dry cheese, are soft, granular, and yellowish, influenced by histotoxic effects of lipopolysaccharides from Mycobacterium tuberculosis.\n\n#### Microscopic Features\n- Necrosed foci appear structureless and eosinophilic with granular debris.\n- Surrounding tissue shows a characteristic granulomatous inflammatory reaction consisting of epithelioid cells and giant cells (of Langhans' or foreign body type), with a peripheral mantle of lymphocytes.\n\n## FAT NECROSIS\n- Refers to focal areas of fat destruction, commonly due to acute pancreatic necrosis or traumatic fat necrosis in the breasts.\n- Pancreatic enzymes leaking from acinar cells and ducts liquefy membranes of fat cells, while lipases split triglycerides into free fatty acids, which combine with calcium to form grossly visible chalky white areas (fat saponification).\n\n### Morphology of Fat Necrosis\n#### Gross Features\n- Fat necrosis appears as yellowish-white and firm deposits with chalky white appearance due to calcium soap formation.\n\n#### Microscopic Features\n- Necrosed fat cells exhibit cloudy appearance surrounded by an inflammatory reaction with mixed inflammatory cells.\n- Calcium soaps appear as amorphous, granular, and basophilic material in tissue sections.\n\n## FIBRINOID NECROSIS\n- Characterized by deposition of fibrin-like material with properties of fibrin, commonly encountered in examples of immunologic tissue injury such as immune complex vasculitis and autoimmune diseases.\n\n### Morphology of Fibrinoid Necrosis\n#### Microscopically\n- Brightly eosinophilic, hyaline-like deposition observed in vessel walls, with necrotic focus surrounded by nuclear debris from neutrophils.\n- Local hemorrhage may occur due to blood vessel rupture.\n\n# APOPTOSIS\n- Defined as a form of coordinated and internally programmed cell death.\n- It is a pathway induced by a tightly regulated suicide program whereby cells destined to die activate enzymes that degrade their own nuclear DNA and associated proteins.\n- Apoptosis mediates cell death in various physiological and pathological processes.\n\n## IMPORTANCE OF APOPTOSIS\n### Physiological Apoptosis\n- Organized cell destruction during tissue sculpting in embryonic development.\n- Physiologic involution in hormone-dependent tissues such as:

  • Endometrial shedding during menstruation
  • Regression of the lactating breast after breastfeeding cessation\n- Normal cell destruction followed by regeneration, as seen in intestinal epithelium.\n- Involution of the thymus in early age.\n\n### Apoptosis in Pathological States\n- Cell death in tumors exposed to chemotherapy.
    • Cell death via cytotoxic T cells in immune reactions (e.g., graft-versus-host disease, rejection reactions).
    • Progressive depletion of CD4+ T cells in AIDS pathogenesis.
    • Pathologic atrophy in organs/tissues after stimuli withdrawal (e.g., prostatic atrophy post-orchiectomy).
    • Degenerative diseases of the CNS like Alzheimer's and Parkinson's disease, or chronic infective dementias.\n\n## APOPTOSIS - MORPHOLOGICAL CHANGES\n- Key morphological changes during apoptosis include:\n - Chromatin condensation\n - Nuclear fragmentation\n - Organelle reduction\n - Mitochondrial leakage\n - Progressive cell shrinkage\n - Plasma membrane blebbing\n - Formation of apoptotic bodies\n - Phagocytosis without inflammation\n\n## APOPTOSIS - BIOCHEMICAL CHANGES\n- Biochemical alterations during apoptosis consist of:\n - Cleavage of chromosomal DNA into fragments\n - Change in plasma membrane composition (phosphatidylserine externalization) \n - Loss of electrical potential across mitochondrial inner membranes\n - Relocation of cytochrome c from mitochondrial intermembrane space to cytosol\n\n# APOPTOSIS - PATHWAYS\n## Death Ligands: Extrinsic and Intrinsic Pathways\n- Extrinsic pathway: involves death receptors like TRAIL receptors, Fas, and TNF receptors interacting with ligands like FasL, TRAIL, TNF.\n- Intrinsic pathway: involves intracellular signaling activated by stress signals leading to mitochondrial release of cytochrome c.\n\n### Components of Apoptotic Pathways\n- Extrinsic Pathway: includes death receptors which have a death domain to signal for apoptosis.\n- Intrinsic Pathway: involves BAX and BAK (pro-apoptotic proteins) and apoptosomes leading to caspase activation for executing cell death.\n\n## EXTRINSIC PATHWAY - DEATH RECEPTORS & LIGANDS\n- Death receptors are members of the TNF receptor superfamily with a critical death domain.\n- Example receptors and ligands include FasR (CD95/APO1) with FasL and TNF receptors with TNF ligands.\n\n### EXTRINSIC PATHWAY - ADAPTOR PROTEINS\n- Apoptotic adaptor proteins, e.g., FADD and TRADD, are essential in regulating pro- and anti-apoptotic signaling pathways, mediating apoptotic signaling from ligand-activated receptors.\n\n### EXTRINSIC PATHWAY - CASPASES\n- Caspases are a family of cysteine proteases critical in apoptosis initiation and execution.\n- Ten major caspases recognized include:
  • Signaling/Initiator caspases: 2, 8, 9, 10
  • Effector/Executioner caspases: 3, 6, 7
  • Inflammatory caspases: 1, 4, 5\n- Caspase 3 (CPP32) plays a central role in the apoptotic cascade.\n\n## INTRINSIC PATHWAY - COMPONENTS\n- The intrinsic pathway is mitochondria-mediated and involves Bcl-2 family proteins that regulate apoptotic events.\n- Bcl-2 family proteins:
  • Anti-apoptotic: Bcl-2, Bcl-x, Bcl-XL, Bcl-w
  • Pro-apoptotic: Bax, Bak, Bid, Bad, Bim, Bik\n- Regulation occurs via alteration in mitochondrial membrane permeability impacting cytochrome c release.\n\n### INTRINSIC PATHWAY - CYTOCHROME C & APOPTOSOME\n- Cytochrome c acts as an electron transport intermediate and binds to Apaf-1 and procaspase-9, together forming an apoptosome.\n- Mitochondrial permeability alterations allow pro-apoptotic proteins to be released into the cytosol.\n\n### INTRINSIC PATHWAY - STIMULI\n- Triggering stimuli for the intrinsic pathway include damages from radiation, absence of growth factors, and cytokines.\n- These lead to mitochondrial outer membrane permeabilization followed by cytochrome c release, activation of caspase-9, and subsequent caspase-3 activation leading to cell death.\n\n## COMPARATIVE ANALYSIS OF APOPTOSIS AND NECROSIS\n### Morphological Criteria\n| Feature | Apoptosis | Necrosis |\n|---|---|---|\n| Loss of membrane integrity | No | Yes |\n| Cell swelling and lysis | No | Yes |\n| Cell shrinkage and formation of apoptotic bodies | Yes | No |\n| Significant inflammatory response | No | Yes |\n| Lack of inflammatory response | Yes | No |\n| Induction by physiological stimuli disturbances | Yes | No |\n| Energy requirement | Yes | No |\n\n### Biochemical Criteria\n| Feature | Apoptosis | Necrosis |\n|---|---|---|\n| Requirement for macromolecular synthesis | Yes | No |\n| Requirement for de novo gene transcription | Yes | No |\n| Nonrandom fragmentation of DNA | Yes | No |\n| Induction by non-physiological disturbances | No | Yes |\n| Lack of energy requirement | No | Yes |\n| Lack of macromolecular synthesis requirement | No | Yes |\n| Random digestion of DNA | No | Yes |\n\n# GANGRENE\n- A potentially life-threatening condition due to critically insufficient blood supply (ischemia) often following injury or infection or in chronic health conditions affecting blood circulation.\n- Primary cause of gangrene is reduced blood supply leading to necrosis.\n- Risk factors include diabetes and long-term smoking.\n\n## TYPES OF GANGRENE\n- Three main forms of gangrene exist:
  • Dry Gangrene
  • Wet Gangrene
  • Gas Gangrene\n- Coagulation necrosis might undergo liquefaction due to putrefactive bacteria in either type of gangrene.\n\n### DRY GANGRENE\n- A form of coagulative necrosis occurring in ischemic tissue with inadequate blood supply.\n- Frequently caused by peripheral artery disease or acute limb ischemia.\n- The affected tissue is dry, shrunken, and dark reddish-black.\n- Complete separation and autoamputation may occur if the gangrenous tissue is not surgically removed.\n- Common sites include toes and feet in elderly patients due to arteriosclerosis, thromboangiitis obliterans, Raynaud’s disease, trauma, and ergot poisoning.\n\n### WET GANGRENE\n- Characterized by bacterial infection with poor prognosis.\n- Infected by saprogenic microorganisms (e.g. Bacillus fusiformis), causing tissue to swell and emit foul odor.\n- Develops rapidly due to venous (or less often, arterial) blood flow obstruction.\n- Affected areas are edematous, soft, putrid, and dark.\n\n### CONTRASTING FEATURES OF DRY AND WET GANGRENE\n| Feature | Dry Gangrene | Wet Gangrene |\n|---|---|---|\n| Site | Commonly limbs | More common in bowel |\n| Blood Supply Mechanism | Arterial occlusion | Venous obstruction, less often arterial |\n| Macroscopy | Organ dry, shrunken, and black | Part moist, soft, swollen, rotten, and dark |\n| Putrefaction | Limited | Marked |\n| Line of demarcation | Present | No clear line |\n| Bacteria | Fail to survive | Numerous present |\n| Prognosis | Generally better | Generally poor |\n\n### GAS GANGRENE\n- A bacterial infection that produces gas within tissues (e.g., Clostridium perfringens).\n- Rapidly spreading due to gaseous production expanding and infiltrating healthy tissue.\n- Treatment is a medical emergency due to quick progression to toxemia and shock.\n- Reported gas composition in one clinical case included 5.9% hydrogen, 3.4% carbon dioxide, 74.5% nitrogen, and 16.1% oxygen.\n\n#### Morphological Features of Gas Gangrene\n##### Gross Features\n- Affected area appears swollen, edematous, painful, and crepitant from gas bubbles.\n- Tissue may become dark black and foul-smelling.\n\n##### Microscopic Features\n- Muscle fibers undergo coagulative necrosis with liquefaction.\n- Numerous gram-positive bacilli are identifiable, with a periphery demonstrating leukocytic infiltration, edema, and common thrombosis in capillaries and veins.\n\n# PATHOLOGIC CALCIFICATION\n- Definition: Abnormal deposits of calcium salts in tissues, except for bones and teeth.\n- Two types of pathological calcification:\n - Dystrophic Calcification: Deposition of calcium in dead or degenerated tissues with normal calcium metabolism.\n - Metastatic Calcification: Occurs in normal tissues with deranged calcium metabolism and hypercalcemia.\n\n## DIFFERENCES BETWEEN DYSTROPHIC AND METASTATIC CALCIFICATION\n| Feature | Dystrophic Calcification | Metastatic Calcification |\n|---|---|---|\n| Definition | Deposits in dead/degenerated tissues | Deposits in normal tissues |\n| Calcium Metabolism | Normal | Deranged |\n| Serum Calcium Level | Normal | Hypercalcemia |\n| Reversibility | Generally irreversible | Reversible upon correction |\n| Causes | Necrosis, old scars, atheromas | Hyperparathyroidism, prolonged immobilization |\n\n# THANK YOU