irreversible cell injury, necrosis, apoptosis, free radicles, reperfusion injury

irreversible cell injury, necrosis, apoptosis, free radicles, reperfusion injury

1. Item-3 Irreversible Cell Injury Dr. A K M Maruf Raza Associate Professor of Pathology Based on Robbins and Cotran 9 th edition

2. Irreversible cell injury: If the injurious stimulus persists or is severe enough, the cell suffers irreversible injury and ultimately undergoes cell death. Two types: i . Necrosis. ii. Apoptosis.

3. Necrosis Necrosis is the result of denaturation of intracellular proteins and enzymatic digestion of the lethally injured cell. Necrotic cells lost membrane integrity and their contents leak out. The enzymes that digest the necrotic cell derived from the lysosomes of that cells.

4. Nuclear changes in Necrosis Necrotic cells show increased eosinophilia (increased pink colour ). Nuclear changes appears in three patterns due to breakdown of DNA: I. Karyolysis . II. Pyknosis . III. Karyorrhexis .

5. Karyolysis is due to loss of DNA. The basophilia (violate color) taken by chromatin fade away. Pyknosis characterized by nuclear shrinkage and increased basophilia (pink colour ). Karyorrhexis , the nuclear fragmentation in pyknotic cells and ultimately disappearance of nucleus.

6. Types of Necrosis: Coagulative necrosis is a form of necrosis in which the architecture of dead tissues is preserved . It is due to denaturation of structural proteins and enzymes. Exm : Necrosis effecting heart, kidney, lungs. Liquefactive necrosis is characterized by digestion of the dead cells, resulting formation of a liquid viscous mass called pus . Exm : Abscess, Boil, necrosis of brain.

7. Types of Necrosis: Gangrenous necrosis is a clinical term applicable generally for necrosis affecting the limp, specially the lower leg that has lost its blood supply and has undergone necrosis. Wet gangrene: When bacterial infection is superimposed there is there is more liquefactive necrosis in the gangrenous limb .

8. Types of Necrosis: Caseous necrosis is a special type of coagulative necrosis present in tuberculous infection (Tuberculosis). The term “ caseous ” ( cheeselike ) is derived from the friable white appearance of the area of necrosis.

9. Types of Necrosis: Fat necrosis refers to focal areas of fat destruction due to release of activated pancreatic lipases into the pancreas and peritoneal cavity. Fat necrosis is two types: i . Enzymatic fat necrosis: occurs in pancreas. ii. Traumatic fat necrosis: in breast, in subcutaneous tissue. Fibrinoid necrosis is a form of necrosis seen in immune reactions involving blood vessels.

10. Apoptosis Apoptosis is genetically regulated cell death induced by regulated suicidal program by injured cells. It is referred to as programmed cell death. Apoptosis is a pathway of cell death in which cells die by activating enzymes that degrade the cells own nuclear DNA and nuclear and cytoplasmic proteins.

11. Causes/ example of Apoptosis Apoptosis in Physiologic Situations: The destruction of cells during embryogenesis. Epithelial Cell loss in the intestinal epithelium, skin. Death of cells that served their purpose such as neutrophils Regression of the lactating breast after lactational period.

12. Causes/ example of Apoptosis Apoptosis in Pathologic Conditions: Radiation, Cytotoxic anticancer drugs and hypoxia can damage DNA. If DNA is not repaired ultimately cells undergo apoptosis. Degenerative diseases of the central nervous system. Cell death in viral infections due to viral cytopathic effect.

13. Morphologic changes in Apoptosis Cell shrinkage. Chromatin condensation. Formation of apoptotic bodies . Phagocytosis of apoptotic cells or cell bodies, usually by macrophages.

14. Mechanisms of Apoptosis Apoptosis results from the activation of enzymes called caspases . Caspases exist as inactive proenzymes within the cells. The process of apoptosis divided into: 1.initiation/activation phase: caspases become catalytically active 2.execution phase: degradation of cellular components occurs.

15. Mechanisms of Apoptosis The activation of caspases depends on balance between pro-apoptotic (BAX, BAC) and anti-apoptotic (BCL2, BCL-XL) proteins. Two pathways of caspase activation: i . The mitochondrial pathway (Mainly) ii. The death receptor pathway

16. Difference between Necrosis and Apoptosis Feature Necrosis Apoptosis Cell size Enlarged (swelling) Reduced (shrinkage) Nucleus Pyknosis karyorrhexis karyolysis Fragmentation into fragments Plasma membrane Disrupted Intact Cellular contents Enzymatic digestion; leak out of cell Intact; may be released in apoptotic bodies Adjacent inflammation Frequent No Physiologic or pathologic role pathologic physiologic and also pathologic

17. Mechanisms of Cell Injury/ Biochemical alteration in cell injury Depletion of ATP. Mitochondrial Damage. Influx of Calcium and Loss of Calcium Homeostasis Accumulation of Oxygen-Derived Free Radicals. Defects in Membrane Permeability. Damage to DNA and Proteins.

18. Mechanisms of Cell Injury

19. ATP depletion Reduction of ATP and decreased ATP synthesis is the most important cause of cell death associated with hypoxic and toxic injury. The major causes of ATP depletion are: i . reduced supply of oxygen and nutrients. ii. mitochondrial damage. iii. actions of some toxins (e.g., cyanide).

20. Mitochondrial damage Mitochondria supply life sustaining energy ATP, its damage causes cell death. Damage to mitochondrial membranes results in opening of the mitochondrial permeability transition pore. Which leads to decreased ATP generation and release of proteins that trigger cell death.

21. Influx of Calcium and Loss of Calcium Homeostasis Calcium ions are important mediators of cell injury. Low level of intracellular calcium protects cells from injury. Ischemia and certain toxins cause an increase in cytosolic calcium concentration , initially from intracellular stores and later influx across the plasma membrane.

22. Influx of Calcium and Loss of Calcium Homeostasis Increased intracellular Ca2+ causes cell injury by several mechanisms: i . The accumulation of Ca2+ in mitochondria results in opening of the mitochondrial permeability transition pore. ii. Increased cytosolic Ca2+ activates a number of lysosomal enzymes. iii. Increased intracellular Ca2+ causes activation of caspases .

23. Influx of Calcium and Loss of Calcium Homeostasis

24. Accumulation of Oxygen-Derived Free Radicals Cell injury induced by free radicals causes cell damage in many pathologic conditions. Such as chemical and radiation injury, ischemia-reperfusion injury, cellular aging, and microbial killing by phagocytes.

25. Defects in Membrane Permeability Membrane damage of cell causes cell injury and cell death. The most important sites of membrane damage in cell injury are: i . Mitochondrial membrane. ii. Plasma membrane. iii. Membranes of lysosomes .

26. Necroptosis Necroptosis is the form of cell death that shares both necrosis and apoptosis. Morphologically it resembles necrosis but by mechanism it is programmed cell death that is Apoptosis.

27. Necroptosis It is associated with cell death in i.Steatohepatitis (Alcoholic fatty liver disease) ii.Acute pancreatitis iii.Reperfusion injury iv.Neurodegenerative diseases such as Parkinson disease.

28. Pyroptosis Pyro (Fever)+ Optosis (Apoptosis). Pyroptosis is the release of fever inducing cytokine IL-1 which bears biochemical similarities with apoptosis. Pyroptosis occurs in cells infected by microbial organism . Which causes death of the infected cell.

29. Autophagy Autophagy is a process in which a cell eats its own contents. Caused by the delivery of cytoplasmic materials to the lysosome for degradation. Autophagy plays a role in human diseases like Cancer, Alzheimer disease and also in ageing .

30. Ischemia-Reperfusion Injury Ischemia +Reperfusion Injury. Restoration of blood flow causes recovery of cells of reversibly injured cells. Restoration of blood flow to ischemic tissues can also increases the injury and cause cell death , it is called Ischemia-Reperfusion Injury . Exmp : Rapid restoration of blood flow in MI can cause more myocardial injury.

31. Ischemia-Reperfusion Injury Why new tissue injury after restoration of blood?: i . Increased generation of reactive oxygen and nitrogen species. ii. Intracellular calcium overload. iii. Repurfusion recruit circulating neutrophils and causes tissue injury. iv. Activation of the complement system.

32. Free Radicals; Accumulation of Oxygen-Derived Free Radicals (Oxidative Stress)

33. Free radicals Free radicals are chemical species that have a single unpaired electron in an outer orbit. Unpaired electrons are highly reactive and attack proteins, lipids, carbohydrates and nucleic acids of cells causing cell damage and cellular ageing. Superoxide anion. hydrogen peroxide, nitric oxide; hydroxyl radical are example of free radicals.

34. Free radicals Reactive oxygen species (ROS) are free radical produced normally in cells during mitochondrial respiration and energy generation. Increased production or decreased scavenging of ROS may lead to an excess of these free radicals, a condition called oxidative stress.

35. Generation of Free Radicals The reduction-oxidation reactions that occur during normal metabolic processes. Absorption of radiant energy (e.g., ultraviolet light, x-rays). Produced in activated leukocytes during inflammation. Enzymatic metabolism of exogenous chemicals or drugs generate free radicals (e.g., CCl4) Nitric oxide (NO) generated by endothelial cells, macrophages act as free radical.

36. Effects of Free Radicals on cell The effects of ROS and other free radicals are cell injury and cellular ageing by: i . Lipid peroxidation in membranes. ii. Oxidative modification of proteins may damage the active sites of enzymes, generate misfolded protein . iii. Free radicals causes DNA damage (single- and double-strand breaks in DNA).

37. Removal of Free Radicals 1. Free radicals are unstable and decay spontaneously. 2. Antioxidants either block free radical formation or inactivate free radicals ( Vit E, Vit A, Vit C). 3. A series of enzymes that breaks down free radicals: i. Catalase inactivates H2O2 ii. Superoxidase dismutases converts superoxide anion iii. Glutathione peroxidase

38. Free radicals Free radicals are chemical species that have a single unpaired electron in an outer orbit. Unpaired electrons are highly reactive and attack proteins, lipids, carbohydrates and nucleic acids of cells.

39. Generation of Free Radicals The reduction-oxidation reactions that occur during normal metabolic processes Absorption of radiant energy (e.g., ultraviolet light, x-rays). Produced in activated leukocytes during inflammation. Enzymatic metabolism of exogenous chemicals or drugs generate free radicals (e.g., CCl4)

40. Removal of Free Radicals Free radicals are inherently unstable and decay spontaneously. Antioxidants A series of enzymes that breaks down free radicals: i.Catalase ii.Superoxidase dismutases iii.Glutathione peroxidase

41. Effects of Free Radicals on cell Lipid peroxidation in membranes. Oxidative modification of proteins . Lesions in DNA (single- and double-strand breaks in DNA, cross-linking of DNA; ultimately causing DNA damage)