1. RHY/CH0576 Biology of Disease CH0576 Irreversible Cell Injury & Death

2. RHY/CH0576 Irreversible Cell Injury Cells can adapt to worsening environmental conditions and persistent injuring factors. However, there is a limit to the extent of the adaptations which are possible If the acute stress is > capacity to adapt then the resulting changes in both structure and function will inevitably lead to cell death

3. RHY/CH0576 Irreversible Cell Injury The theoretical ‘point of no return’ is passed. The injury becomes irreversible The cell will inevitably proceed to cell death. Cell death is almost always accompanied by a series of morphological changes which can be recognised in the light microscope

4. RHY/CH0576 Irreversible Injury & Cell Death These recognisable changes are usually referred to as Coagulative Necrosis. We are unable to recognise when a cell is irreversibly injured until it is dead, and the feature of necrosis are apparent.

5. RHY/CH0576 Morphology of Coagulative Necrosis Coagulative necrosis involves changes in the cytoplasm, nucleus and membrane. When stained with H & E, the cell cytoplasm is much more eosinophilic than normal. Initially the nucleus of a necrotic cell shows clumping of the chromatin, followed by a redistribution around the periphery of the nuclear membrane.

6. RHY/CH0576 Nuclear Changes in CN. The nucleus becomes smaller and stains more basophilic as the chromatin within it continues to clump This is referred to as PYKNOSIS. Basophilia indicating the end of DNA transcription

7. RHY/CH0576 Normal v Pyknotic Cell

8. RHY/CH0576 Nuclear changes in CN. Necrotic process continues, with the action of nucleases, causing the nucleus to fragment. The fragments become scattered throughout the cytoplasm. –KARYORRHEXIS

9. RHY/CH0576 Karyorrhexis

10. RHY/CH0576 Nuclear Changes in CN. The pyknotic or fragmented nucleus may be actively extruded from the cell or it may undergo further and complete dissolution, a process known as:- –KARYOLYSIS

11. RHY/CH0576 Normal v Necrotic Cell

12. RHY/CH0576 Pathogenesis of Necrosis The necrotic cell is left as a mass of partly denatured protein, still having the same rough cellular outline as the surrounding cells. The cytoplasm is deeply eosinophilic. Coagulative necrosis is the same no matter what the cause of cell death: virus, radiation or ischaemia, for example.

13. RHY/CH0576 Pathogenesis of Necrosis A characteristic of living cells is that they maintain large differences between their internal and external environments. These differences are maintained by the plasma membrane. With cell death, these characteristic differences in ionic concentrations are dissipated or lost.

14. RHY/CH0576 Pathogenesis of Necrosis One of the most significant gradients maintained across a living cell membrane is that of Ca 2+. The concentration of Ca 2+ in the external fluid is in the millimolar range. Concentration within the cell is around 10,000 times lower.

15. RHY/CH0576 Pathogenesis of Necrosis This gradient is actively maintained. Cell death is accompanied by an accumulation of Ca 2+ within the cell. Calcium ions have a wide range of biological functions and their accumulation may account for many of the features of coagulative necrosis.

16. RHY/CH0576 Pathogenesis of Necrosis The sequence of events leading to CN may be: –Irreversible cell injury and death –loss of membrane’s ability to maintain the calcium gradients. –Influx and accumulation of Ca 2+ –Morphologic appearance of Coagulative Necrosis.

17. RHY/CH0576 Morphology of Necrosis Several different patterns of necrosis are described. These largely reflect various macroscopic appearances of the dead tissues. These include:- Coagulative Liquefactive Fat necrosis Gummatous Haemorrhagic Fibrinoid Caseous

18. RHY/CH0576 Coagulative Necrosis This describes dead tissue which appears pale and firm - giving the appearance almost of cooked meat! Even though the cells are dead, much of the cellular outline and tissue architecture can still be recognised Tissues with relatively low levels of lysosomes exhibit this form.

19. RHY/CH0576 Coagulative Necrosis The most common cause of CN is ischaemia due to the occlusion of the arterial blood supply to a tissue. In some cases of CN, the proteins and enzymes which are released from dead cells can be used as a diagnostic indicator or marker of specific disease. Their presence in blood indicating specific cellular damage.

20. RHY/CH0576 Coagulative Necrosis In order for a particular protein or enzyme assay to be of use as a diagnostic aid, the substance must satisfy two major criteria: –It must have a restricted cellular distribution –It must be normally present in blood in only low concentrations, making an elevation in concentration significant.

21. RHY/CH0576 Diagnostic Assays A number of fairly routine diagnostic clinical chemistry assays for the following, rely on this process:- –Myocardial infarct –Liver damage –Striated muscle –Exocrine pancreas

22. RHY/CH0576 Coagulative Necrosis In coagulative necrosis the dead cells remain in situ long enough to be recognised and identified. In most cases the necrotic debris is eventually removed as a consequence of the inflammatory reaction. In cases where the are large areas of coagulative necrosis the necrotic tissue may remain in place for years.

23. RHY/CH0576 Coagulative Necrosis Examples would include occlusion of a coronary artery and the resultant infarction of a large area of the myocardium. The central area of necrosis may be inaccessible to the inflammatory reaction and the necrotic debris remains in situ.

24. RHY/CH0576 Coagulative Necrosis This explains how, on post mortem studies, previous infarcts are evident as fibrous scars. In most cases regeneration and repair mechanisms are responsible for the active removal of necrotic tissue. Unfortunately, the heart is composed of a permanent tissue - cardiac muscle.

25. RHY/CH0576 Coagulative Necrosis Coagulative necrosis in area of the kidney Ischaemia has led to an infarction Tissue architecture is maintained despite all of the cells in the area being dead

26. RHY/CH0576 Normal v Coagulative Necrosis

27. RHY/CH0576 Liquefactive Necrosis This pattern of necrosis describes tissue which appears semi-liquid. The appearance is due to the dissolution of the necrotic tissue under the influence of powerful hydrolytic enzymes. Two main instances: –Necrosis in the brain –Necrosis due to bacterial infection.

28. RHY/CH0576 Liquefactive Necrosis Liquefactive necrosis in a cerebral infarct No residual tissue architecture is retained. The area of the brain is transformed into a semi- liquid mass of protein with numerous macrophages

29. RHY/CH0576 Fat Necrosis This affects adipose tissue and is most commonly the result of either: –Physical trauma to adipose tissue e.g. breast –Pancreatitis. Unique feature in this form of necrosis is the presence of triglycerides released from damaged fat cells.

30. RHY/CH0576 Fat Necrosis - Pancreas The process begins with the inappropriate release of digestive enzymes. These are normally restricted to: –pancreatic acinar cells –pancreatic ducts –small intestine. They are released inappropriately from damaged pancreatic acinar cells.

31. RHY/CH0576 Fat Necrosis The enzymes gain access to the extracellular compartment. They commence to digest the tissue of the pancreas itself as well as the surrounding tissue, especially adipose cells. Phospholipases and proteases released attack the plasma membranes of the fat cells. Stored triglycerides are released.

32. RHY/CH0576 Fat Necrosis The pancreatic enzymes then digest the triglycerides in to free fatty acids. These precipitate in the form of ‘calcium soaps’. These accumulate as amorphous, basophilic deposits at the edge of irregular islands of necrotic adipose cells.

33. RHY/CH0576 Fat Necrosis On macrosciopic examination (left) fat necrosis appears as chalky-white areas, embedded in otherwise normal tissue. Histologically these areas of fat necrosis are composed of large areas of necrotic fat, usually around 5 mm diameter, with surrounding areas of reactive inflammation (right) Foci of fat necrosis Necrotic Fat Inflammation

34. RHY/CH0576 Caseous Necrosis This form of necrosis is highly characteristic of Tuberculosis. The lesions associated with TB are tuberculous granulomas or tubercles. In the centre of the granulomas the chronic inflammatory cells (mononuclear cells) which are mediating the response against the infection are killed along with the tissue cells.

35. RHY/CH0576 Caseous Necrosis In caseous necrosis the necrotic cells don’t retain their cellular outlines nor are they completely lysed as in liquefactive necrosis. The dead cells persist as amorphous, coarsely granular eosinophilic debris. Macroscopically the debris appears greyish white and crumbly. It has an appearance resembling crumbly cheese - hence ‘caseous’ necrosis.

36. RHY/CH0576 Caseous Necrosis This form of necrosis is not seen at the centre of granulomas caused by other agents. This highly characteristic pattern of necrosis is thought to be due to the toxic effects of the unusual cell wall of the mycobacterium, which contains complex waxes, known as peptidoglycolipids. Viable mycobacteria are present within the necrotic debris.

37. RHY/CH0576 Caseous Necrosis - Tuberculosis CN AM L GC A tuberculous granuloma with central caseous necrosis

38. RHY/CH0576 Gummatous Necrosis This describes dead tissue that is firm and rubbery. None of the original tissue architecture can be seen histologically. The dead cells form an amorphous mass. Seen in syphillis due to spirochaete T.pallidum N

39. RHY/CH0576 Haemorrhagic Necrosis This describes necrotic tissues which are engorged or suffused with extravascular red blood cells. This pattern of necrosis is seen particularly when cell death is due to a blockage of the venous drainage from the tissue e.g. torsion of the testis. Congestion of the tissue by blood with a result of failure in arterial perfusion  ischaemia.

40. RHY/CH0576 Haemorrhagic Necrosis - Testicular torsion Torsion of the testis, due to torsion or twisting of the spermatic cord. Venous return is blocked Blood cannot escape the tissue which becomes engorged. Arterial perfusion fails as tissue is full of venous blood  ischaemia.

41. RHY/CH0576 Fibrinoid Necrosis - Vasculitis Term used to describe the appearance of arteries in cases of vasculitis or in severe hypertension. Plasma proteins, and in particular fibrin, become deposited in the damaged necrotic vessel wall producing marked eosionophilia FN