RHY/CH0576

Biology of Disease CH0576

Irreversible Cell Injury & Death

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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

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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

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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.

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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.

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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

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Normal v Pyknotic Cell

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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

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Karyorrhexis

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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

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Normal v Necrotic Cell

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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.

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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.

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Pathogenesis of Necrosis

• One of the most significant gradients maintained across a living cell membrane is that of Ca2+.

• The concentration of Ca2+ in the external fluid is in the millimolar range.

• Concentration within the cell is around 10,000 times lower.

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Pathogenesis of Necrosis

• This gradient is actively maintained.• Cell death is accompanied by an

accumulation of Ca2+ 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.

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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 Ca2+

– Morphologic appearance of Coagulative Necrosis.

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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

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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.

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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.

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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.

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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

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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.

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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.

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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.

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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

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Normal v Coagulative Necrosis

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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.

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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

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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.

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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.

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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.

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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.

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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

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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.

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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.

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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.

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Caseous Necrosis - Tuberculosis

CN

AM

L

GC

A tuberculous granuloma with central caseous necrosis

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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

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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.

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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.

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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