Dr Muath Al salek

1. Cell injury
Moath Al Salek
Prosthodontics resident

2. Cellular responses and adaptations to stress
Overview of cellular injury
Mechanisms of cell injury
Cell death

4.  Normal cell needs special conditions
“environment” to function properly.
 (Everything should be titrated to our needs).
 Cells try to adapt to surrounding stimuli or
changes  so it can survive.

5.  The normal condition or the standard environment that
the cell looks to live in is called: (Homeo-stasis).
 Homeo : home or environment.
 stasis: stable or standing still or fixed .
 Things that might change around the cell: PH, Temp.,
Electrolytes level, Glucose.

7. Adaptation: hypertrophy, hyperplasia, atrophy,
metaplasia.
Injury: reversible and irreversible (cell death).
Intracellular accumulation; calcification
Cellular aging.

8. Reversible changes in size, number, phenotype,
metabolic activity or function in response to changes
in their environment.
Adaptation can be both physiologic (we want to
happen) and/or pathologic (disease).

9. Hypertrophy is an increase in cell size resulting in
increase in the size of the organ.
Alone in nondividing cells (cardiac myocytes or
muscles).
Coexisting with hyperplasia in dividing cells (skin or
GI tract cells).
Physiologic vs pathologic.

10. Muscle

11. Heart: left ventricle hypertrophy.

12. Increased functional demand (workload) or stimulation by
hormones or growth factors.
Mechanism: increased production of cellular
structural proteins and organelles.
There is a limit for hypertrophy.
Subcellular organelle may undergo selective
hypertrophy. (drugs causing smooth ER hypertrophy).

13. Increased number of cells resulting in increased mass
of the organ or tissue.
Takes place in cells capable of dividing.
Physiologic vs pathologic.

14. • Physiological Hyperplasia (hormonally induced or
compensatory), Examples:
– Uterine enlargement during pregnancy
– Female breast in puberty & lactation
– Compensatory hyperplasia in partial liver resection.

15. • Pathological
– Hyperplasia of the endometrium (excessive hormone
stimulation) and prostate hyperplasia.
– Wound healing (Effects of growth factors).
– Infection by papillomavirus (skin warts).
• Hyperplasia can be a fertile soil for development
of malignancy.

16. Mechanism: it is the result of growth factor-driven
proliferation of mature cells.
In some cases by increased output of new cells from
tissue stem cells.
Not increased work load as in Hypertrophy.

17. Reduced size of cell, tissue or organ as a result from
loss of cell substance (size and number).
Physiologic :
Embryonic development.
involuting gravid uterus.

18. Pathologic:
- Decreased workload (Disuse atrophy)
- Loss of innervation (Denervation atrophy)
- Diminished blood supply.
- Inadequate nutrition.
- Loss of endocrine stimulation.
- Aging.

19. Atrophy muscle fibers.

20. Brain (frontal lobe)

21. Mechanisms:
- decreased protein synthesis and increased protein
degradation.
- Reduced metabolic activity, which causes decreased
protein synthesis also.
- Ubiquitin-proteasome pathway: protein binds organells 
signals (kill me)  decrease # of organells.
- Autophagy (self eating) to find sources of protein.
(starvation).

22. Metaplasia is a “reversible” change in which one
differentiated cell type (epithelial or
mesenchymal) is replaced by another cell type.
New epithelium is better in dealing with the
current stress or irritation.
Persistence of factors causing metaplasia may
lead to progression into malignant
transformation.

23. Replacement of ciliated columnar epithelium by
stratified squamous epithelium in the respiratory
tract of a smoker.

24. Mechanism: reprogramming of stem cells that are
known to exist in normal tissues or undifferentiated
mesenchymal cells present in connective tissue.
Signals generated by cytokines, growth factors and
extracellular matrix promote expression of
genes toward a new differentiation.

25. Some functions might be lost.
Malignant transformation if the cause for metaplasia
persists.

26. If cells are no longer able to adapt the stress OR
exposed to damaging agents from the start.
Reversible.
Irreversible (death).

27. Hypertrophy: increased cell and organ size, often in
response to increased workload; induced by
mechanical stress and by growth factors; occurs in
tissues incapable of cell division.
Hyperplasia: increased cell numbers in response to
hormones and other growth factors; occurs in tissues
whose cells are able to divide

28. Atrophy: decreased cell and organ size, as a result of
decreased nutrient supply or disuse; associated with
decreased synthesis and increased proteolytic
breakdown of cellular organelles
Metaplasia: change in phenotype of differentiated
cells, often a response to chronic irritation that makes
cells better able to withstand the stress; usually
induced by altered differentiation pathway of tissue
stem cells; may result in reduced functions or
increased propensity for malignant transformation.

30. Stress (if severe, prolonged or damaging) leads to
Injury.
Stress Reversible Injury Irreversible
Irreversible Injury (cell death). Necrosis and
Apoptosis

31.  Oxygen deprivation.
 Physical agents
 Chemical agents
 Infectious agents
 Immunologic reaction
 Genatic derangement
 Nutritional imbalance
 Aging

32.  Oxygen Deprivation: Hypoxia: deficiency of O2.
Causes:
 Ischemia
 Inadequate oxygenation: cardiorespiratory failure.
 Decreased oxygen carrying capacity: anemia, carbon
monoxide poisoning or blood loss.

33. Physical agents: mechanical trauma, extremes of
temperature, changes in atmospheric pressure,
radiation and electric shock.
Chemical agents and drugs: Glucose or salt, oxygen,
arsenic compounds, cyanide, mercuric salts,
insecticides, herbicides, asbestos, alcohol, smoking
and therapeutic drugs.

34. Infectious agents: viruses, bacteria, fungi and
parasites.
Immunologic reactions: to external stimuli and
endogenous self antigens (Autoimmune & allergy).

35. Genetic derangement: Chromosomal anomalies, gene
alteration or simple amino acid alteration.
Nutritional imbalance: Protein calorie deficiencies,
vitamin deficiencies, Psychiatric disorders(Anorexia
nervosa), excess food, type of food.
Aging: replicative and repair abilities.

36. Stress effect:
1- Changes at the molecular or biochemical level.
2- Changes of function.
3- After certain time morphological changes will appear.

37. Changes can be detected by histochemical or
ultrastructural techniques, microscopy or grossly.
Usually morphological changes of reversible injury
take shorter time to develop than those of irreversible
cell injury.

38. Generalized swelling of the cell: failure of energy-
dependent “ion pumps” in the plasma membrane
result in disturbances in ionic and fluid homeostasis.
 It is usually the first manifestatioin.
 Another names hydropic change or vacuolar
degeneration.

39. Plasma membrane alterations: blebs, blunting or loss of
villi and loosening of intercellular attachments.
Mitochondrial changes: swelling and appearance of small
amorphous densities.
Nuclear alterations: nuclear chromatin clumping.

40. Dilatation of ER and detachment of polysomes
(ribosomes) with possibility of myelin figure
formation in the cytoplasm.

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43. Result from denaturation of intracellular proteins and
enzymatic digestion of cells.
Loss of membrane integrity.
Digestion enzymes: from lysosomes of dying cells
and from leukocytes (inflammatory response).
Structural changes need time to develop.

44. Vacuolation due to digestion of cytoplasmic organells.
Plasma and organelle membrane discontinuities.

45. Myelin figures: aggregates of damaged cell
membranes (phospholipids). They either
phagocytosed by other cells or further degraded into
fatty acids and calcify.

46. Marked dilatation of mitochondria and appearance of
large densities.
Nuclear changes: breakdown of DNA
- Karyolysis: loss of DNA, fade of basophilia.
- Pyknosis: nuclear shrinkage and increased
basophilia.
- Karyorhexis: fragmentation of the pyknotic
nucleus.
- Disappearance of the nucleus.

49. The cellular response to injurious stimuli depends on
the nature of the injury, its duration and its severity.
The consequences of injurious stimuli depend on the
type, status, genetic makeup and adaptability of the
injured cell.
Cell injury results from different biochemical
mechanisms acting on several essential cellular
components (mitochondria, calcium homeostasis,
plasma membranes and DNA.

50. Any injurious stimulus may simultaneously trigger
multiple interconnected mechanisms that damage
cells.

51. Usually in hypoxic and chemical injuries.
Sources : oxidative phosphorelation of ADP in the
mitochondria and Glycolytic pathway using Glucose.
The major causes of ATP depletion are reduced
supply of oxygen and nutrients, mitochondrial
damage and the actions of some toxins (Cyanide).

52. Tissues with a greater glycolytic capacity (liver) are
more able to survive loss of oxygen and decreased
oxidative phosphorylation better than are tissues with
limited capacity for glycolysis (brain).

54. Low oxygen situation results in misfolding of proteins
which trigger a cellular response called the unfolded
protein response that may lead to cell death
(Activation of apoptosis).

55. Supplies ATP (energy) to the cell.
Damaged by Calcium influx, reactive oxygen species,
radiation, oxygen deprivation, toxins and mutations
in mitochondrial genes.
Consequences of mitochondrial damage: Formation
of mitochondrial permeability transition pore which
leads to loss of membrane potential, failure of
phosphorylation and ATP depletion and then
necrosis.

56. • Consequences of mitochondrial damage: Release of
cytochrome c into the cytosol that activate apoptosis
(death).
Failure of oxidative phosphorylation leads to
ATP depletion and formation of reactive
oxygen species.

58. Depleting extracellular Ca protects the cell from
injury and delays it.
Cytosolic Ca concentration is very low and is present
intracellularly in mitochondria and ER.
Injury will lead to increase cytosolic Ca.
Consequences of Ca increase: opening of
mitochondrial permeability transition pore, and
activation of a number of enzymes (phospholipases,
proteases, endonucleases & ATPases)

59. Consequences of Ca increase: induction of apoptosis
by direct activation of caspases and increasing
mitochondrial permeability.

61. It is important in chemical and radiation injuries,
ischemia-reperfusion injury, cellular aging and
microbial killing by phagocytosis.
Free radicals: chemical species that have a single
unpaired electron in the outer orbital.
Unstable atoms, react with inorganic and organic
chemicals (proteins, lipids, carbohyd.)
Initiate autocatalytic reactions..... Creation of more
radicals (propagation).

63. One of the oxygen derived free radicals.
Produced normally in small amounts and removed by
defence mechanisms.
Once the ROS amount increases this will lead to what
so called oxidative stress.
Oxidative stress : cell injury, cancer, aging and some
degenerative diseases like Alzheimer. Also ROS are
produced by leukocytes and macrophages in
inflammation.

64. Reduction oxydation reactions.
Normal mitochondrial respiration; molecular Oxygen
is reduced by reacting with H2 to generate two water
molecules.
By products are: superoxide anion, hydrogen peroxide
(H2O2) and hydroxyl ions (OH).
Absorption of radiant energy.
UV light and X-rays. Hydrolyze water into OH & H
free radicals.

65. Production by leukocytes.
Plasma membrane multiprotein complex using
NADPH oxidase.
Intracellular oxidases such as xanthine oxidase
generate superoxide anion.
Enzymatic metabolism of exogenous chemicals or
drugs.
Not ROS but similar. Ex CCL4 ---- CCL3

66. Transition metals.
Iron and copper.
Fenton reaction (H2O2+Fe²⁺ Fe³⁺+OH•+OH⁻)
Ferric iron should be reduced to ferrous iron to
participate in Fenton reaction. This reaction is
enhanced by superoxide anion and so sources of iron
and superoxides may participate in oxidative cell
damage.

67. Nitric oxide (NO).
Endothelial cells, macrophages, neurons and other
cell types.
Can act as a free radical and can also be converted to
highly reactive peroxynitrite anion as well as NO2 and
NO3.

68. Decay spontaneously.
Antioxidants: Vitamin E and A, ascorbic acid and
glutathione in the cytosol.
Binding proteins.
Enzymes: Catalase-----H2O2 ----- O2 and H2O,
Superoxide dismutase------ superoxide anion
----H2O2, Glutathione peroxidase---- H2O2 ---H2O or
OH------ H2O. Reduced Glutathione level is important
in cell safety.

69. Lipid peroxidation in membranes. Oxidative damage
of the double bonds in the polyunsaturated fatty acids
resulting in formation of peroxides which are unstable
and lead to membrane damage.
Oxidative modification of proteins. Damage the active
sites on enzymes, change the structures of proteins
and enhance proteosomal degradation of unfolded
proteins.

70. Lesions in DNA. Single and double strand breaks in
DNA. Oxidative DNA damage has been implicated in
cell aging and in malignant transformation of cells.
Radicals are involved in both necrosis and apoptosis.

73. Selective and overt membrane damage is a constant
feature in all forms of cell injury except Apoptosis.
Causes include ischemia (ATP depletion and calcium
mediated activation of phospholipases), direct
damage (bacterial toxins, viral proteins, lytic
complement components, physical and chemical
agents).

74. Reactive oxygen species: lipid peroxidation.
Decreased phospholipids synthesis: as a consequence
of defective mitochondrial function or hypoxia. This
affects all cellular membranes including mitochondria
themselves.
Increased phospholipids breakdown: activation of
endogenous phospholipases due to Ca⁺² resulting in
accumulation of lipid breakdown products.

75. • Lipid Breakdown products: include unesterified free
fatty acids, acyl carnitine and lysophospholipids
which have a detergent effect on membranes causing
changes in permeability and electrophysiologic
alterations.
Cytoskeletal abnormalities: activation of proteases by
high Ca⁺² causes damage to the elements of
cytoskeleton.

76. Most important sites of membrane damage:
mitochondrial, plasma membrane and lysosomal.
Lysosomes contain many degrading enzymes like
RNases, DNases, proteases.....

77. Cells, usually, have mechanisms to repair DNA
damage but if the damage is severe the cells initiate a
suicide program results in cell death by Apoptosis.
Accumulation of misfolded proteins triggers
Apoptosis.

78. The identification of factors that determine when
reversible injury becomes irreversible and progresses
to cell death would be very useful so we may be able
to identify strategies to prevent permanent
consequences of cell injury.
Leakage of intracellular proteins into blood through
damaged membranes provides a means of detecting
tissue damage. CK & troponin in MI and ALT, AST
&ALK in liver.

79. Most common type of injury in clinical medicine.
Hypoxia: anaerobic glycolysis
Ischemia: delivery of substrates is also compromised.
Ischemia is more rapidly damaging than hypoxia in
the absence of ischemia.

80. Low O2 leads to loss of oxidative phosphorylation and
decreased generation of ATP.
Na/K and Ca⁺² pumps failure.
Progressive loss of glycogen and decreased protein
synthesis.
Loss of function though the cell is not yet dead.

81. Cytoskeleton abnormalities; blebs and loss of villi.
Formation of myelin figures and swollen organelles.
To this point changes are reversible.
After that, severe swelling to the mitochondria,
extensive damage to the plasma membranes, myelin
figures formation and swelling of lysosomes.

82. Large densities develop in the mitochondria.
Massive influx of Ca⁺² happens especially if the
ischemic area is reperfused.
Death is mainly by necrosis but apoptosis also takes
place.
Dead cells may become replaced by large masses of
myelin figures which are either phagocytosed or
degraded more into fatty acids and may become
calcified.

83. Restoration of blood flow to ischemic tissues can
promote recovery if they are reversibly injured.
In certain situations, reperfusion paradoxically
exacerbates injury (more dead cells in addition to the
already irreversibly injured cells).
 Mechanisms:

84. Reoxygenation: increased regeneration of reactive
oxygen and nitrogen species from parenchymal and
endothelial cells and leukocytes. Ca⁺² influx.
Inflammation response mediated by cytokines which
recruits more leukocytes and more injury. Applying of
Anti-cytokines might aid in decreasing the unwanted
effects of inflammation.

85. Activation of the complement system: Some IgM
antibodies are deposited in ischemic tissues for
unknown reasons and once the blood is restored
complement proteins bind to those antibodies and
lead to their activation and so more injury.

86. Major problem. Drugs.
Liver as a major site of drug metabolism is a target for
drug toxicity.
Mechanisms:
Directly by combining with critical molecular
component. Example mercuric chloride poisoning
bind to the sulfhydryl groups of cell membrane
proteins causing increased permeability. More in GIT
and kidney.

87. Cyanide poisons mitochondrial cytochrome oxidase
and inhibits oxidative phosphorylation
Most chemicals are not biologically active and need to
be converted into active forms (toxic metabolites)
which usually takes place in liver ( cytochrome P-450
mixed-function oxidases). Free radical formation and
lipid peroxidation. CCl4 is converted to CCl3 • which
causes lipid peroxidation and decrease export of lipids
(Fatty change).

88. Acetaminophen (paracetamol) converted to to toxic
products in liver leading to injury.

89. Cell death (necrosis & apoptosis)

90. Coagulative necrosis: preservation of the architecture
of dead tissue for at least some days.
Denaturation of structural proteins and enzymes.
Eosinophilic anucleated cells.
Cells are removed by inflammatory leukocytes.

91. Ischemia in any organ except the brain may lead to
coagulative necrosis.
Infarction: localised area of coagulative necrosis.

93. Liquefactive necrosis: digestion of dead cells resulting
into a liquid viscous mass.
In focal bacterial or fungal infections and in hypoxic
death in central nervous system.
Creamy yellow due to accumulation of dead
leukocytes (pus).

95. Gangrenous necrosis: not a distinctive pattern. Used
clinically in describing lower limb coagulative
necrosis secondary to ischemia.
Once infected by bacteria it becomes wet gangrene
(liquefaction).

96. Caseous necrosis: White cheeselike friable necrosis.
Tuberculosis
Collection of fragmented or lysed cells with
amorphous granular eosinophilic debris surrounded
by histiocytes (macrophages), known as granuloma.

98. Fat necrosis: usually used in clinical terms and it is
not a specific type.
Necrosis (destruction) of fat.
Typical example: pancreatic enzymes (lipases) release
in acute pancreatitis.
The fatty acids result from the breakdown of fat
combine with calcium leading to the formation of
white chalky areas (Saponification).

100. Fibrinoid necrosis: immune reactions involving blood
vessels.
Complexes of antigens and antibodies deposited in
the walls of arteries.
Immune complexes deposits along with fibrin result
in a bright pink material on H&E.
Example: vasculitis.

102. Phagocytosis.
Replacement by scar.
Regeneration.
Calcification.

104.  Apoptosis is a pathway of cell death in which cells
activate enzymes that degrade the cells’ own nuclear
DNA and nuclear and cytoplasmic proteins.

105. • Pathway of cell death induced by a “suicide” program
in which activation of degrading enzymes takes place.
• Apoptotic cells break into fragments called “apoptotic
bodies”, which contain portions of the cytoplasm and
nucleus.
• Apoptotic bodies will become targets for phagocytosis
before their contents leak out and so there would be
no inflammatory reaction.

106. Occurs normally (physiological) or pathologicaly.
Physiologic situations:
- Embryogenesis.
- Involution of hormone-dependent tissues upon
hormone withdrawal.

107. Pathologic situations:
DNA damage, accumulation of misfolded proteins
(Excessive accumulation of these proteins in the ER
called ER stress).
Certain infections (viral ones).

108. Cell shrinkage: dense cytoplasm, tightly packed
organelles.
Chromatin condensation: peripherally under the nuclear
membrane.
Formation of cytoplasmic blebs and apoptotic bodies:
blebbing then fragmentation into membrane bound
apoptotic bodies composed of cytoplasm and tightly packed
organelles with or without nuclear fragments.

109. Phagocytosis of apoptotic cells or bodies by
macrophages.
The cells rapidly shrink, form cytoplasmic buds, and
fragment into apoptotic bodies
composed of membrane-bound vesicles of cytosol and
organelles.
On H&E apoptotic cell appears intensely eosinophilic
or shrunken basophilic fragment surrounded by halo.

112. Apoptosis results from the activation of enzymes called
caspases.
The activation of caspases depends on a finely tuned
balance between production of pro- and anti-
apoptotic proteins.
Two distinct pathways converge on caspase
activation: the mitochondrial pathway
and the death receptor pathway.

113. • Activation of Caspases:
- Two types: initiators (caspase 9&8) and executioners
(caspase 3&6).
• DNA and Protein breakdown: DNA breaks down into
large 50 to 300 kilobase pieces.
- Ca⁺² & Mg⁺² dependent endonucleases break DNA into
fragments that are multiples of 180 to 200 base pairs.
DNA ladders on electrophoresis.

114.  Membrane alterations and recognition: changes
making cells recognisable by phagocytes.
- Movement of some phospholipids
(phosphatidylserine) from the inner leaflet to the
outer leaflet of the membrane which are able to bind
to receptors on phagocytes.

115. All cells contain intrinsic mechanisms that signal
death or survival signals.
Apoptosis results from an imbalance in these signals.
Initiation: intrinsic and extrinsic.
Execution.

116. • Major mechanism.
• Release of mitochondrial molecules into the cytosol
(cytochrome c).
• Release is controlled by “pro” and “anti” apoptotic
proteins called (Bcl) family.
• Bcl family; 20 members.
• Growth factors and other survival signals stimulate
production of anti-apoptotic proteins (Bcl-2, Bclx & Mcl-1)
in cytoplasm and mitochondrial membranes.

117. Stress (loss of survival signals, DNA damage...)
activates some sensors.
Sensors of damage (BH3-only proteins) are Bcl family
members including Bim, Bid & Bad.
Sensors activate proapoptosis factors (Bax & Bak)
leading to formation of channels and leakage of
mitochondrial proteins (cytochrome c) and activation
of caspases.

118. Cytochrome c binds to Apaf-1(apoptosis activating
factor) forming apoptosome which binds to caspase-
9 (the critical initiator caspase).
Other mitochondrial proteins (Smac/DIABLO) enter
the cytoplasm blocking the inhibitors of apoptosis
(Bcl-2 and Bcl-xl) the function of which is block the
activation of caspases.

120. Initiated by plasma membrane death receptors (TNF
receptor family).
Death domain: cytoplasmic structure involved in
protein-protein interactions.
TNF-1 with related protein called Fas (CD95)
expressed on the surface of many cells.
 Its ligand FasL is expressed on activated T-cells.

121. Final step after the initiation phase.
Caspases 3 & 6.
Activation of DNase, degradation of structural
components of nuclear matrix (fragmentation of
nuclei).

122. Growth factor deprivation.(Uterus after delivery).
DNA damage. Radiation or chemicals mediated DNA
damage called genotoxic stress.

124. - Regulated mechanism of cell death that serves to
eliminate unwanted and irrevesibly damaged cells, with
the least possible host reaction.
- Characterized by enzymatic degradation of proteins
and DNA, initiated by caspases; and by recognition and
removal of dead cells by phagocytes.

125.  Mitochondrial (intrinsic) pathway is triggered by
loss of survival signals, DNA damage and accumulation
of misfolded
proteins (ER stress); associated with leakage of
pro-apoptotic proteins from mitochondrial membrane
into the cytoplasm, where they trigger caspase
activation.

126. Inhibited by anti-apoptotic members of the Bcl
family, which are induced by survival signals
including growth factors.

127.  Death receptor (extrinsic) pathway is responsible
for elimination of self-reactive lymphocytes and
damage by cytotoxic T lymphocytes; is initiated by
engagement of
death receptors (members of the TNF receptor
family) by ligands on adjacent cells.

128.  Thank you.