This document summarizes the key mechanisms of cellular injury: 1. Depletion of ATP disrupts cellular processes and leads to necrosis as mitochondria and lysosomes are damaged. ATP depletion occurs due to reduced oxygen/nutrients, mitochondrial damage, or toxins. 2. Influx of calcium into cells activates destructive enzymes and can induce apoptosis. Increased cytosolic calcium occurs when ATP-dependent calcium pumps fail during ischemia or toxicity. 3. Accumulation of reactive oxygen species damages lipids, proteins, and DNA, causing cell injury. ROS are generated during oxidative phosphorylation and inflammation and cause oxidative stress.

2. MECHANISM OF
CELLULAR INJURY

3. PREPARED BY :-
ASAVER ALAMGIR
ROLL # 01
JAVERIA AKHTER
ROLL # 15

4. OUTLINE
Introduction
Depletion of ATP
Influx of Calcium
Mitochondrial Damage and Dysfunction
Accumulation of Oxygen-Derived Free
Radicals
Defects in Membrane Permeability
Damage to DNA and Protein

5. INTRODUCTION
GENERAL PRINCIPLES RELEVANT TO MOST
FORMS OF CELL INJURY :-
1. Cellular injury depends on type of injury, its
duration and its severity. e.g
brief ischemic duration reversible injury
long ischemic duration irreversible injury
2. Consequences of an injurious stimulus depends
on the type, status, adaptability and genetic
makeup of the injured cell .

6. 3 Cell injury due to functional and biochemical
abnormalities in several essential cellular
components.
4 Multiple biochemical alterations by any
injurious insult.

8. ATP: A NECESSITY !!!
ATP is produced by:
 Oxidative phosphorylation
 Glycolytic pathway
ATP is required for all synthetic and degradative
processes within the cell including:
 Membrane transport
 Protein synthesis
 Lipogenesis and
 Deacylation-reacylation reaction
A healthy human burns 50-75 kg of ATP everyday!!!

9. DEPLETION OF ATP
The major causes of ATP depletion are:
• Reduced oxygen nutrient supply
• Mitochondrial damage
• Actions of toxins (e.g cyanide)
Tissue with greater glycolytic capacity (e.g liver)
survive loss of oxygen and decreased oxidative
phosphorylation better than those tissues with
limited capacity for glycolysis (e.g brain)

10. EFFECTS OF ATP DEPLETION
• Reduction in activity of Na-K pump
• Increase in anaerobic glycolysis
• Failure of ATP-dependant Ca++ pumps
Prolonged or worsen depletion of ATP causes
structural disruption of protein synthetic
apparatus.
Ultimately, there is irreversible damage to
mitochondrial and lysosomal membrane, and
the cell undergoes necrosis

12. MITOCHONDRIAL DAMAGE AND
DYSFUNCTION
• Failure of oxidative phosphorylation leads to ATP
depletion ultimately leading to necrosis
• Abnormal oxidative phosphorylation also leads to
formation of ROS which have many deleterious
effects
• Mitochondrial damage is often associated with the
formation of high-conductance channel in
mitochondrial membrane called as mitochondrial
permeability transition pore leading to loss of
mitochondrial potential changes

13. • The mitochondria contain several proteins that
when released into the cytoplasm , tell the cell
there is internal injury and activate a pathway of
apoptosis.

15. INFLUX OF CALCIUM
• Cytosolic calcium concentration is 10,000 times
lower than the extracellular and intracellular
(mitochondrial and ER) calcium concentration
• Cytosolic free calcium is maintained by ATP-
dependant calcium transporters
• Ischemia and toxins cause an increase in
cytosolic calcium concentration
• Increase cytosolic calcium activates enzymes
with deleterious cellular effects

16. • These enzymes include:
1. Phospholipases (cause membrane damage)
2. Proteases ( breakdown both membrane and
cytoskeleton proteins)
3. Endonucleases (cause DNA and chromatin
fragmentation)
4. ATPases (hastening ATP depletion )
• Increase intracellular calcium may also induce
apoptosis , by direct activation of caspases and
by increasing mitochondrial permeability

18. FREE RADICALS
Free radicals are chemical species with single
unpaired electron in outer orbital . Such
chemical states are:
1. Extremely unstable
2. Highly reactive
When generated in cell they attack nucleic acids
and variety of cellular proteins and lipids. They
also initiate chain reaction.

19. REACTIVE OXYGEN SPECIES
(ROS)
• ROS are type of oxygen-derived free radicals whose
role in cell injury is well established
• The circumstances involving damage by free
radicals include:
I. Ischemia reperfusions
II. Chemical and radiation injury
III. Toxicity from oxygen and other gases
IV. Cellular aging
V. Microbial killing by phagocytes
VI. Tissue injury caused by inflammatory cells

21. TYPES AND PATH OF PRODUCTION OF
ROS
1. By partial reduction oxygen during redox
reaction that occur during mitochondrial
respiration highly reactive but short lived toxic
intermediates are generated which include:
 Superoxide which is converted to H2O2
spontaneously and by action of superoxide
dimutase. In the presence of metal such as Fe++
H2O2 is converted to highly reactive hydroxyl
radical by Fenton reaction.
2. ROS is produced in phagocytic leukocytes
mainly neutrophils and macrophages

22. RESPIRATORY AND OXIDATIVE
BURST
ROS is generated in phagosomes &
phagolysosomes of leukocytes by process called
respiratory burst.
In this process of phagosome membrane
catalyses generation of superoxide which is
converted to H2O2 which is then converted to
highly reactive compound hypochlorite by the
enzyme myeloperoxidase which is present in
leukocytes

23. Nitric oxide is another reactive free radical
produced in leukocytes and other cell. It can
react with superoxide to form a highly reactive
compound peroxynitrite , which also
participates in cell injury.
The damage caused by free radicals is determined
by their rates of production and removal.

24. OXIDATIVE STRESS
When the production of ROS increases the result is an
excess of these free radicals , leading to a condition
called oxidative stress.
Free radical generation is increased under several
circumstances :
• Absorption of radiant energy (ionizing radiation can
hydrolyze water into hydroxyl and hydrogen free
radicals)
• Enzymatic metabolism of exogenous chemicals e.g
CCl4
• Inflammation in which free radicals are produced
by leukocytes

25. REMOVAL OF FREE RADICALS
1. Free radicals are unstable & decay spontaneously.
2. There are non-enzymatic & enzymatic systems that
contribute to inactivation radicals
 Superoxide is catalysed by superoxide dimutase.
 Hydrogen peroxide is catalysed by glutathione
peroxidase 1
2 GSH + H2O2 ---> GS-SG + 2 H2O
 Catalase is one of the most active enzyme capable
of degrading millions of molecules of H2O2 per
second.

26. Are inEndogenous & exogenous anti-oxidants (e.g
vitamins E , A , C & beta-carrotenes) block the
formation of free radicals.
Reactive oxygen species cause cell injury by three
main reactions:
1. Lipid peroxidation of membranes
2. Cross linking & other changes in proteins
3. DNA damage
 Low concentrations of ROS are also involved in
various signalling pathways in cells thus are
involved in various physiologic reactions to avoid
their harmful effects ,their intracellular
concentrations are tightly regulated in healthy cell.

28. DEFECTS IN MEMBRANE PERMEABILITY
Plasma membrane can be damaged by:
• ischemia
• Various microbial toxins
• Lytic complement components
• Physical & chemical agents.
Several biochemical mechanisms may contribute to
membrane damage
• Decreased phospholipid synthesis
• Increased phospholipid breakdown
• ROS cause injury to cell membrane by lipid
peroxidation

29. • Cytoskeletal abnormalities
• Lipid breakdown products.
Most important sites of membrane damage
during cell injury are:
1. Mitochondrial membrane
2. Plasma membrane
3. Membranes of lysosomes

31. DAMAGE TO DNA & PROTEINS
• Cells have mechanism to repair damage to
DNA but if damage is too severe the cell
initiates suicide program and dies by
apoptosis
• Accumulation of improperly folded proteins
or external triggers such as free radicals may
initiate a similar reaction
These mechanisms of cell injury typically cause
APOPTOSIS

33. REFERENCE: ROBBINS BASIC
PATHOLOGY
9TH EDITION

34. THANK YOU!!!