Apoptosis is a programmed cell death process that occurs in physiological and pathological conditions. It is mediated by caspases and involves two main pathways - the intrinsic mitochondrial pathway and the extrinsic death receptor pathway. Both pathways activate initiator and executioner caspases that degrade cellular proteins and lead to characteristic morphological changes including cell shrinkage, nuclear fragmentation, and formation of apoptotic bodies that are phagocytosed without inflammation. Apoptosis is regulated and differs from necrosis in being an energy-dependent process without inflammatory response.

2. Dr Asma Rasheed
Assistant Professor
Pathology

3. LEARNING OBJECTIVES
 Define Apoptosis
 Enlist clinical examples of apoptosis in physiological
conditions
 Enlist clinical examples of apoptosis in pathological
conditions
 Describe mechanisms of apoptosis
 Tabulate differences between necrosis and apoptosis

4. Definition
 Apoptosis is a well controlled programmed individual
cell death that is caspase mediated and leads to
fragmentation of the cell and organelles into
numerous small buds, which are then engulfed by
macrophages without surrounding inflammation.

6. Why should a cell commit suicide?
Programmed cell death is
as needed for proper
normal development as
mitosis is.

7. 1-The resorption of the tadpole
tail in frog .
2- The formation of the fingers
and toes of the fetus requires
the removal, by apoptosis.
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Apoptosis in Physiological condition:

8. 3- Programmed destruction during embryogenesis
4- Involution of hormone dependent tissues upon
hormone withdrawal such as
 Endometrial cell breakdown during the menstrual
cycle
 Ovarian follicular atresia in menopause
 Regression of lactating breast after weaning.

9. 5- Cell loss in proliferating cell populations such as
 Immature lymphocytes
 Thymus that fail to express useful antigen receptors.
6- Elimination of harmful self- reactive lymphocytes
either before or after they have completed their
maturation.
7- Death of host cells that have served their useful
purpose, neutrophils and lymphocytes .

10. Apoptosis in bud
formation during
which many
interdigital cells
die. They are stained
black by a TUNEL
method
Incomplete differentiation
in two toes due to lack of
apoptosis
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11. Apoptosis in pathological conditions
1- DNA damage:
 Radiation, cytotoxic anticancer drugs, hypoxia can
damage DNA via free radicals.
2- Accumulation of misfolded proteins
3- Cell death in certain infections
 HIV infections, viral hepatitis.
4- Pathological atrophy in parenchymal organs
 Duct obstruction(pancreas, parotid gland)
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12. Mechanisms of apoptosis
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13. Stimuli for Apoptotic Cell Death

14. What are death receptor activators ?
 Tumor necrosis factor alpha (TNF-)
 Lymphotoxin (TNF-β)
 Fas ligand (FasL)

15. What is a Caspase?
 A specific feature of apoptosis is the activation of several
members of a family of cysteine proteases named “caspases.”
 The caspase family has more than 10 members and is divided
functionally into two groups- initiator and executioner.
 Initiator caspases are caspase 8 and 9.
 Executioner caspases are caspase 3 and caspase 6.
 Caspases exist as inactive pro-enzymes called zymogens, and
must undergo an enzymatic cleavage to become active .

16. Other Important Proteins
Bcl-2 family helps regulate the activation of procaspases.
 Inhibitors - Bcl-2, Bcl-X, Mcl-1(antiapoptotic)
 Promoters – Bad, Bax, Bak (pro-apoptotic)

17. Phases of Apoptosis
The process of apoptosis is divided into
1) Initiation phase: during which some caspases
become catalytically active.
2) Execution phase: during which other caspases
trigger the degradation of critical cellular components.

18. 1- Initiation phase
 Signals from two distinct pathways:
1) The intrinsic, or mitochondrial pathway
2) The extrinsic or death receptor-initiated pathway
 Both pathways are induced by distinct stimuli and
involve different sets of proteins.
 Both pathways converge to activate caspases which are
the actual mediators of cell death.

19. Extrinsic or death
receptor-initiated
pathway.

20. Mechanism (Extrinsic)
 Death Receptors- cell surface receptor that transmit
apoptotic signals, initiated by ligands.
 Ligands: Fas ligand, Tumor Necrosis Factor (TNF)
alpha, and Tumor necrosis (TNF)-related apoptosis-
inducing ligand TRAIL.
 Lead to generation of ceramide
 Result in large number of clustering of death receptors.
 This helps strengthen the apoptotic signaling.

21.  Death domain
 Conformation change in the intracellular domain of the
receptor.
 Allows recruitment of various apoptotic proteins to the
receptor.
 Final step- recruitment of caspase 8
 Resulting the activation of caspase 8 and the initiation
of apoptosis.

22. Death Inducing Signaling
Complex

23. The extrinsic (death receptor-initiated) pathway of apoptosis, illustrated by
the events following Fas engagement. FADD= Fas associated death domain.

24. INTRINSIC OR
MITOCHONDRIAL
PATHWAY

27. Mechanism (Intrinsic)
 Mitochondria contains proteins like Apoptosis
Inducing Factor (AIF), Smac/ DIABLO and
Cytochrome c that regulate cell death.
 These proteins are release through a pore called
Permeability Transition (PT) pore
 Form by pro-apoptotic members of Bcl-2 family
(activated by apoptotic signals)

28.  The release of Cytochrome leads to recruitment of pro-
caspase 9 and Apoptotic protease activating factor 1
(Apaf-1).
 This forms the apoptosome
 Followed by the activation of caspase 9 which in turn
results in the induction of apoptosis.

30. The intrinsic (mitochondrial) pathway of apoptosis. Death agonists cause changes in the inner
mitochondrial membrane, resulting in the mitochondrial permeability transition (MPT) and
release of cytochrome c and other pro-apoptotic proteins into the cytosol, which activate
caspases. AIF= Apoptosis inhibitory factor; IAPs= Inhibitors of apoptosis proteins; Apaf-1=
apoptosis protease activating factor

31. Execution phase
•Mediated by caspase 3 and 6
•Caspase 8 and 9 in initiation phase activate
caspases 3 and 6
•They break down cytoskeleton protein
that results in bleb formation and endonuclease
activation ( break down of nucleus)

32.  Loss of membrane integrity
 Begins with swelling of
cytoplasm and mitochondria
 Ends with total cell lysis, no
vesicle formation, complete
lysis
 Disintegration (swelling) of
organelles
 Membrane blebbing, but no
loss of integrity
 Begins with shrinking of
cytoplasm and condensation
of nucleus
 Ends with fragmentation of
cell into smaller bodies
 Mitochondria become leaky
due to pore formation .

33.  Loss of regulation of ion
homeostasis
 No energy requirement
 Random digestion of
DNA.DNA fragmentation
(late event of death)
 Tightly regulated process
 Energy (ATP)-dependent
 Non-random mono- and
oligonucleosomal length
fragmentation of DNA(ladder
type pattern)
 Release of various factors into
cytoplasm by mitochondria
 Activation of caspase cascade

34.  Affects groups of contiguous
cells
 Evoked by non-physiological
stimuli(complement attack,
lytic viruses, hypothermia,
hypoxia, ischemia, metabolic
poisons)
 Phagocytosis by
macrophages
 Significant inflammatory
response
 Affects individual cells
 Induced by physiological
stimuli (lack of growth
factors, changes in hormonal
environment.
 Phagocytosis by adjacent
cells or macrophages
 No inflammatory response

35. SUMMARY
Four stages of apoptosis have been defined:
i. Commitment to death by extracellular or intracellular
triggers/signals
ii. Cell killing (execution) by activation of intracellular
proteases (caspases)
iii. Engulfment of cell corpse by other cells
iv. Degradation of the cell corpse within the lysosomes of
phagocytic cells