Apoptosis

Slide 1 · Apoptosis

Regulated cell death with membrane-contained fragmentation

Execution requires activation of intracellular proteases and DNA-degrading enzymes.

Fragments are recognized and rapidly engulfed, limiting inflammation.

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Distinguish apoptosis from necrosis using mechanism and morphology.

Explain physiologic and pathologic triggers for apoptotic cell death.

Trace intrinsic and extrinsic pathways to executioner caspases.

Relate apoptotic-body clearance to minimal tissue inflammation.

Mechanism → morphology → clinicopathologic consequences

Slide 3 · A cell-directed process of self-destruction

Intrinsic enzymes degrade cellular proteins and nuclear DNA.

Apoptotic bodies retain plasma membrane around cellular fragments.

Surface signals recruit phagocytes before contents escape.

The tissue reaction is therefore usually inconspicuous.

The defining feature is controlled dismantling, not simple cell disappearance.

Slide 4 · The four linked events in apoptosis

Death stimulus

Caspase activation

Ordered dismantling

Efferocytosis

Pathway initiation and execution lead to efficient removal of the dying cell.

Slide 5 · Why apoptosis does not resemble necrosis

Apoptosis

Necrosis

Membrane-bound cellular fragments

Early phagocytosis

Little surrounding inflammation

Loss of membrane integrity

Leakage of enzymes and contents

Local inflammatory reaction

Slide 6 · Programmed cell death: A contextual term

Developmental cell removal requires tightly timed elimination

Apoptosis also occurs in adult tissue homeostasis.

Pathologic apoptosis removes irreparably damaged cells.

Slide 7 · Physiologic apoptosis regulates tissue cell numbers

Cell deletion matches ongoing production or changing tissue needs.

Supernumerary cells are removed during organ development.

Hormone-responsive tissues regress after hormonal withdrawal.

Immune and epithelial populations are continuously pruned.

Cell death is integral to normal tissue maintenance.

Slide 8 · Embryonic remodeling depends on cell removal

Transient structure

Developmental signal

Selective apoptosis

Remodeled tissue

Cells no longer required for the maturing structure are eliminated.

Slide 9 · Hormone withdrawal induces physiologic involution

Endometrial cells are lost during the menstrual cycle.

Ovarian follicles undergo atresia when hormonal support changes.

Lactating mammary tissue regresses after weaning.

Withdrawal of survival signals permits apoptotic machinery to engage.

Hormone dependence connects endocrine physiology to controlled cell loss.

Slide 10 · Epithelial homeostasis balances gain and loss

Cell production

Epithelial renewal

Apoptotic deletion

Stable population

Intestinal crypt epithelium illustrates high-turnover tissue equilibrium.

Slide 11 · Immature lymphocyte selection removes failed cells

Developing lymphocytes must assemble functional antigen receptors.

Cells that fail this requirement are eliminated by apoptosis.

Selection operates in bone marrow and thymus.

Germinal-center B-cell populations also undergo regulated deletion.

Failed immune-cell development need not produce necrotic injury.

Slide 12 · Central immune tolerance requires cell deletion

Self-reactive lymphocyte

Recognition of self

Apoptotic deletion

Reduced autoreactivity

Removal of potentially harmful lymphocytes is one defense against autoimmunity.

Slide 13 · Neutrophil removal helps resolve acute inflammation

Neutrophils act for a limited interval at inflammatory sites.

Spent neutrophils subsequently undergo apoptosis.

Phagocytes clear the fragments without major content spillage.

Cell removal facilitates the resolution of an inflammatory episode.

The cessation of inflammation requires active cell clearance.

Slide 14 · Immune contraction follows a completed response

Expanded effector lymphocyte populations are no longer needed.

Apoptosis contracts the population after antigen control.

Cells are removed while surrounding tissue is relatively preserved.

A failure of effective deletion may allow persistent lymphocytes.

Physiologic apoptosis limits the duration of immune responses.

Slide 15 · Survival signals protect against unnecessary death

Signals that preserve cells

Events favoring deletion

Growth factors support viability.

Extracellular-matrix contacts contribute survival cues.

Anti-apoptotic proteins maintain mitochondrial integrity.

Withdrawal of growth support

Loss of requisite cell contacts

Acquisition of pro-apoptotic signals

Slide 16 · Cells integrate competing survival and death signals

Apoptosis depends on the balance of opposing molecular influences

Survival cues favor anti-apoptotic BCL2-family activity.

Cell stress activates sensors that favor BAX/BAK-mediated membrane permeabilization.

Slide 17 · Pathologic apoptosis minimizes collateral damage

Cells injured beyond repair are selectively eliminated.

DNA lesions or misfolded proteins may initiate the response.

Viral infection and immune cytotoxicity are further triggers.

Parenchymal cells may be lost after prolonged duct obstruction.

Apoptosis is protective in some settings yet contributes to tissue loss in others.

Slide 18 · Severe DNA damage can trigger deletion

Radiation or drug injury

DNA damage

Failure of repair

Apoptosis

Deletion prevents survival of cells harboring potentially oncogenic mutations.

Slide 19 · Genotoxic insults threaten genomic integrity

Radiation can damage DNA directly or through reactive species.

Cytotoxic anticancer drugs can produce DNA lesions.

Damage-repair mechanisms may restore cellular viability.

When repair fails, intrinsic apoptotic pathways remove the cell.

Whether a cell survives depends in part on the ability to repair its DNA.

Slide 20 · Misfolded proteins couple ER stress to death

Protein misfolding

ER stress response

Persistent unresolved stress

Intrinsic apoptosis

Severe unresolved proteostasis stress can ultimately cause cell loss.

Slide 21 · Infections can engage more than one apoptotic trigger

Some viruses directly injure infected cells.

Immune cells can induce apoptosis in infected targets.

The response limits reservoirs of intracellular pathogens.

Extensive apoptotic deletion may nevertheless damage tissues.

Viral hepatitis illustrates host-mediated hepatocyte injury.

Slide 22 · Cytotoxic T lymphocytes remove infected targets

Recognition of viral antigen

Cytotoxic T-cell response

Target-cell apoptosis

Reduced infected-cell reservoir

Apoptotic killing protects the host but can contribute to organ injury.

Slide 23 · Immune cytotoxicity also targets noninfected cells

Cytotoxic T lymphocytes may eliminate transformed tumor cells.

Immune recognition causes cell-death pathways in targets.

The same cytotoxic capacity underlies cellular allograft rejection.

Graft-versus-host disease also causes immune-mediated tissue injury.

One cytotoxic mechanism can be protective or pathologic depending on its target.

Slide 24 · Duct obstruction can lead to parenchymal atrophy

Duct obstruction

Loss of functional support

Apoptotic cell deletion

Parenchymal atrophy

Examples include pancreas, parotid gland, and kidney.

Slide 25 · Physiologic and pathologic contexts

Physiologic context

Pathologic context

Developmental remodeling

Radiation-induced DNA injury

Hormone-dependent involution

Persistent misfolded-protein stress

Epithelial cell turnover

Virus-associated tissue damage

Lymphocyte selection

Immune injury to grafts

Resolution of inflammation

Obstruction-associated atrophy

Slide 26 · Morphologic progression: The cellular overview

Viable cell

Shrinkage

Membrane blebs

Apoptotic bodies

Schematic — Original schematic of successive morphologic changes; Not a histologic specimen.

Slide 27 · Apoptotic cells become smaller and denser

Cell volume decreases, unlike the early swelling of necrosis.

Cytoplasm becomes densely eosinophilic on H&E.

Intracellular organelles remain relatively intact initially.

Organelles become more closely packed as the cell shrinks.

Cell shrinkage is a characteristic early morphologic cue.

Slide 28 · Condensed chromatin is a dominant nuclear clue

Dispersed chromatin

Condensed nuclear material

Schematic — In apoptosis, chromatin aggregates into dense peripheral masses near the nuclear envelope.

Slide 29 · Chromatin condensation precedes nuclear breakdown

Condensed chromatin forms dense masses of varying shapes.

The chromatin often lies beneath the nuclear membrane.

Later, the nucleus may break into multiple pieces.

Fragmentation is spatially confined within apoptotic structures.

Chromatin condensation is emphasized as a particularly characteristic feature.

Slide 30 · Membrane blebs precede cell fragmentation

Cell shrinkage

Surface blebbing

Discrete fragments

Schematic — The cell surface forms blebs and then separates into membrane-bound bodies.

Slide 31 · Apoptotic bodies package cellular material

Fragments contain cytoplasm and densely packed organelles.

Some, but not all, bodies also carry fragments of nuclei.

Each fragment remains bounded by a cellular membrane.

Membrane confinement reduces uncontrolled tissue exposure.

An apoptotic body is a fragment of a dead cell, not an intact surviving cell.

Slide 32 · Phagocytosis completes morphologic removal

Apoptotic body

Surface recognition

Phagocytic engulfment

Lysosomal degradation

Macrophages and other phagocytes efficiently dispose of the remnants.

Slide 33 · What an apoptotic cell looks like on H&E

A round or oval shrunken cellular profile

Intensely eosinophilic cytoplasm

Condensed or fragmented, deeply basophilic chromatin

Usually little surrounding inflammatory reaction

Descriptions are; No photomicrograph is reproduced in this deck.

Slide 34 · Apoptosis versus necrosis: Microscopic clues

Feature

Apoptosis

Necrosis

Cell volume

Shrinkage

Swelling

Plasma membrane

Maintained as fragments

Disrupted

Nucleus

Condensation; Fragments

Pyknosis, karyorrhexis, karyolysis

Contents

Membrane-contained

Leaked into tissue

Inflammation

Typically minimal

Usually present

Slide 35 · A section may underestimate ongoing apoptosis

Apoptotic shrinkage and breakup can occur rapidly.

Phagocytosis promptly removes the individual fragments.

Transient events may disappear between sampling and fixation.

An absence of inflammation offers few secondary clues.

A low visible apoptotic-body count does not establish absent apoptotic activity.

Slide 36 · Caspases execute controlled cell disassembly

Caspases are cysteine proteases that cleave substrates after aspartate residues

They are produced as inactive precursors.

Proteolytic activation initiates an amplifying enzymatic cascade.

Slide 37 · Initiator and executioner caspases have different roles

Initiator caspases are activated by upstream signaling assemblies.

Caspase-9 is central to the intrinsic pathway.

Caspase-8 or -10 is activated by death receptor signaling.

Executioner caspases such as caspase-3 and -6 cleave cellular substrates.

Initiation differs; The final execution program converges.

Slide 38 · Two routes converge on a common execution program

Intrinsic pathway

Initiator caspase

Executioner caspases

Cell fragmentation

Extrinsic signaling reaches the same terminal proteolytic machinery.

Slide 39 · The initiation phase and the execution phase

Initiation

Execution

Distinct cellular death signals

Formation of pathway-specific complexes

Activation of initiator caspases

Rapid activation of downstream caspases

Protein and nuclear component degradation

Fragmentation and clearance

Slide 40 · Common triggers of intrinsic apoptosis

Withdrawal of essential survival factors

DNA damage that cannot be adequately repaired

Accumulation of misfolded proteins and ER stress

Other cellular injuries sufficient to activate BH3-only sensors

Mitochondrial apoptosis operates in many physiologic and pathologic situations.

Slide 41 · Mitochondrial outer membrane permeabilization is pivotal

Permeabilization releases normally sequestered death-promoting molecules

Cytochrome c functions in mitochondrial energy metabolism.

In the cytosol, released cytochrome c contributes to caspase activation.

Slide 42 · The BCL2 family: Three functional groups

Group

Representative members

Principal function

Anti-apoptotic

BCL2, BCL-XL, MCL1

Preserve mitochondrial integrity

Effector proteins

BAX, BAK

Promote membrane permeabilization

BH3-only sensors

BAD, BIM, BID, PUMA, NOXA

Respond to death signals; Engage effectors

Slide 43 · BCL2-family proteins govern the life–death threshold

Family members are distinguished by BCL2 homology domains.

Pro-survival proteins oppose mitochondrial membrane permeabilization.

BAX and BAK are pro-apoptotic effector molecules.

Stress-responsive BH3-only proteins shift the balance toward death.

The relevant output is permeability of the outer mitochondrial membrane.

Slide 44 · BCL2, BCL-XL, and MCL1 protect cells

These proteins are major anti-apoptotic BCL2-family members.

They help prevent release of cytochrome c and related factors.

They occur at outer mitochondrial membranes and other sites.

Survival signals can support anti-apoptotic protein expression.

Protection is functional rather than a simple fixed on/off gene state.

Slide 45 · BAX and BAK produce the mitochondrial commitment step

BH3-only activation

BAX/BAK activation

BAX/BAK oligomerization

Outer membrane permeabilization

The exact biophysical pore-forming mechanism is not completely settled.

Slide 46 · BH3-only proteins sense cellular stress

Textbook examples include BAD, BIM, BID, PUMA, and NOXA.

Stress can increase their abundance or activity.

They can activate BAX/BAK and oppose BCL2/BCL-XL.

Their combined actions favor cytochrome c release.

BH3-only proteins link diverse injuries to the common mitochondrial response.

Slide 47 · Growth factor withdrawal can activate apoptosis

Less survival signaling

Reduced anti-apoptotic protection

BH3-only influence

BAX/BAK activation

Apoptosis

Loss of trophic signals removes protection that previously held the pathway in check.

Slide 48 · Survival and death influences at mitochondria

Survival bias

Death bias

Growth-factor signaling present

BCL2-family protection maintained

Cytochrome c retained in mitochondria

DNA injury or ER stress persists

BH3-only activity increases

BAX/BAK permits cytochrome c escape

Slide 49 · Cytochrome c initiates apoptosome assembly

Cytochrome c release

Cytosolic APAF-1 binding

Apoptosome assembly

Caspase-9 activation

The apoptosome is a multiprotein platform for initiation of caspase signaling.

Slide 50 · Caspase-9 launches the intrinsic caspase cascade

Cytochrome c–APAF-1 complexes recruit procaspase-9.

Proximity promotes caspase-9 activation.

Active caspase-9 activates downstream caspases.

Caspase-3 is one of the key executioner enzymes.

The cytosolic apoptosome connects mitochondrial injury to proteolysis.

Slide 51 · Mitochondrial proteins also release caspase brakes

Mitochondrial injury

Smac/DIABLO release

IAP neutralization

Less caspase inhibition

IAPs physiologically restrain unwanted caspase activation.

Slide 52 · Inhibitor-of-apoptosis proteins restrain death

IAPs oppose inappropriate caspase activity in viable cells.

Mitochondrial Smac/DIABLO can antagonize IAP function.

Removing the inhibitory influence facilitates executioner action.

Apoptotic commitment reflects activating and inhibitory mechanisms.

The caspase cascade is regulated by more than its initiator complexes.

Slide 53 · Intrinsic pathway from injury to execution

Stress or signal withdrawal

BH3-only protein activity

BAX/BAK; Cytochrome c release

APAF-1 apoptosome; Caspase-9

Executioner caspases

Key logic: Internal stress changes mitochondrial outer-membrane permeability, allowing cytochrome c to activate the APAF-1/caspase-9 platform. Anti-apoptotic BCL2 proteins oppose this transition.

Slide 54 · Death receptors initiate an extracellularly triggered route

Death receptors are members of the TNF receptor superfamily.

A cytoplasmic death domain recruits signaling adaptors.

Fas (CD95) and TNFR1 are major examples.

Receptor engagement can trigger caspase activation.

Death-receptor signaling is not identical to all TNF-family receptor signaling.

Slide 55 · Fas ligand initiates death receptor signaling

FasL engagement

Fas receptor clustering

FADD recruitment

Caspase-8 activation

The cytoplasmic death domain transmits the receptor-associated death signal.

Slide 56 · Fas-mediated lymphocyte deletion supports tolerance

Fas is present on multiple cell types.

FasL is expressed by certain activated T cells.

Engagement can help eliminate self-reactive lymphocytes.

The same system contributes to CTL killing of targets.

Extrinsic apoptosis has a prominent immune-system role.

Slide 57 · FADD organizes the death-inducing complex

Fas receptor clustering juxtaposes intracellular death domains.

FADD binds these receptor-associated domains.

FADD recruits procaspase-8 or procaspase-10 molecules.

Proximity allows processing into active initiator caspases.

The death-inducing signaling complex is a protein assembly, not a membrane pore.

Slide 58 · The extrinsic route reaches the same executioners

FasL–Fas interaction

FADD-linked complex

Caspase-8 or -10

Executioner caspases

Cell dismantling

Death receptor engagement changes the initiation machinery but not the broad end point.

Slide 59 · FLIP can inhibit death-receptor apoptosis

FLIP interacts with the procaspase-8 activation machinery.

It can interfere with assembly of productive FADD–caspase complexes.

Some normal cells use FLIP as a protective mechanism.

Certain viruses also express molecules that antagonize Fas killing.

Receptor engagement alone does not always guarantee target-cell apoptosis.

Slide 60 · Fas signaling, from membrane to nucleus

Fas ligand / Fas receptor

FADD adaptor complex

Caspase-8 activation

Executioner caspases

Protein and DNA cleavage

At the surface, receptor aggregation recruits FADD; Initiator caspase processing then drives a downstream caspase cascade. This is distinct from cytochrome c–APAF-1 signaling.

Slide 61 · BID provides cross-talk to mitochondria

Caspase-8 activation

BID cleavage/activation

Mitochondrial engagement

Amplified apoptosis

Hepatocytes and pancreatic β-cells as examples of this linkage.

Slide 62 · Intrinsic and extrinsic apoptosis compared

Parameter

Intrinsic

Extrinsic

Usual starting signal

Internal stress / signal withdrawal

Death ligand / receptor binding

Main initiating platform

Cytochrome c–APAF-1 apoptosome

FADD-dependent receptor complex

Initiator caspase

Caspase-9

Caspase-8 or -10

Central regulatory proteins

BCL2 family; IAPs

Fas, FADD, FLIP

Common endpoint

Executioner caspases

Executioner caspases

Slide 63 · Executioner caspases target the cell interior

Activated caspase-3 and -6 cleave multiple cellular proteins.

Nuclear scaffolding proteins are dismantled.

An inhibitor of DNase is cleaved, enabling DNA fragmentation.

Proteolysis and nucleolysis cooperate in cell disassembly.

Biochemical degradation explains the appearance of nuclear fragments.

Slide 64 · Caspases activate DNA fragmentation

Executioner caspase

DNase inhibitor cleavage

Active DNase

Nuclear DNA degradation

The execution phase converts a death decision into irreversible structural changes.

Slide 65 · How the nucleus fragments during apoptosis

Executioner enzymes degrade nuclear structural elements.

Activated nucleases cleave nuclear DNA.

Chromatin compacts and breaks into discrete fragments.

Fragments may become enclosed within apoptotic bodies.

Morphologic nuclear segmentation reflects a coordinated biochemical process.

Slide 66 · Membrane blebbing remains incompletely explained

Not every morphologic step is mechanistically resolved

Caspases clearly degrade many structural proteins.

The precise formation of blebs and apoptotic bodies is less well defined.

Slide 67 · Phosphatidylserine externalization signals disposal

Viable membrane: PS inside

Apoptotic membrane: PS outside

Phagocyte receptor recognition

Engulfment

Phosphatidylserine on the outer leaflet is a central “eat-me” signal.

Slide 68 · Apoptotic cells attract and identify themselves to phagocytes

Dying cells can release soluble phagocyte-recruiting factors.

Exposed phosphatidylserine promotes target recognition.

Macrophage-associated bridging proteins may aid adhesion.

Multiple recognition signals make clearance efficient.

“Find-me” and “eat-me” are distinct but cooperating functions.

Slide 69 · Opsonins can aid apoptotic-body recognition

Natural antibodies may coat apoptotic fragments.

Complement proteins including C1q can associate with these bodies.

Phagocyte receptors recognize the coating molecules.

This adds redundant pathways for prompt engulfment.

The receptor–ligand repertoire extends beyond phosphatidylserine alone.

Slide 70 · Efferocytosis completes apoptosis without spillage

Apoptotic body recognition

Macrophage engulfment

Lysosomal digestion

Little content release

Rapid efferocytosis explains why cell loss can be histologically inconspicuous.

Slide 71 · Clearance also limits inflammatory cytokine production

Macrophages ingest apoptotic cells before membrane rupture.

Engulfment is associated with reduced proinflammatory cytokines.

This suppressive milieu helps prevent collateral tissue injury.

Efficient clearance is therefore an active anti-inflammatory process.

Limited inflammation is a combined consequence of containment and clearance.

Slide 72 · Application: Radiation-induced cell loss

After radiation exposure, a cell accumulates DNA damage that cannot be repaired, yet its plasma membrane has not ruptured.

Which major apoptotic pathway is expected to dominate?

Intrinsic (mitochondrial) pathway.

DNA injury engages stress sensors that favor BAX/BAK and cytochrome c release.

Slide 73 · Application: Self-reactive T-cell deletion

An activated autoreactive lymphocyte expresses Fas and interacts with a T cell expressing Fas ligand.

Which initiator caspase is associated with the receptor complex?

Caspase-8 (or caspase-10).

FADD recruits the procaspases to a death-inducing signaling complex.

Slide 74 · Application: An eosinophilic shrunken cell

A tissue section shows an isolated small round cell with intensely eosinophilic cytoplasm and dense fragmented chromatin, with no surrounding neutrophil influx.

What mechanism best fits these observations?

Apoptosis.

Shrinkage, chromatin condensation, and absent substantial inflammation support this mechanism.

Slide 75 · Application: Why tissue inflammation is absent

A biopsy contains isolated apoptotic bodies and relatively preserved adjacent parenchyma.

Which event best explains the limited tissue reaction?

Timely recognition and efferocytosis.

Membrane-contained bodies are swallowed before releasing their contents.

Slide 76 · Interpretation pitfalls when recognizing apoptosis

Observation

Appropriate interpretation

Few visible apoptotic figures

Rapid clearance may underestimate cumulative death

No inflammation

Consistent with apoptosis, but not independently diagnostic

Fragmented nuclear chromatin

Assess cell shrinkage and membrane-bound bodies

Caspase activation

Supports active execution machinery

Large necrotic zone

Consider membrane breakdown and necrosis instead

Slide 77 · Five mechanisms worth remembering

Intrinsic injury acts through BH3-only proteins and BAX/BAK.

Cytosolic cytochrome c and APAF-1 activate caspase-9.

Fas/FADD signaling typically engages caspase-8 or -10.

Executioner caspases degrade proteins and enable DNA cleavage.

Both pathways end in membrane-contained fragments that phagocytes remove.

Slide 78 · Core references and source boundaries

Robbins, Cotran & Kumar: Pathologic Basis of Disease

11th edition (2026) · Elsevier

Sections covered: apoptosis, its causes, morphology, signaling pathways, and removal of dead cells.