ApoptosisProgrammed cell

death

OBJECTIVESDEFINITION, PHYSIOLOGIC AND PATHOLOGIC CONDITIONS.

DESCRIBE THE MORPHOLOGY AND DISCUSS THE POSSIBLE MECHANISMS.

COMPARE BETWEEN APOPTOSIS AND NECROSIS.

Apoptosis = “Falling away from”

Apoptosis is responsible for the programmed

cell death in several important physiologic and

pathologic processes.

ApoptosisApoptosis : :

Programmed cell death that involves one cellor small group of cells

APOPTOSIS, PHYSIOLOGIC APOPTOSIS, PHYSIOLOGIC CONDITIONSCONDITIONS1- Destruction of unwanted cells during embryogenesis.

2- Mature tissue homeostasis, in proliferating cells.

3- Involution of hormone-dependent tissue as shedding of the endometrium during the menstrual cycle and regression of the breast following weaning.

4- Elimination of harmful self-reactive lymphocytes.

*** Failure of cells to undergo physiologic apoptosis may result in developmental defects, autoimmune diseases and others.

APOPTOSIS, PATHOLOGIC APOPTOSIS, PATHOLOGIC CONDITIONSCONDITIONS

Apoptosis eliminates injured cells that are not able to repair

1- DNA damage, as in radiations; cytotoxic drugs; and hypoxia, to prevent malignant transformations.

2- Cell death in certain infections as viral infections by response from cytotoxic T-lymphocytes.

3- In tumors and transplant rejection.

4- Pathological atrophy in parynchemal organs after duct obstruction: pancreas, parotid, and kidney.

Morphology of apoptosisMorphology of apoptosis

Usually involves single or small clusters of cells.

Microscopically, apoptotic cells appear as round or oval masses with intense eosinophilic cytoplasm.

1.The nuclear chromatin undergoes condensation and fragmentation.

2.The cells rapidly shrink, forming cytoplasmic buds and then fragment into apoptotic bodies composed of membrane-bound vesicles of cytoplasm and organelles.

3. Digestion of apoptotic bodies by macrophages.

4. Lack of inflammation in the surrounding.

Apoptosis of a keratinocyte in skin (arrow)

Apoptosis may be histologically inapparent, Apoptosis may be histologically inapparent,

possible reasons include:possible reasons include:

1- Apoptotic fragments are rapidly phagocytozed

and degraded.

2- Apoptosis does not induce inflammatory

reaction, thus it becomes difficult to be recognised

microscopically.

Mechanisms of apoptosisMechanisms of apoptosis

1- Signalling

2- Control and integration

3- Execution

4- Removal of dead cells

The basic processes include four separable but overlapping components:

1- Signalling Variety of signals (intrinsic & extrinsic) induce apoptosis:

1. Intrinsic (as in development)

2. Lack of growth factor

3. Toxin from cytotoxic T cells

4. Radiation, heat, chemicals

5. Receptor-ligand interactions:Tumor necrosis factor (TNF) family of plasma

membrane receptors (e.g., FAS receptor) is a major initiator of death signals.

Specific proteins connect the original death signals to the final execution program.

Two pathways are involved in this process:

1- Direct transmission of death signals by specific adapter proteins to the executive mechanism.

2- Regulation of mitochondrial permeability by members of BCL-2 family of proteins.

2- Control and integration2- Control and integration

Mitochondrial BCL-2 family members (BCL-X ) inhibit and others (BAX & BAD or BAK) promote pore formation in mitochondria:

Promotion leads to less ATP production with mitochondrial swelling.

Outer mito. membrane permeability increases leading to a release of apoptotic trigger (cytochrome c) into cytosol.

Cytochrome c binds some cytosolic proteins, activating them and then eventually induces caspase activation.

Caspases, when activated, begin proteolytic events that kill the cell.

3- Execution3- Execution

Distinctive biochemical events resulting from activity of catabolic enzymes in the cytosol.

1- Protein cleavage by caspases:

Caspases:

Specific proteases with an active site cysteine.

Activation of one or more caspase enzyme leads

to cascade of activation of other proteases.

E.g., activation of endonuclease result in DNA

fragmentation

2- Proteins are cross-linked by transglutaminase

Forming condensed shell that readily fragments

into apoptotic bodies.

3- DNA breakdown

giving 180 – 200 base pair fragments, by action of endonucleases.

This may be visualized as a distinctive “Ladders” of DNA fragments on electrophoresis.

This laddering pattern is not specific for apoptosis. Necrosis usually gives a more random pattern of fragmentation, forming a smear.

Gel electrophoresis of DNA extracted from cultured cells:

1- Lane a, Viable control cells

2- Lane b, Extensive apoptosis, with laddered DNA demonstrating fragmentation

3- Lane c, necrosis

However, DNA laddering is not diagnostic for apoptosis as it may be seen in early stages of necrosis as well!

Apoptotic bodies have plasma membrane

surface markers that facilitate uptake and

degradation by adjacent cells or by

phagocytes.

A “flip” of inner plasma membrane

phosphatidylserine to the outer surface is a

sufficient signal to attract other cells for

phagocytosis without the harmful secondary

effects of inflammation.

4- Removal of dead 4- Removal of dead cellscells

COMPARISON BETWEEN COMPARISON BETWEEN NECROSIS & NECROSIS &

APOPTOSISAPOPTOSISFeatureNecrosisApoptosis

Cell sizeswellingshrinkage

NucleusPyknosis, karyorrhexis, karyolysis

Fragmentation to apoptotic bodies

Plasma membranedisruptedintact

Cell contentsLeak out of cellintact

Adjacent inflammation

frequentno

Physiologic or pathologic role

pathologicPhysiologic and can be pathologic

THANK

YOU DR/MONA KAMAL