APOPTOSIS and NECROSIS

The Morphology of Apoptosis Engulfment of the cell corpse Chromosomes condense and fragment Nuclear membrane breaks down Apoptotic body formation Cytoplasm shrinks

Difference Between Apoptosis and Necrosis Necrosis (pathological cell death): dying cells swell and lyse; toxic contents leak out and result in inflammatory response. Apoptosis (physiological or programmed cell death): dying cells shrink, are engulfed by other cells, leave no trace, and don’t result in harmful outcomes

Forms of cell death "Classic" Necrosis Apoptosis Mitotic catastrophe Passive Active Passive Pathological Physiological or Pathological pathological Swelling, lysis Condensation, Swelling, lysis cross-linking Dissipates Phagocytosed Dissipates Inflammation No inflammation Inflammation Externally induced Internally or Internally induced externally induced

APOPTOSIS Evolutionarily conserved Occurs in all multicellular animals studies (plants too!) Stages and genes conserved from nematodes (worms) and flies to mice and humans

Functions of apoptosis Sculpt body structures, e.g. hand digit Serve some function but no longer needed e.g. tadpole tail of frog. Needed in one sex but not another e.g. Mullerian duct important for female is eliminated in males by apoptosis. Produced in excess, e.g. extra neurons are removed by apoptosis during neurogenesis. Serve in immune system as a defense mechanism to get rid of harmful or damaged cells.

STAGES OF CLASSIC APOPTOSIS Healthy cell DEATH SIGNAL (extrinsic or intrinsic) Commitment to die (reversible) EXECUTION (irreversible) Dead cell (condensed, crosslinked) ENGULFMENT (macrophages, neighboring cells) DEGRADATION

STAGES OF CLASSIC APOPTOSIS Genetically controlled: Caenorhabditis elegans soil nematode (worm) ces2 ces1 ced9 ced3,4 Dead cell Healthy cell Committed cell BCL2 Caspases (proteases) C. elegans genes == mammalian genes

Cells are balanced between life and death DAMAGE Physiological death signals DEATH SIGNAL PROAPOPTOTIC PROTEINS (dozens!) ANTIAPOPTOTIC PROTEINS (dozens!) DEATH

APOPTOSIS: important in embryogenesis Morphogenesis (eliminates excess cells): Selection (eliminates non-functional cells):

APOPTOSIS: important in embryogenesis Immunity (eliminates dangerous cells): Self antigen recognizing cell Organ size (eliminates excess cells):

APOPTOSIS: important in adults Tissue remodeling (eliminates cells no longer needed): Apoptosis Late pregnancy, lactation Involution (non-pregnant, non-lactating) Virgin mammary gland - Testosterone Apoptosis Prostate gland

APOPTOSIS: important in adults Tissue remodeling (eliminates cells no longer needed): Apoptosis Resting lymphocytes + antigen (e.g. infection) - antigen (e.g. recovery) Steroid immunosuppressants: kill lymphocytes by apoptosis Lymphocytes poised to die by apoptosis

X APOPTOSIS: important in adults Maintains organ size and function: + cell division Cells lost by apoptosis are replaced by cell division (remember limited replicative potential of normal cells restricts how many times this can occur before tissue renewal declines)

APOPTOSIS: control Receptor pathway (physiological): Death FAS ligand TNF Death receptors: (FAS, TNF-R, etc) Death domains Adaptor proteins Pro-caspase 8 (inactive) Caspase 8 (active) Pro-execution caspase (inactive) Execution caspase (active) MITOCHONDRIA Death

APOPTOSIS: control Intrinsic pathway (damage): Mitochondria Death Cytochrome c release BAX BAK BOK BCL-Xs BAD BID B IK BIM NIP3 BNIP3 BCL-2 BCL-XL BCL-W MCL1 BFL1 DIVA NR-13 Several viral proteins Pro-caspase 9 cleavage Pro-execution caspase (3) cleavage Caspase (3) cleavage of cellular proteins, nuclease activation, etc. Death

APOPTOSIS: control Physiological Intrinsic receptor pathway damage pathway MITOCHONDRIAL SIGNALS Caspase cleavage cascade Orderly cleavage of proteins and DNA CROSSLINKING OF CELL CORPSES; ENGULFMENT (no inflammation)

APOPTOSIS: Role in Disease TOO MUCH: Tissue atrophy Neurodegeneration Thin skin etc TOO LITTLE: Hyperplasia Cancer Athersclerosis etc

APOPTOSIS: Role in Disease Neurodegeneration Neurons are post-mitotic (cannot replace themselves; neuronal stem cell replacement is inefficient) Neuronal death caused by loss of proper connections, loss of proper growth factors (e.g. NGF), and/or damage (especially oxidative damage) Neuronal dysfunction or damage results in loss of synapses or loss of cell bodies (synaptosis, can be reversible; apopsosis, irreversible) PARKINSON'S DISEASE ALZHEIMER'S DISEASE HUNTINGTON'S DISEASE etc.

APOPTOSIS: Role in Disease Cancer Apoptosis eliminates damaged cells (damage => mutations => cancer Tumor suppressor p53 controls senescence and apoptosis responses to damage Most cancer cells are defective in apoptotic response (damaged, mutant cells survive) High levels of anti-apoptotic proteins or Low levels of pro-apoptotic proteins ===> CANCER

APOPTOSIS: Role in Disease AGING Aging --> both too much and too little apoptosis (evidence for both) Too much (accumulated oxidative damage?) ---> tissue degeneration Too little (defective sensors, signals? ---> dysfunctional cells accumulate hyperplasia (precancerous lesions)

OPTIMAL FUNCTION (HEALTH) APOPTOSIS AGING APOPTOSIS Neurodegeneration, cancer, …..

NECROSIS

Development of Necrosis (2 mechanisms) irreversible damage to mitochondria (failure of ATP generation) ↓ anaerobic respiration lactic acid accumulation ↓ pH in intracellalar matrix (↑ acidity) activation of lysosomal enzymes proteolytic digestion of cell dead tissue cleaned away damage to cell membrane ↓ loss of phospholipids & damage due to lipid breakdown material cytoskeleton detached from cell membrane injury allows enzymes to escape into ECF Influx of Ca2+ ions Ca2+ causes permanent damage to mitochondria, inhibits cellular enzyme action & denatures protein characteristic cell changes (coagulative necrosis)

Cytoplasmic changes 1- Increased eosinophilia: Increased binding of eosin to the denatured protein Loss of basophilia of RNA 2- Glassy homogenous appearance: due to loss of glycogen particles. 3- Vacuolated cytoplasm: due to enzymatic degradation of the organelles.

Nuclear changes Pyknosis – shrinking & condensation Karyorrhexis – rupture of nuclear membrane Karyolysis – basophilia gradually fades

Types of Necrosis Coagulative necrosis (Ischaemia Infarction) Colliquative necrosis (Liquefactive necrosis) Caseous necrosis Fat necrosis Enzymatic fat necrosis Traumatic fat necrosis Gangrenous necrosis (putrefactive infection)

Coagulative necrosis

Many nuclei have become pyknotic (shrunken and dark) and have then undergone karorrhexis (fragmentation) and karyolysis (dissolution). The cytoplasm and cell borders are not recognizable.

Here is myocardium in which the cells are dying Here is myocardium in which the cells are dying. The nuclei of the myocardial fibers are being lost. The cytoplasm is losing its structure, because no well-defined cross-striations are seen.

Liquefactive necrosis The necrotic tissue is rapidly liquefied. It occurs in: 1- Infarctions of the brain: due to high lipid and fluid content. 2- Pyogenic abscess: due to proteolytic enzymes released by pus cells 3- Amoebic abscess: due to liquefactive enzymes released by the parasite

Fat Necrosis Enzymatic Fat Necrosis Traumatic Fat Necrosis acute pancreatitis ↓ enzymes escape into surrounding tissue lipases hydrolyse true fat into glycerol & fatty acids fatty acids saponify or form soaps (white opaque masses or plaques) diagnostic of acute pancreatitis calcification may occur later rupture of cell membrane ↓ release natural fat into tissues phagocytosed by macrophages foamy appearance chronic inflammatory reaction fibrosis & scarring (no enzymatic fat breakdown)

Acute pancreatitis

Fat necrosis

Caseation necrosis Distinctive type of necrosis in which there is coagulative necrosis with slow liquefaction