Lecture 26 Flashcards
Cell Injury & Death
1
Q
stages of cellular response to stress and injurious stimuli
A
normal cell is in homeostasis
- stress leads to adaptation, if unable to adapt -> cell injury
- injurious stimulus leads to cell injury
- if cell injury is mild and transient, it is reversible and can go back to being a normal cell
- if cell injury is severe and progessive, it is irreversible and can lead to necrosis or apoptosis
pg 616
2
Q
common causes of cell injury
A
- oxygen deprivation -> anoxia or ischemia
- physical, chemical, environmental agents and drugs
- infectious agents
- immunologic reactions
- genetic abnormalities
- nutritional imbalances
pg 617
3
Q
progession of cell injury to death
A
- all stresses and noxious influences evert their effects first at the molecular or biochemical level
- time lag b/n stress and morphologic changes: early changes are subtle (histochemical, ultrastructural, or biochemical), changes visible by light microscopy or the naked eye may take hours to develop
- morphologic manifestations of necrosis take more time to develop than those of reversible damage
- various forms of cell death have been identified with each relying on a diff sibset of proteins for the activation and execution of their respective pathways
pg 618
4
Q
biochemical and morphologic changes in cell injury
A
in reversible cell injury, cell function declines rapidly
in irreversible cell injury, there are biochemical alterations leading to cell death, ultrastructural changes, light microscopic changes, and gross morphologic changes (all which take some time to occur)
pg 619
5
Q
reversible cell injury
A
features seen in reversibly injured cells:
- generalized swelling of the cell and its organelles (result from influx of water -> failure of Na+K+ATPase pump; causes pallor, increased turgor, and increased weight of affected organ)
- blebbing of the plasma membrane (leads to disintegration of membrane in irreversible injury)
- detachment of ribosomes from the endoplasmic reticulum (ER)
- clumping of nuclear chromatin
- fatty change (occurs in organs involved in lipid metabolism; toxic injury disrupts metabolic pathways and leads to rapid accumulation of triglyceride-filled lipid vacuoles)
- small clear vacuoles may be seen w/in the cytoplasm
pg 620
6
Q
morphologic differences between necrosis and apoptosis
A
necrosis
- cell SWELLING (at this point it is reversible)
- progressive injury leading to breakdown of plasma membrane (no longer reversible)
- mitochondrial leakage leads to inflammation
- unplanned! BAD for the cell and environment!
apoptosis
- cell SHRINKING
- cellular fragmentation
- phagocytosis of apoptotic cells and fragments by macrophages
- NO inflammation
- controlled! good for environment
7
Q
morphology of reversible cell injury: ultrastructural changes
A
- plasma membrane alterations -> blebbing, blunting, and loss of microvilli (first thing to go)
- mitochondrial changes -> swelling and appearance of small amorphous densities
- accumulation of “myelin figures” in the cytosol composed of phospholipids derived from damaged cellular membranes
- dilation of the ER with detachment of polysomes
- nuclear alterations with disaggregation of granular and fibrillar elements
pg 623
8
Q
common mechanisms of cell death
A
- unprogrammed: necrosis (result of injury -> catastrophe)
- programmed: split into caspase dependent and independent
- caspase dependent: apoptosis (intrinsic pathway, extrinsic pathway) and pyroptosis
- caspase independent: necroptosis, ferroptosis, autophagy
pg 625
9
Q
necrosis
A
characterized by:
- cellular swelling (as opposed to shrinkage)
- denaturation of cellular proteins
- leakage of cellular contents through damaged membranes eliciting a host reaction (inflammation)
- enzymatic digestion of the lethally injured cell by lysosomal enzymes
pg 628
10
Q
leakage of cellular contents through damaged membranes eliciting a host reaction (inflammation)
A
- some specific substances released from injured cells have been called damage-associated molecular patterns (DAMPs)
- these include ATP (released from damaged mitochondria), uric acid (a breakdown product of DNA), and numerous other molecules that are normally confined within healthy cells and whose release is an indicator of severe cell injury
- basis for blood tests that detect tissue-specific cellular injury
pg 628
11
Q
morphology of necrosis
A
- necrotic cells show increased eosinophilia (PINK) in H&E stains -> attributable in part to the loss of cytoplasmic RNA (binds BLUE stain) and in part to accumulation of denatured cytoplasmic proteins (which bind the red dye eosin)
- necrotic cells may have a glassy homogenous appearance, mainly as a result of the loss of glycogen particles
- when enzymes have digested the cell’s organelles, the cytoplasm becomes vacuolated and appears moth-eaten -> looks like holes exist
- dead cells may be replaced by large whorled phospholipid precipitates called myelin figures: either phagocytosed by other cells or further degraded into fatty acids; calcification of FA residues results in deposition of calcium-rich precipitates
pg 629
12
Q
morphology of necrosis - nuclear changes
A
nuclear changes appear in one of three patterns, all due to breakdown of DNA
* karyolysis: basophilia of the chromatin may fade (a change that presumably reflects loss of DNA because of enzymatic degradation by endonucleases)
* pyknosis: characterized by nuclear shrinkage and increased basophilia; here the chromatin condenses into a dense, shrunken basophilic mass (blueish color)
* karyorrhexis: pyknotic nucleus undergoes fragmentation
* with the passage of time (1-2 days), the nucleus in the necrotic cell totally disappears
pg 630
13
Q
morphology of coagulative necrosis
A
- form of necrosis in which the architecture of dead tissue is preserved for a span of at least some days
- affected tissue has a firm texture
- injury denatures not only structural proteins, but also enzymes and so blocks the proteolysis of the dead cells (intensely eosinophilic cells with indistinct or reddish nuclei may persist)
- necrotic cells are broken down by the action of lysosomal enzymes derived from infiltrating leukocytes, which also remove the debris of the dead cells by phagocytosis
- ischemia caused by obstruction in a vessel may lead to coagulative necrosis of the supplied tissue in all organs except the brain
- a localized area of coagulative necrosis is called an infarct
- common in the kidneys
pg 632
14
Q
morphology of liquefactive necrosis
A
- characterized by digestion of the dead cells, resulting in transformation of the tissue into a viscous liquid
- seen in focal bacterial or fungal infections because microbes stimulate the accumulation of leukocytes and the liberation of enzymes from these cells
- necrotic material is frequently creamy yellow because of the presence of pus
- hypoxic death of cells within the CNS often manifests this way
- ischemic lesions in BRAIN -> breakdown of lipids
pg 633
15
Q
morphology of gangrenous necrosis
A
- not a specific pattern of cell death, but the term is commonly used in clinical practice
- usually applied to a limb, generally the lower leg, that has lost its blood supply and has undergone necrosis (typically coagulative necrosis) involving multiple tissue planes
- when bacterial infection is superimposed, there is more liquefactive necrosis because of the actions of degradative enzymes in the bacteria and the attracted leukocytes (giving rise to so-called wet gangrene -> has an inflammatory component)
- occurs in the fingers and toes
pg 634
16
Q
morphology of caseous necrosis
A
- encountered most often in foci of tuberculous infection
- term caseous (cheese-like) is derived from the friable white appearance of the area of necrosis
- microscopically, necrotic area appears as a structureless collection of fragmented or lysed cells and amorphous granular debris enclosed within a distinctive inflammatory border; this appearance is characteristic of a granuloma (surrounds dead cells)
- commonly occurs in the lung
pg 635
17
Q
morphology of fat necrosis
A
- refers to focal area of fat destruction, typically resulting from release of activated pancreatic lipases into the substance of the pancreas and the peritoneal cavity
- occurs in acute pancreatitis -> pancreatic enzymes leak out of damaged acinar cells and liquefy the membranes of fat cells in the peritoneum, releasing triglyceride esters that are split by pancreatic lipases, FAs are generated that combine with Ca to produce grossly visible chalky-white areas (fat saponification)
- can also occur in breast tissue due to injury
pg 636
18
Q
morphology of fibrinois necrosis
A
- a special form of vascular damage usually seen in immune reactions involving blood vessels
- typically occurs when complexes of antigens and antibodies are deposited in the walls of arteries
- deposits of these immune complexes, together with plasma proteins that have leaked out, result in a bright pink and amorphous appearance in H&E stains called “fibrinoid”
- commonly occurs in arteries
pg 637
19
Q
apoptosis
A
- regulated mechanism of cell death usually with no external influence
- serves to eliminate unwanted and irreparably damaged cells, with the least possible host reaction (inflammation)
- characterized by enzymatic degradation of proteins and DNA
- followed by recognition and removal of dead cells by phagocytes
- all pathways result from the activation of enzymes called caspases -> inactive proenzymes and must undergo enzymatic cleavage to become active, active caspases are marker for cells undergoing apoptosis
- phases of apoptosis -> initiation phase: some caspases become catalytically active and unleash a cascade of other caspases; execution phase: terminal caspases trigger cellular fragmentation
- NO INFLAMMATION
pg 640
20
Q
physiologic causes of apoptosis
A
- removal of supernumerary cells during development
- involution of hormone-dependent tissues on hormone withdrawal (ex: menstruation)
- cell turnover in proliferating cell populations
- elimination of potentially harmful self-reactive lymphocytes to prevent immune reactions against one’s own
- death of host cells that have served their useful purpose
pg 641
21
Q
pathologic causes of apoptosis
A
- DNA damage (and ultimately cancer)
- accumulation of misfolded proteins (ex: Alzheimer’s)
- infections
- pathologic atrophy in the parenchymal organs after duct obstruction, such as occurs in the pancreas, parotid gland, and kidney
pg 641
22
Q
apoptosis initiation
A
three major pathways
- mitochondrial (intrinsic) pathway: controlled by the equilibrium of the different Bcl-2 (B cell lymphoma) family members which can be disrupted by various stimuli leading to cell death
- death receptor (extrinsic) pathway: members of the TNF (tumor necrosis factor) superfamily (TNFSF) can induce cell death by binding to their cell surface receptors and activating a deathly signaling cascade
- granzyme B/perforin pathway: facilitated by caspase-like protease granzyme B (not as important as other two)
pg 642
23
Q
morphologic/biochemical changes in apoptosis
A
- cell shrinkage
- chromatin condensation
- formation of cytoplasmic blebs and apoptotic bodies
- organelle loss
- phagocytosis of apoptotic cells or cell bodies, usually by macrophages (as opposed to necrosis which has inflammation by PMLs)
- during apoptotic death, the cell breaks into small membrane-surrounded fragments (apoptotic bodies)
- in H&E stained tissue, the apoptotic cell appears as a round or oval mass of intensely eosinophilic cytoplasm with fragments of dense nuclear chromatin
- intense redness of cytoplasm and blueness of nuclei
pg 643
24
Q
mitochondrial (intrinsic) pathway of apoptosis
A
- also known as mitochondrial suicide
- increased permeability of the mitochondrial outer membrane releases death-inducing (pro-apoptotic) molecules into the cytoplasm
- release of pro-apoptotic proteins such as cytochrome c (CRITICAL POINT) is determined by the integrity of the outer mitochondrial membrane, which is tighly controlled by the BCL2 family of proteins
pg 645
25
Q
BCL2 superfamily
A
- anti-apoptotic Bcl-2 proteins
- pro-apoptotic BH3-only (BH -> Bcl-2 homology) proteins
- death effectors: Bax (Bcl-2-associated X protein), Bak (Bcl-2 homologous antagonist/killer), Bok (Bcl-2-related ovarian killer)
pg 645
26
Q
mitochondrial (intrinsic) pathway of apoptosis pt 2
A
- cell viability is maintained by the induction of anti-apoptotic proteins such as Bcl-2 by survival signals; these proteins maintain the integrity of mitochondrial membranes and prevent leakage of mitochondrial proteins
- loss of survival signals, DNA damage, and other insults activate sensors that antagonize the anti-apoptotic proteins and actiavte the pro-apoptotic proteins Bax and Bak, which form channels in the mitochondrial membrane; the subsequent leakage of cytochrome C leads to caspase activation and apoptosis
- in viable cell: Bcl-2 keeps channels closed -> no leakage of cytochrome c
- in apoptotic cell: BH3 only -> leakage of cytochrome c -> activation of caspases -> apoptosis
pg 646
27
Q
cytochrome C
A
once released into the cytosol, binds to APAF-1 (apoptosis activating factor-1) and caspase-9, forming apoptosome
28
Q
mitochondrial (intrinsic) pathway of apoptosis pt 3
A
- apoptosome binds to caspase-9, the critical intiator caspase of the mitochondrial pathway, and promotes its autocatalytic cleavage, generating catalytically active forms of the enzyme
- active caspase-9 then triggers a cascade of caspase activation by cleaving and thereby activating other pro-caspases which mediate the execution phase of apoptosis
- other mitochondrial proteins like Smac/DIABLO (BLOCK INHIBITORS) enter the cytoplasm, where they bind to and neutralize cytoplasmic proteins that function as physiologic inhibitors of apoptosis (IAPs)
pg 647
29
Q
extrinsic (death receptor-initiated) pathway of apoptosis
A
initiated by engagement of plasma membrane death receptors which are members of the tumor necrosis factor (TNF) receptor family that contain a cytoplasmic domain involved in protein-protein interactions
- this death domain is essential for delivering apoptotic signals
- best-known death receptors: type I TNF receptor (TNFR1) and Fas (CD95)
pg 649
30
Q
extrinsic apoptosis as illustrated by Fas receptor
A
- ligand for Fas is called Fas ligand (FasL)
- FasL is expressed on T cells that recognize self antigens
- when FasL binds Fas, 3+ molecules of Fas are brought together and their cytoplasmic death domains form a binding site for an adaptor protein called FADD (Fas-associated death domain)
- FADD binds inactive caspase-8 brinding multiple caspase molecules and leading to autocatalytic cleavage and generation of active caspase-8
- active caspase-8 initiates the same executioner sequence as in intrinsic pathway
- pathway can be inhibited by a protein called FLIP which binds to pro-caspase-8 thereby blocking FADD binding
- some viruses and normal cells produce FLIP as a mechanism to protect themselves from Fas-mediated apoptosis
pg 650
31
Q
granzyme B (CTL) pathway of apoptosis
A
- CTLs (cytotoxic T lymphocytes) and NK cells mediate cell killing by inducing the formation of perforin pores in targets allowing the entry of the caspase-like protease granzyme
- granzyme B can either directly activate executioner caspase 3 or cleave Bid (a BH3 domain only pro-apoptotic member) triggering the intrinsic apoptotic pathway
pg 651
32
Q
the execution phase of apoptosis
A
- intrinsic and extrinsic pathways converge to activate a caspase cascade that mediates the final phase of apoptosis (intrinsic -> initiator caspase-9; extrinsic -> caspase-8 and 10)
- caspases trigger the rapid and sequential activation of the executioner caspases, such as caspase-3 and caspase-6, which then act on many cellular components
- numerous ligands induced on apoptotic cells serve as “eat me” signals and are recognized by receptors on phagocytes that bind and engulf these cells
- production of pro-inflammatory cytokines is reduced in macrophages that have ingested apoptotic cells
- together with rapid clearance, this limits inflammatory reactions, even in the face of extensive apoptosis
pg 652
33
Q
pyroptosis
A
- form of apoptosis accompanied by the release of the fever-inducing cytokine IL-1
- occurs in cells infected by microbes
- involves activation of caspase-1
- caspase-1 cleaves the precursor form of IL-1 to generate biologically active IL-1
- IL-1 is a mediator of many aspects of inflammation, including leukocyte recruitment and fever
- caspase-1 along with other closely related caspases also cause death of the infected cell by formation of membrane pores
- injurious agents recognized by inflammasomes (complex of pattern recognition receptors, procaspase 1 and other molecules) -> activate caspase 1(from procaspase 1) -> eventually leads to pyroptosis
pg 655
34
Q
necroptosis
A
- resembles necrosis morphologically, but like apoptosis is a generically controlled form of cell death
- triggered by ligation of TNFR1 and by proteins foudn in RNA and DNA viruses
- depends on the RIPK1 and RIPK3 complex; RIPK1-RIPK3 signaling leads to the phosphorylation of MLKL (mixed lineage kinase domain-like protein) which then forms pores in the plasma membrane
- physiologic necroptosis occurs during the formation of the mammalian bone growth plate
- in pathologic states, associated with cell death in steatohepatitis, acute pancreatitis, ischemia-reperfusion injury, and neurodegenerative diseases such as Parkinson disease
- necroptosis also acts as a backup mechanism in host defense against certain viruses that encode caspase inhibitors
- viral infections use this pathway
pg 658
35
Q
ferroptosis
A
- triggered when excessive intracellular levels of iron or reactive oxygen species overwhelm the glutathione-dependent antioxidant defenses to cause unchecked membrane lipid peroxidation (KEY) -> causes loss of plasma membrane permeability, which ultimately leads to cell death resembling necrosis
- regulated by specific signals and can be prevented by reducing iron levels
- ultrastructurally, loss of mitochondrial cristae and ruptured outer mitochondrial membrane
- linked to cell death in a variety of human pathologies, including cancer, neurodegenerative diseases, and stroke
pg 660
36
Q
autophagy
A
- involves sequestration of cellular organelles into cytoplasmic autophagic vacuoles (autophagosomes) that fuse with lysosomes and digest enclosed material -> protective mechanism for survival
- adaptive response
- enhanced during nutrient deprivation, allowing the cell to cannibalize itself to survive
- implicated in many physiologic states and pathologic processes
pg 662
