1. Cell Injury

Qbank - 1. Cell Injury


Cellular Adaptations


n -> n

  • Physiologic:
    Uterus after parturition
    Organ atrophy (fetal development)
  • Pathologic:
    Denervation e.g. : Polio
    Inadequate nutrition e.g.: PEM
    Chronic Ischemic Atrophy e.g.: Senile dementia (Alzheimer’s)

Atrophy is now considered Pre-malignant instead of Benign

[[Pasted image 20210627214134.png]]


n -> N

  • Increased cellular protein production
  • Mechanical sensors (integrins?) or Agonists or Growth factors -> Signal Transduction pathways -> Transcription factors -> Increased cellular protein synthesis
  • Physiologic
    Uterine hypertrophy during pregnancy
    Skeltal muscles
  • Pathologic:
    Cardiac hypertrophy (in hypertension/valvular disease) - left ventricle most affected


n -> 10n

  • Physiologic:
    Uterus and breast (during puberty and pregnancy
    Bone Marrow (hemolytic anemia)
  • Pathologic
    Prostatic Hyperplasia (due to increased secretion of Dihydrotestosterone (DHT)) -> Increased prostate cells -> Periurethral zone enlarges as prostate size increases-> Urethra compression BENIGN
  • Endometrial Hyperplasia

When uncontrolled hyperplasia occurs it may progress to cancer, thus endometrial hyperplasia is pre-malignant as uncontrolled irregular hyperplasia will lead to malignancies.


n -> h

  • Epithelial Metaplasia
    e.g: Pseudostratified ciliated columnar Epithelium to squamous in the lungs (due to smoking) (Squamous metaplasia) –––––––––––––––––––> Squamous cancer

  • Vitamin A deficiency can also result in metaplasia. It is necessary for differentiation of specialized epithelial surfaces such as the conjunctiva covering the eye.

    • In vitamin A deficiency, the thin squamous lining ofthe conjunctiva undergoes metaplasia into stratified keratinizing squamous epithelium. This change is called keratomalacia

    [[Pasted image 20210624103225.png]]

  • Acid reflux causes squamous to columnar epithelia (intestinal columnar metaplasia
    Barret’s esophagus - Gastroesophageal reflux disease (Goblet cells are present secretes mucin identified by Alcian blue stain [Goblet cells appear blue in color]

  • Connective Tissue Metaplasia
    Myositis Ossificans
    Hemorhhage/traumatic injury in muscle -> stemcells differentiate into other mesenchymal cells -> muscle replaced like bone-like tissue

H - Hypoxic Hypoxia A - Anemic Hypoxia S - Stagnant Hypoxia H - Histotoxic Hypoxia

  • Low partial pressure of O2 in blood - HYPOXEMIA SaO2 is < 90 % PaO2 < 60mmHg |||| SaO2 - percentage of RBC bound to O2 ||| PaO2 - pressure of O2 inside arterioles


Hypoxic hypoxia

Hypoxic hypoxia - less O2 enters the blood stream due to

  • less partial pressure of O2’
  • Pulmonary pathology impairing diffusion • High altitudes
    • ILD Interstitial Lung Disease) • Right -> Left Shunt

Anemic hypoxia

Anemic Hypoxia - functional Hb is reduced in number E.g:

  • Altered Hb

  • CO poisonig -> CO-Hb headache, cherry red color present

  • Met-Hb - - [[Pasted image 20210618184600.png]] - Associated with Cyt B5 reductase deficiency - Fe2 binds O2 -> Met-Hb has Fe3 - Chocolate colored blood with cyanosis - Methemoglobemia is treated with Methylene Blue IV and vitamin C

  • PaO2 and SaO2 is normal (as SaO2 is qualitative rather than quantitative) in quantiative anemias, while SaO2 is decreased in qualitative anemias

Stagnant Hypoxia

Ischemia - arterial obstruction - most common reason for development of hypoxia Congestion - venous obstruction

Histotoxic hypoxia

 - Cell or a tissue is exposed to toxins leading to inability to use O2
- Cytochrome inhibited by cyanide (associated with IV Nitroprusside long duration - usually given by microdrip set
- Cyanide posioning - brick red Rx -
- Vit B12 [methylcobalamine] converts into cyanocobaiamine which is water soluble, excerted by kidneys - Amyl nitrate - Sodium Thiosufate

Reversible injury

Hallmark: Cellular swelling


  • Onset - CNS 2 minutes > Cardiac Fiber 20-30 minutes > Skeletal Muscle fiber > Fibroblast In the CNS, Purkinje cells of the cerebellum is most susceptible
    Cerebral cortical neurons 3,5,6 layers
    In the cardiac tissue - Sub endocardial area in the most sensitive area
    GIT - Splenic flexure and rectosigmoid colon There is a time lag between stress and onset of morphologic changes

e.g. Angina is reversible injury

Mitochondria is the first organelle that’s affected from hypoxia, leading to decreased ATP. - Cell Membrane - Endoplasmic Reticulum - Misc. Changes - Amorphous/mitochondrial density occurs due to calcium accumulation due to decreased ATP

// Cell Membrane:

  • Na+K+ Pump reduced
    • Na+ increased
    • Water content inside increased as water follows
  • Leads to Cellular Swelling (hydropic change) - 1st microscopic feature
    • Bleb formation
    • Microvilli formation
  • Myelin figures | denatured phospholipid from cell membrane or membrane of organelles
    • Seen in ICM and ECM
      • Resembles of Myelin sheath surrounding neuron

[[Pasted image 20210618190851.png]]

// Endoplasmic Reticulum Decreased ATP leads to

  • Swelling of the RER
    • Ribosome breaks away (polyribosomes become monoribosomes)
    • Decreased protein synthesis
  • Accumulation of misfolded proteins due to loss/impaired secondary and tertiary structures

Accumulation of misfolded proteins leads the ER to produce an Unfolderd Protein Response activating

  • Increased chaperone production
  • Decreased protein translation
  • Proteosomal degradation of misfolded proteins This leads to reduction in misfolded proteins. If ER is unable to cope with this -> ER stress which leads to activation of caspase -> Apoptosis

// Miscellaneous changes

  • Increased glycolysis (anaerobic pathway) leading to decreased glycogen
    • Increased accumulation of lactic acid and pyruvic acid leading of Acidosis Nuclear changes:
  • Chromatin clumping due to acidosis

[[Pasted image 20210618191049.png]]

  • Liver becomes fatty

Irreversible Cell Injury

Hallmark: Membrane damage of:

  • Plasma membrane
  • Inner mitochondrial membrane
  • Lysosome membrane
  • Loss of nucleus

e.g: Myocardial Infarction Two important patterns

  • Apoptosis
  • Necrosis


Due to persistent reversible injury -> Mitochondrial Dysfunction

  • leakage of cytochrome C into cytoplasm
  • This leads to Apoptosis
    • Pyknosis (ink-dot appearance) is seen

Apoptosis - Falling of leaves

  • **Caspase **dependent Programmed Cell death
  • Controlled cell death - by genes
  • May affect single or multiple cells

Types of genes

  • Anti-apoptotic - Contain 4 domains (BH1-4)

    • BCL-2 [Associated with follicular lymphoma] ^565948
    • BCL-XL
    • MCL-1 gene [Associated with development of resistance to chemotherapy]
    • Sex steroids [Drug that inhibits apoptosis]

    L - genes - Anti-apoptotic except Xs

  • Pro apoptoptic - Contains 3 domains (BH1-3)

    • BAK
    • BAX
    • BCL-Xs
    • P53
    • Glucocorticoids [Drug that promotes apoptosis]
  • Regulated (by Sensors) apoptosis initiators - Contains BH-3 domain (1 domain only)

    • When upregulated and activate they can initiate apoptosis, by directly activating the two critical pro-apoptotic family members, BAX and BAK
    • BH3-only proteins may also bind to and block the function of BCL2 and BCL-XL
    • BAD
    • BiM
    • BiD
    • PUMA
    • NOXA

Types of Apoptosis

  • Physiological
    • Embryogenesis
      • removal of tail cells in developing fetus
      • separation of finger due to death of cells present between fingers [Syndactemy - presence of webbed fingers due to non-apoptosis]
      • Menstrual cycle - endometrial shedding
      • Post-Infection -> Leukocytes are killed off
      • Self-reactive B and T cells (lymphocytes) are resigned through Apoptosis
  • Pathological
    • DNA damage such as exposure to DNA damage
    • Duct obstruction ( in kidneys, salivary glands, pancreas etc)
    • Viral infection (such as of hepatocytes) | CD8+ T-cell mediated killing
      • Councilmann bodies (Apoptitic bodies) are an indication of apoptosis #hepatitis
    • Transplant rejection / Graft vs Host disease
    • Retinitis pigemontosa
      • Excessive apoptosis of Rod pigmentation [causes Night Blindness]
      • (rods -> dim vision | cones -> daylight vision/color vision)


  • Normal mitochondria has Cyt-C
  • BCL-2 inhibits movement of Cyt-C by keeping outer mitochondrial membrane impermeable
  • BCL-2 recieves signals from growth factors
  • Whenever there’s a deficiency of growth factor (cellular injury, DNA damage, decreased hormonal stimulation, expression of BCL-2 decreases
  • At the same time, BAK/BAX is expressed -> Increases permeability of Mitochondria
  • This causes Cyt-C leaks into cytoplasm
Affects single/small groupAffects large group
Physiological or pathologicalAlways pathological
Intrinsic Pathway
  • Leakage of Cytochrome C causes activation of APAF-1 (Apoptosis Protease Activating Factor-1) [Cyt-C + APAF-1 = Apoptosome]
  • This leads to activation of Caspases [Cysteine containing protease enzymes - Attacking Aspartic Acid residues | C-Asp-Ase]
    • Initiator caspase: Caspase-9
    • Executioner caspases- Caspase-3/6/7

Intiator - Nine

  • Executioner caspases activate
    • Proteases - breaks down cytoskeleton
    • Endonucleases - breaks down DNA
Extrinsic Pathway

Receptor-ligand pathway (e.g. seen in T-Cell Selection) Receptors: - Type 1 TNF-R (TNFR1) - FAS-R a.k.a CD95

  • On cell injury, ligands for the above receptors are released. FasL is expressed on T cells that recognize self antigens and on some CTLs that kill virus-infected and tumor cells.
  • Whenever TNF-alpha or FAS-L/Cd95-L attaches to the receptor, the receptor trimerizes
  • This activates is FADD [Fas Associated Death Domain]
  • This converts Pro-Caspase 8 -> Caspase 8 (in humans, it’s caspase 10 || Homo sapien - 10 letters)

Eight - Extrinisic initiator

  • This leads to activation of Executioner Pathways
  • Extrinsic apoptosis pathway can be inhibited by a protein called FLIP, which binds to procaspase-8
  • Executioner caspases activate
    • Proteases
    • Endonucleases

[[Pasted image 20210620134806.png]]

Sailent features

  • Cell size - Shrinkage vs Necrosis had swelling | Earliest feature
  • No inflammation occurs | As the cell membrane remains intact


  • Due to activation of endonucleases, Chromatin fragmentation occurs -> Chromatin condensation //HALLMARK FEATURE
  • On gel electrophoresis - Step-ladder appearance
  • Tunel technique [TdT terminaldeoxy nucleotide transferase - UTP - Nick End Labelling] - This is a diagnostic
    • DUTP dye is a dye that attaches to 3’-OH of DNA
    • Since there’s multiple fragments, higher intensifed brown color
  • Staining: Annexin V is used
  • Apoptotic cells have flipped molecules (such as phosphatidylserine) that are normally present within the cell membrane
  • These flipped molecules attract phagocytic cells immediately
  • Thus there is no enzyme leakage as there’s no time for it. -> Cell membrane are intact
  • Annexin V attaches to the flipped molecules ** Molecules that flip are** > - Phospitidyl serine - Attaches to ANNEXIN V > - C1q > - Thrombospondin

AIF - Apoptosis Initiating Factor is secreted in traumatic neuronal injury instead of APAF - Apoptosis Activating Factor

 These do not inititate Caspases
 They directly activate Endonucleases and Proteases
  • Di-amibino-phenyl-indole (DAPI) - useful for detection of apoptosis | Another is DUTP - tunnel technique

  • There may be interconnections between EP and IP, for instance, in hepatocytes and pancreatic β cells, caspase-8 produced by Fas signaling cleaves and activates the BH3-only protein BID, which then feeds into the mitochondrial pathway

Cytotoxic CD8+ T-Cell Pathway
  • CD8+ recognizes antigens on MHC class 1 and then kill them
  • C8+ T cell secretes perforins in membrane of target cell and create pores
  • Granzyme enters pores and activates caspases


Excessive apoptosis

  • Neurodegenerative disorders
  • Viral infections Reduced Apoptosis
  • Autoimmune disorders

p53 -> activates when there’s DNA damage

  • causes cell death in mutated DNA cells
  • located on chromosme 17 (Executioner - 3 + 6 + 7 + 1)
  • 50% of human cancer are due to p53 mutation

[[Pasted image 20210620140207.png]]


  • It’s the spectrum of changes in the morphological appearance of tissue seen after cell death
  • Local acute inflammation is induced to clear the damaged cells
  • Due to reduced ATP supply, Calcium increases as pumps (in cell and within sarcoplasmic reticulum) fails as intracellular stores open up and also an influx through concentration gradient.
  • Increased calcium leads to 1. Mitochondrial swelling with amorphous densities in the matrix. This is due to transient pore formation -> Cell death 2. Enzyme activation of intracellular enzymes - Lysosomal enzymes - Phospholipase activation -> cell membrane damage due to breakage of phospholipids - ATPase -> reduced ATP - Proteases -> Cell Damage - Endonucleases - Nucleic acid -> leads to Pyknosis (ink dot like) Karyorrhexis (destructive fragmentation) and karyorrlysis

[[Pasted image 20210618181638.png]]

[[Pasted image 20210618181244.png]]

[[Pasted image 20210618181250.png]]

Smeared pattern during cell necrosis compared to step-laddern (apoptosis).

100 Normal pattern is a single band

  • Abundant myelin figures, eosinophilia, pyknosis, karyorrhexis, karyorrlysis - Main features

Types of Necrosis

Coagulative necrosis
  • Commonest pattern of necrosis seen microscopically
  • Commonest reason is Ischemia
  • Due to severe lactic acidosis resulting from lack of O2, denaturation/inactivation of hydrolytic enzymes occurs This prevents hydrolysis and coagulation of cellular proteins occurs Thus the structural outline of the cell is preserved


  • Tombstone appearance
  • CNS ischemia is not associated with coagulative necrosis
  • Commonly associated with neutrophilic infiltration - for clearing the dead cells

Zenker’s degeneration is a type of necrosis

  • Associated with Typhoid, it is the coagulative necrosis in the Skeletal Muscle

Infarct - localized area undergoing (coagulative) necrosis

  • Usually wedge/triangular area and pale in color usually
  • Apex points towards area of ischemia
  • Two types
    • White - seen in (solid) organs with end arterial blood supply
    • Red - seen in organs with (soft) loose connective tissue Seen in organs that have dual supply such as lungs, liver Usually because of venous drain blockage (testicular torsion)
Liquefactive necrosis
  • A.k.a Colliquative necrosis
  • Seen in focal bacterial infections and fungal infections
  • Here activation of hydrolytic enzymes occurs - creamy yellow pus
    • CNS ischemia -> damage of microglial cells leading to Hydrolytic enzyme present within getting activated
    • Abscess -> Walled off area of dead tissue -> Hydrolytic enzymes present in neutrophils liquefact it
    • Pancreatitis - enzymes in pancreas are blocked from flowing into duct, thus pancreatic liquefaction occurs
    • Also seen in infections (pus formation such as in Staph. aureus)
      • Microbes stimulate the accumulation of leukocytes and the liberation of enzymes from these cells.

Caseous necrosis
  • Cottage-Cheese-like necrosis
  • Coagulative + Liquefactive necrosis
  • Coagulative necrosis is predominant contributor. However cell structural outline is not preserved
    • Organisms associated with high lipid content
      • Coci of tuberculous infection -> Ghon’s complex is a lesion seen in the lung that is caused by tuberculosis
      • Fungi
        • Histoplasmosis
        • Coccidiodomycosis/valley fever
      • Syphillis
  • Granulomatous reaction - Monocyte and lymphocyte infiltration is seen rather than neutrophils
  • Granuloma - a structureless collection of fragmented or lysed cells and amorphous (no definite shape) granular debris enclosed within a distinctive inflammatory border.
  • Langhans Giant Cells are associated Tuberculosis and formed by fusion of macrophages (epitheloid cells) and surrounding cells
    • They are not the same as Langerhans cells

Fat necrosis
  • Seen in organs containing high fat or lipase activation
    • Injury to breast tissue
    • Injury to omentum tissue
    • Pancreatitis-mediated damage of pancreatic fat
      • Acute pancreatitis is usually caused from gall stones or alcohol (both of them together cause 80% of pancreatitis)
      • Gall stones -> Lipase activation due to cholecystitis -> Lipids turn into fatty acids
  • saponification: Fatty acids combine, fat converts to soap through lipase with calcium deposits | DYSTROPHIC

Hallmark: Chalk-like yellow white deposit When there’s lower serum calcium, there’s higher inflammation

[[Pasted image 20210619180910.png]]

If cellular debris is not promptly destroyed, they provide a nidus for deposiition of calcium salts and other minerals. This phenomenon is dystrophic #calcification

Fibrinoid necrosis
  • Antigen-antibody (immune complexes) are deposited in the endothelium along with fibrin

  • Entry/leakage of plasma protein into the endothelium, forming deposits

  • Together these deposits result in bright pink and amorphous appearance (fibrinoid/fibrin-like)

    • Malignant hypertension
    • Pneuatic fever: Aschoff body
    • Preeclempsia - fibrinoid necrosis of placenta as a consequence of Preeclempsia
    • Vasculitis syndrome
    • Type 3 hypersensitivity [Ppolyarteritis nodosa, Henoch-Schonlein purpura (HSP)]

    [[Pasted image 20210619181936.png]]

    • Pinkish appearance inside of the vessel

#GANGRENE Surgical term

  • Dry Gangrene (mummified)
    • Blood supply reduced (ischemia)
    • Usually coagulative necrosis
  • Wet gangrene
    • Ischemia + Secondary infection
    • Usually liquifactive necrosis superimposed over coagulative
      • Gas gangrene
        • Subtype of wet gangrene
        • Associated with Clostridium welchi


  • Infections -> Intracellular/cytoplasmic receptors (Nod Like receptors) attach to the infective agent
  • Forms Inflammasome which activate Caspase 1 or Caspase 11
    • Caspase 1 -> cleaves precursor of Interleukin-1 (IL-1) -> Pyrexia/Fever
    • Caspase 11 -> Causes Cell death
  • Autoinflammatory syndrome - There’s overactivity of NLRs -> Periodic fever syndrome
    • Treated by IL-1 Antagonists

Morphological features

  • Cellular swelling (unlike Apoptosis)
  • Damage of plasma membrane -> thus Inflammation is present


  • A.k.a Programmed necrosis
  • Caspase independent Cell death
  • TNF attaches to TNF-R1
  • This leads to activation of Receptor associated kinase (RIPK1 and RIPK3)
  • Leads to
    • Formation of Reactive oxygen species
    • Decreased ATP
    • No activation of caspase
    • Cell membrane damage occurs
      • RIPK3 phosphorylates MLKL (a cytoplasmic protein) -> MLKL translocates to Plasma membrane -> Membrane disruption


  • During formation of Mammalian bone growth plate


  • Pancreatitis

  • Steatohepatitis

  • Ischemia-Repurfusion injury

  • Neurodegenrative disorders Parkinsonism

  • CMV (Cytomegalovirus) produces FLIP which inhibits Caspase-8

    • Thus necroptosis is done


  • Triggered when excessive intracellular levels of iron or reactive oxygen species overwhelm the glutathione-dependent antioxidant defenses to cause unchecked membrane lipid peroxidation
  • The overall effect is the loss of plasma membrane permeability

Free Radical Injury

  • Free radicals molecule having single unpaired electron in an outer orbit
  • Highly reactive molecule
  • Can be generated physiologically/pathologically


  • Physiologically: During Oxidative Phosphorylation - during partial reduction of O2 leads to formation of superoxide Pathologically
  • Radiation exposure
  • Reperfusion injury
    • When reperfusion occurs, inflammatory cells also reaches the dying tissue along with oxygen which leads to generation of free radicals
  • Heavy metal injury such as Copper and Iron (Cu and Fe)
    • As they exist in dual state oxidative state
    • Fe excessive - hemochromatosis - tissue damage
    • Cu - Wilson’s disease - tissue damage
  • Infections
    • Oxidative Stress from inability to detoxify - overwhelming attack on detoxifying enzymes
    • Inflammation: Oxidative Stress - Oxygen-dependent HOCl generation - usually generated by NADPH oxidase/Xanthine oxidase
  • Chemicals and drugs
    • CCL4 -> CCL3**·** [Carbon tetrachloride is used in dry cleaning industry - damages the liver in the P450 sites -> Apolipoprotein syntheis is impaired thus there is fatty liver]
    • Paracetamol -> Liver necrosis due to free radical generation
    • Nitric oxide

O2 -> O2·- (Superoxide) -> H2O2 -> OH· (hydroxy radical)

  • Causes damage to DNA/Proteins/Lipids
  • Lipids undergo peroxidation results in generating of more free radicals
  • Thus free radical injury is an auto-catalytic reaction
  • Fenton’s reaction:

[[Pasted image 20210622133355.png]] Generates hydroxy radical - Fe3+ is more in intracellular thus the produced Fe3+ is then reduced by a superoxide (O2·)

  • Most dangerous: hydroxyl radical


  • Anti-oxidants such as:
    • Vitamins:

      • Vitamin A, C, E
    • Plasma proteins:

      • Transferrin/lactoferrin (with Fe)
      • Ceruloplasmin (with Cu)
    • Enzymes:

      [[Pasted image 20210622133653.png]]

      • Super Oxide Dismutase
        • When present in the mitochondri Mg is a co-factor
        • When present in the cytoplasm, Cu and Zn is a co-factor
          • Mutation in SOD1 leads to ALS (Amyotrophic Lateral Sclerosis)
      • Catalase
      • Glutathione Peroxidase catalyzes this reaction:

      [[Pasted image 20210622133902.png]] GS-SG:GSH ratio is thus higher if there’s oxidative stress

Pathologic effects

  • Lipid peroxidation in membranes -> Interactions with lipids lead to formation of peroxides which are again reactive. Thus it is an autocatalytic chain propagation reaction.
  • Oxidative modification of proteins -> Breakdown/misfolding
  • DNA damage - Lesion in DNA -> Mutations




  • Black pigment -> endogenous
  • Provides hair and skin color
  • Derived from tyrosine
  • Identified by Masson-Fontana stain [Not specific]

[[Pasted image 20210623161412.png]]

  • More specific: Dopa-oxidase method


  • Most common: Coal dust
    • Asymptomatic mostly - causes Anthracosis
    • Progressive massive pigmentation
  • Tattooing
#hepatitis #calcification #GANGRENE