Cellular changes and adaptive responses

• Definition: decreased blood supply that cannot meet the oxygen demands of an organ or tissue.
• Pathogenesis
  • Decreased arterial perfusion → e.g., atherosclerosis, thromboembolism
  • Decreased venous drainage → e.g., Budd-Chiari syndrome, testicular torsion, ovarian torsion
  • Shock
    • Hypovolemic shock (e.g., hemorrhage) → decreased intravascular volume → decreased delivery of oxygen to tissue → ischemia
    • Cardiogenic shock (e.g., cardiac tamponade) → decreased left ventricular function → decreased forward flow of blood → decreased delivery of oxygen to tissue → ischemia
    • Distributive shock (e.g., septic, neurogenic, and anaphylactic shock) → systemic vasodilation → peripheral pooling of blood → decreased delivery of oxygen to tissue → ischemia
    • For more specific details see the article on shock.

Organs most susceptible to ischemia
Organ	Specific structure	Clinical significance
Brain	Watershed areas Anterior cerebral artery Middle cerebral artery Posterior cerebral artery Purkinje cells (cerebellum) Pyramidal cells (hippocampus and neocortex)	Ischemic stroke
Heart	Subendocardial tissue of the left ventricle	Stable angina CAD Unstable angina NSTEMI STEMI
Kidney	Proximal tubule (straight segment in the medulla) Thick ascending limb (in the medulla)	Acute kidney injury
Liver	Region around the central vein (zone III)	Centrilobular necrosis (e.g., Budd-Chiari syndrome, shock liver in trauma)
Bowel	Watershed areas Splenic flexure → confluence of blood supply between the SMA and IMA Rectum → confluence of blood supply between the superior rectal artery (branch of the IMA) and the middle and inferior rectal artery	Ischemic colitis (colonic ischemia) Acute mesenteric ischemia Chronic mesenteric ischemia

• Ischemic tolerance time, after which irreversible tissue damage begins to take place
  • Skin: 12 h
  • Musculature: 6–8 h
  • Neural tissue: 2–4 h

References:^[2]

Reperfusion injury is tissue damage that occurs when blood flow is restored to a previously ischemic environment (typically > 6 h for most tissues).

• Pathophysiology
  • Oxygen is reintroduced to the previously ischemic tissue (oxygen toxicity) → activated endothelial cells and leukocytes generate reactive oxygen species (ROS)
  • ROS
    • Cause damage to:
      • DNA (via fragmentation)
      • Cell membranes: via direct damage and lipid peroxidation → increased permeability
        • → ↑ intracellular Ca²⁺ → activates numerous enzymes causing damage to the cytoskeleton, nuclear chromatin, activates apoptosis
        • → leakage of cellular proteins → apoptosis and necrosis
      • Mitochondrial membranes: via lipid peroxidation and transition pores → increased permeability
        • → cytochrome c escapes from mitochondria and activate caspases → apoptosis
        • → increased permeability to small molecules → draw in water → swelling → rupture → apoptosis and necrosis
      • Cellular proteins
      • Microvessels: microvascular injury → increased permeability of capillaries and arterioles → increased diffusion and fluid filtration → tissue swelling
    • Recruit and activate platelets → increase coagulation
    • Recruit and activate WBCs → worsen the immune response started by ischemia
  • WBCs infiltrate the area (in response to ischemic cell damage) → release cytokines → inflammatory response
  • WBCs bind to the endothelium of small capillaries → capillary obstruction → worsening of ischemia
• Occurrence: spontaneously or secondary to intervention (e.g., PCI or thrombolytic therapy)
  • Acute coronary syndrome
  • Ischemic stroke (reperfusion injury results in intraparenchymal hemorrhage)
  • Peripheral vascular disease
• Pathology: : results in a red infarct
• Possible complications (depending on the location of ischemia/reperfusion injury)
  • Acidosis, hyperkalemia → cardiac arrhythmia
  • Rhabdomyolysis → myoglobinemia → crush syndrome
  • Ischemia/reperfusion edema → compartment syndrome
  • Massive edema → hypovolemic shock
  • DIC (disseminated intravascular coagulation), multiorgan dysfunction
• Management
  • Monitoring and symptomatic treatment
  • If affecting limb that is beyond recovery → amputation

• Definition: programmed cell death (physiological cell turnover)
• Etiology:
  • DNA damage
  • Hypoxia and other types of exogenous damage to the cell (radicals, irradiation, toxins)
  • Growth factor withdrawal
  • Specific signals such as TNF-alpha and ligands (TRAIL, FasL) activate the apoptotic program of the cells via binding to death receptors (DR 4/5, Fas, TNF-R).
  • Cytotoxic T cells, which recognize a pathogen on the target cell
  • Denervation
• Characteristics
  • ATP-dependent physiological process
    • Occurs e.g., during the involution of the thymus
  • Usually affects individual cells and not groups of cells (in contrast to necrosis)
  • No inflammatory response or cellular swelling (in contrast to necrosis)

Signaling cascade

Apoptosis can be initiated via two different pathways: the extrinsic pathway (through external stimuli) or the intrinsic pathway (through internal stimuli).

• General sequence of events: stimulus → activation of initiator caspases → activation of executioner caspases → apoptosis
• Caspases: enzymes from the group “Cysteine-ASpartic ProteASES” that cleave proteins and peptides and attack the cell membrane, nucleus, and cytoplasm
  • Association: Caspase 8 is not only a part of the extrinsic pathway but also stimulates the intrinsic pathway by altering the permeability of the inner mitochondrial membrane.
• Extrinsic pathway (death receptor pathway)
  1. Extracellular ligands (e.g., TNF-α, TRAIL, or FasL) bind to a death receptor on the cell surface.
  2. The receptor-ligand complex activates initiator caspases such as caspase 8.
  3. Initiator caspase activates executioner caspases such as caspase 3.
• Intrinsic pathway (mitochondrial pathway): intracellular stimuli activate the mitochondrial pathway
  1. p53 is activated through DNA damage (e.g., chemical toxins, radiation).
  2. p53 leads to an intracellular increase of proapoptotic proteins of the Bcl-2 family (e.g., Bax or Bad).
  3. These proteins increase the permeability of the mitochondrial outer membrane, e.g., through the formation of a membrane channel by the heterodimer Bax/Bad.
  4. Cytochrome c enters the cytosol from mitochondria through the membrane.
  5. Cytochrome c binds to APAF-1 (apoptotic protease activating factor-1) in the cytosol, forming a wheel-like structure, known as an apoptosome.
  6. The complex of cytochrome c and APAF-1 converts procaspase 9 into active caspase 9.
  7. Caspase 9 activates executioner caspases such as caspase 3.

Histopathological findings

1. Shrunken and irregularly shaped cells with membrane blebbing
2. The cell detaches from other cells or the extracellular matrix.
3. Nuclear changes (pyknosis, karyorrhexis, karyolysis)
4. Degradation of the cell: the cytoplasm and cell organelles form small bubbles and the endonucleases degrade the chromatin in the nucleus, resulting in nuclear fragmentation and apoptotic bodies
5. Phagocytosis by macrophages
6. DNA laddering (fragments in multiples of 180 base pairs) is seen on gel electrophoresis and can be used as a sensitive marker for apoptosis

Proteins of the Bcl-2 family can have opposite effects, e.g., Bad and Bax have a proapoptotic effect, whereas Bcl-2 and Bcl-xL have an antiapoptotic effect!

Abnormal regulation of apoptosis

Tumor suppressor genes that regulate the cell cycle and cell death can mutate and allow cells to remain alive even if they have abnormal genes that can cause cancer. Some examples include:

• Follicular lymphoma → translocation t(14;18) → Bcl-2 (regulator of apoptosis) on chromosome 18 is translocated to the immunoglobulin heavy chain locus on chromosome 14 → overexpression of Bcl-2 → abnormal lymphocytes never died and produce cancer.
• Burkitt lymphoma → translocation t (8;14) → c-myc (nuclear regulator protein) on chromosome 8 is translocated to the immunoglobulin heavy chain locus on chromosome 14 → overexpression of c-myc → associates with Bcl-2 → overexpression of c-myc and Bcl-2 → lymphoma
• Cervical cancer
  • Infection with high-risk strains of HPV (e.g., HPV-16 and HPV-18)
    • → HPV encodes for protein E6 → E6 binds to p53 → inactivation of p53 → inability of p53 to arrest the cell cycle and to activate DNA repair genes → proliferation of abnormal cells → low-grade dysplasia → high-grade dysplasia → carcinoma in situ → invasive cervical carcinoma
    • → HPV encodes for protein E7 → E7 binds to Rb → inability of Rb to bind to E2F and arrest the cell cycle → proliferation of abnormal cells → low-grade dysplasia → high-grade dysplasia → carcinoma in situ → invasive cervical carcinoma

• Short description: collective term for unprogrammed cell death and tissue destruction
• Characteristics
  • Never physiologically induced
  • Always associated with an inflammatory reaction
• Process: : cell damage → nuclear changes (pyknosis, karyorrhexis, karyolysis) → cell swelling, cell wall protrusions, cell organelle destruction → cell bursts → inflammation → degradation of the necrotic tissue by leukocytes → organization of granulation tissue

Types of necrosis

	Definition	Pathophysiology	Microscopic appearance	Example
Coagulative necrosis	A type of necrosis caused by tissue ischemia that occurs in most tissues except the brain	Decreased oxygen delivery → decreased ATP Anaerobic metabolism → increased lactic acid production → decreased pH → denaturation of proteins (including proteolytic enzymes) → cell death Impaired Na+/K+-ATPase → ↑ intracellular Na+ → ↑ intracellular H2O → cell swelling	Preserved, anuclear, eosinophilic cellular architecture. H&E staining: eosinophilia	Myocardial, splenic, hepatic, and renal infarction Gangrene Organ damage caused by acidic solutions
Liquefactive necrosis	A type of necrosis with liquefaction/softening of the affected tissue	Release of hydrolytic enzymes from neutrophilic lysosomes that digest the affected tissue	Macrophages and cellular debris (early) followed by cystic spaces or cavitations (late) In bacterial infections → cellular debris and neutrophils	Bacterial abscesses (purulent infection) Stroke Pancreatitis (due to enzymatic damage to the parenchyma) Organ damage caused by alkaline solutions
Fibrinoid necrosis	A type of necrosis characterized by small blood vessel injury with accumulation of fibrin and other plasma proteins	Vessel wall damage (e.g., type III hypersensitivity reaction in immune complex vasculitis) → fragmentation of collagneous and elastic fibers	Vessel wall damage with fragments of embedded cellular debris, serum, and fibrin Necrotic areas stain intense red	Polyarteritis nodosa Rheumatoid arthritis Peptic ulcer disease Preeclampsia Hypertensive emergency
Caseous necrosis	A type of necrosis characterized by granular debris that results from macrophages walling off a pathogen	Macrophages, epitheloid cells, and multinucleated giant cells (Langhans giant cell) surround a focus of infection	Cellular debris in a granular pattern, epitheloid cells and multinucleated giant cells that form granulomas	Tuberculosis Histoplasmosis Nocardiosis
Fat necrosis	A type of necrosis in which adipose cells die off prematurely	Release of lipase and triglycerides from the cytoplasm of damaged cells → lipase breaks down triglycerides → fatty acids bind calcium → saponification (esp. in acute pancreatitis)	Adipocytes with no nuclei Saponification: dark blue appearance on H&E staining	Traumatic breast injury Acute pancreatitis (due to saponification of peripancreatic fat)
Gangrenous necrosis	A subtype of coagulative necrosis most commonly seen in the limbs	Dry gangrene: caused by ischemia Wet gangrene: caused by superinfection of dry gangrene	Dry gangrene: shows coagulative necrosis Wet gangrene: shows coagulative and liquefactive necrosis	Peripheral arterial disease Acute limb ischemia Sepsis Clostridium perfringens: gas gangrene