Comprehensive Notes on Cellular Injury, Necrosis, and Apoptosis

  • Etiology, Pathogenesis, and Homeostasis

    • Etiology: underlying causes & modifying factors responsible for initiation & progression of a disease (Why a disease arises).

    • Pathogenesis: mechanisms of development & progression of disease (cellular & molecular changes, specific functional & structural abnormalities that characterize any particular disease). (How a disease develops).

    • Cells actively interact with their environment, constantly adjusting their structure & function to accommodate changing demands & extracellular stresses.

    • Homeostasis: the intracellular (IC) milieu of cells is normally tightly regulated & remains fairly constant. Defined as the tendency toward a relatively stable equilibrium between interdependent elements, especially as maintained by physiological processes.

  • Adaptation, Injury, and Cell Fate

    • When cells encounter physiologic stresses or injurious conditions, they adapt to a new steady state to preserve viability & function.

    • If adaptive capacity is exceeded or external stress is too harmful, cellular injury occurs.

    • Reversible injury: cells return to normal if the damaging stimulus is removed.

    • Irreversible injury & cell death: if the stress is severe, persistent, or rapid, injury progresses to cell death.

    • Importance of cell death:

    • a) Crucial event in the evolution of diseases.

    • b) Normal & essential in embryogenesis.

    • c) Development of organs.

    • d) Maintenance of tissue homeostasis.

  • STEPS IN THE EVOLUTION OF DISEASE

    • (Overview of how injury leads to disease; details depend on the initiating event and tissue context.)

  • CAUSES OF CELL INJURY

    • There are several broad categories of insults that can injure cells. Major categories include: 1) Hypoxia & ischemia

      • Hypoxia: oxygen deficiency to tissues.

      • Ischemia: reduced blood supply, leading to deficiency of essential nutrients & buildup of toxic metabolites in tissues.

      • Causes of hypoxia:

      • i) Ischemia from arterial obstruction

      • ii) Inadequate oxygenation of the blood (lung diseases)

      • iii) Reduction in the oxygen-carrying capacity of blood (anemia & carbon monoxide poisoning)

    2) Toxins: toxic agents encountered daily (air pollutants, insecticides, cigarette smoke, drugs, etc.).
    3) Infectious agents: pathogens (viruses, bacteria, fungi, protozoa).
    4) Immunologic reactions: immune defense against pathogens can cause cell/tissue injury (autoimmune & allergic reactions).
    5) Genetic abnormalities: congenital malformations, deficiency of functional proteins (e.g., enzymes in inborn errors of metabolism).
    6) Nutritional imbalances: protein–calorie insufficiency, obesity, type 2 diabetes mellitus, atherosclerosis.
    7) Physical agents: trauma, extremes of temperature, radiation, electric shock, and sudden changes in atmospheric pressure.
    8) Aging: cellular senescence reduces the ability to respond to stress, leading to cell and organismal death.

  • SEQUENCE OF EVENTS IN CELL INJURY & CELL DEATH

    • (Progression from initial insult to cellular outcomes; reversible steps may recover if stress is removed, while persistent injury progresses toward irreversible injury and death.)

  • REVERSIBLE CELL INJURY

    • Stage where deranged function & morphology can return to normal if damaging stimulus is removed.

    • Key cellular change: cells & intracellular organelles become swollen due to failure of energy-dependent ion pumps and loss of ionic/fluid homeostasis.

    • MORPHOLOGY: two main features

    • 1) Cellular swelling

    • 2) Fatty change (lipid accumulation)

    • Details of cellular swelling:

    • Increased permeability of the plasma membrane leads to cytoplasmic swelling.

    • Consequences include pallor (from capillary compression), increased turgor, and increased organ weight.

    • Hydropic change / vacuolar degeneration:

    • Microscopy shows small, clear vacuoles within the cytoplasm.

    • This pattern represents non-lethal injury.

    • Fatty change:

    • Lipid vacuoles containing triglycerides in the cytoplasm, commonly seen in organs involved with lipid metabolism (e.g., liver).

    • Other subcellular changes in reversible injury:

    • Eosinophilia (red staining due to protein denaturation)

    • Intracellular changes

    • Plasma membrane blebbing, blunting, or distortion of microvilli & loosening of intercellular attachments

    • Mitochondrial swelling & phospholipid-rich amorphous densities

    • Dilation of the endoplasmic reticulum (ER) with detachment of ribosomes & dissociation of polysomes

    • Nuclear chromatin clumping

  • MORPHOLOGY OF REVERSIBLE INJURY & IRREVERSIBLE INJURY

    • Reversible injury demonstrates viable cells with surface blebs, swelling, and eosinophilia; no net loss of nuclei.

    • Irreversible injury leads to necrosis/apoptosis; loss of nuclei & fragmentation of cells with leakage of contents.

  • IRREVERSIBLE INJURY & CELL DEATH

    • No definitive morphologic/biochemical correlates of irreversibility, but three main features are consistently observed:
      1) Inability to restore mitochondrial function (oxidative phosphorylation) even after resolution of the original injury.
      2) Loss of structure & function of plasma and intracellular membranes.
      3) Loss of DNA & chromatin structural integrity.

  • CELL DEATH: NECROSIS vs APOPTOSIS

    • Necrosis: severe, rapid, uncontrolled cell death with loss of membrane integrity and leakage of cellular contents; elicits inflammation.

    • Apoptosis: regulated cell death via a precise molecular pathway; can be therapeutically targeted and is essential in development; typically does not provoke inflammation.

    • When features of both necrosis and apoptosis are present, the process is often termed necroptosis.

  • NECROSIS

    • Definition: a form of cell death in which cellular membranes are disrupted, leading to leakage of enzymes and digestion of the cell; elicits local inflammation due to released intracellular contents.

    • Often follows reversible injury that cannot be rectified.

    • Biochemical mechanisms:

    • 1) Failure of energy generation (ATP) due to reduced oxygen supply or mitochondrial damage

    • 2) Damage to cellular membranes (plasma & lysosomal membranes) with leakage of contents

    • 3) Irreversible damage to cellular lipids, proteins & nucleic acids due to ROS (reactive oxygen species)

  • MORPHOLOGY OF NECROSIS

    • Cytoplasmic changes: increased eosinophilia, glassy appearance, myelin figures, vacuolated/moth-eaten cytoplasm; electron microscopy shows disrupted membranes, mitochondrial swelling with dense amorphous densities, lysosomal disruption, intracytoplasmic myelin figures.

    • Nuclear changes: patterns of DNA/chromatin breakdown include

    • 1) Pyknosis: nuclear shrinkage & increased basophilia (DNA condensation)

    • 2) Karyorrhexis: fragmentation of the pyknotic nucleus

    • 3) Karyolysis: fading of basophilia due to DNase activity; nucleus may disappear within 1–2 days.

  • MORPHOLOGIC PATTERNS OF TISSUE NECROSIS

    • 1) Coagulative necrosis:

    • preserves tissue architecture for days after cell death; tissue becomes firm; common in infarcts due to ischemia in solid organs except the brain.

    • 2) Liquefactive necrosis:

    • common in focal bacterial & fungal infections; leukocyte enzymes digest tissue; brain infarcts often show liquefactive necrosis; dead tissue becomes viscous liquid and is cleared by phagocytes; pus when inflammation is acute.

    • 3) Caseous necrosis:

    • seen in tuberculosis; cheese-like material grossly; amorphous granular pink appearance in H&E; architecture obliterated; granulomatous inflammation with macrophages.

    • 4) Fat necrosis:

    • focal fat destruction, often in pancreatitis due to unleashed pancreatic lipases; chalky white areas due to fat saponification.

    • 5) Gangrenous necrosis:

    • not a distinct pattern of cell death; typically refers to a limb with loss of blood supply; dry gangrene (coagulative) vs wet gangrene (superimposed infection causing liquefactive necrosis).

    • 6) Fibrinoid necrosis (fibrin-like):

    • immune reactions with immune complex deposition in vessel walls; seen in severe hypertension & polyarteritis nodosa.

  • APOPTOSIS

    • A pathway of cell death in which cells activate enzymes that degrade their own DNA, nuclear, and cytoplasmic proteins.

    • Plasma membrane remains intact but blebs into apoptotic bodies that are highly engulfable by phagocytes, with little leakage of cellular contents and minimal inflammatory response.

    • Occurs in cells with intrinsic abnormalities and as part of normal development and homeostasis (physiologic apoptosis).

    • Unlike necrosis, apoptosis is not inherently pathological and can occur in healthy tissues.

  • CELULAR ALTERATIONS IN APOPTOSIS

    • (Cellular remodeling includes chromatin condensation, DNA fragmentation, and formation of apoptotic bodies.)

  • CAUSES OF APOPTOSIS

    • Physiologic apoptosis:
      1) Normal development: cells die and are replaced (e.g., embryogenesis, organ formation).
      2) Highly proliferative, hormone-responsive tissues undergo cyclical proliferation & cell loss.
      3) Immune system: elimination of excess leukocytes after immune responses and removal of self-reactive lymphocytes.

    • Pathologic apoptosis:
      1) Elimination of cells damaged beyond repair.
      2) Accumulation of misfolded proteins triggers apoptosis.
      3) Infections, especially virus-infected cells, can trigger apoptosis.

  • PHYSIOLOGIC CONDITIONS & MECHANISMS OF APOPTOSIS

    • Embryogenesis: loss of growth factor (GF) signaling triggers apoptosis.

    • Turnover of proliferative tissues (e.g., intestinal epithelium, lymphocytes in bone marrow & thymus).

    • Involution of hormone-dependent tissues (e.g., endometrium) due to decreased hormone signaling.

    • Decline of leukocyte numbers at the end of immune/inflammatory responses due to loss of survival signals.

    • Elimination of potentially harmful self-reactive lymphocytes through strong recognition of self-antigens, activating mitochondria- and death-receptor–mediated pathways.

  • PATHOLOGIC CONDITIONS & MECHANISMS OF APOPTOSIS

    • DNA damage can activate pro-apoptotic proteins.

    • Accumulation of misfolded proteins activates pro-apoptotic pathways.

    • Infections with viruses can trigger apoptosis.

  • MECHANISMS OF APOPTOSIS (Biochemical Pathways)

    • Apoptosis is regulated by caspases; two main pathways converge on caspase activation:

    • 1) Mitochondrial (intrinsic) pathway

    • 2) Death receptor (extrinsic) pathway

    • End result: clear apoptotic bodies by phagocytosis, with minimal inflammation.

    • Mitochondrial (intrinsic) pathway (responsible for most physiologic & pathologic situations):

    • When mitochondrial membranes become permeable, cytochrome c is released into the cytoplasm, triggering caspase activation and apoptotic death.

    • Bcl-2 & Bcl-xL proteins regulate mitochondrial membrane permeability, keeping Bax & Bak in check under normal conditions.

    • In response to loss of growth factor signaling, DNA damage, or misfolded protein accumulation, BH3-only sensors activate Bax/Bak, promoting pore formation in the mitochondrial membrane.

    • Cytochrome c, with cofactors, activates Caspase-9, initiating a caspase cascade that leads to nuclear fragmentation and apoptotic body formation.

    • Death receptor (extrinsic) pathway:

    • Many cells express death receptors (e.g., TNF receptor family, Fas/CD95).

    • Fas ligand (FasL) is expressed mainly on activated T lymphocytes.

    • Crosslinking Fas with FasL recruits adaptor proteins via the death domain, activating caspase-8, which then activates downstream caspases.

    • This pathway is involved in eliminating self-reactive lymphocytes and in killing target cells by cytotoxic T lymphocytes (CTLs) expressing FasL.

    • After activation of caspase-9 or caspase-8, a caspase cascade degrades cellular proteins and DNA, producing the characteristic fragmented apoptotic cells.

  • CLEARANCE OF APOPTOTIC CELLS

    • Apoptotic cells and fragments send phagocytic signals to attract phagocytes.

    • Phosphatidylserine, normally on the inner leaflet of the plasma membrane, flips to the outer leaflet in apoptotic cells and is recognized by macrophages for phagocytosis.

    • Dying cells secrete soluble factors that recruit phagocytes.

    • Prompt clearance minimizes inflammation and tissue damage.

  • MORPHOLOGY OF APOPTOSIS

    • Microscopy: nuclei show progression of chromatin condensation and aggregation, ending with karyorrhexis.

    • Molecular level: DNA fragmentation.

    • Cells rapidly shrink, form cytoplasmic buds, and fragment into apoptotic bodies (membrane-bound fragments of cytosol & organelles).

  • OTHER PATHWAYS OF CELL DEATH

    • 1) Necroptosis: features of both necrosis and apoptosis; initiated by TNF receptor engagement; RIP kinases are activated, leading to dissolution of the cell similar to necrosis.

    • 2) Pyroptosis: associated with inflammasome activation; caspases are activated, inducing inflammation via cytokine production; name derives from apoptosis+fever (pyro).

  • AUTOPHAGY (SELF-EATING)

    • A survival mechanism during nutrient deprivation; lysosomal digestion of the cell’s own components.

    • Isolation of cytosolic organelles and portions of cytosol within ER-derived autophagic vacuoles; vacuoles fuse with lysosomes to form autophagolysosomes where lysosomal enzymes digest contents.

    • If starvation persists, autophagy may lead to apoptotic cell death.

    • Seen in ischemic injury, myopathies, inflammatory bowel disease, and cancer.

  • FEATURES OF NECROSIS & APOPTOSIS (COMPARISON)

    • Cell size:

    • Necrosis: enlarged (swelling)

    • Apoptosis: reduced (shrinkage)

    • Nucleus:

    • Necrosis: pyknosis → karyorrhexis → karyolysis

    • Apoptosis: fragmentation into nucleosome-sized pieces

    • Plasma membrane:

    • Necrosis: disrupted

    • Apoptosis: intact; altered lipid arrangement in apoptotic cells

    • Cellular contents:

    • Necrosis: enzymatic digestion; contents may leak out

    • Apoptosis: contents remain intact within apoptotic bodies or are enclosed

    • Adjacent inflammation:

    • Necrosis: common/inflammation-present

    • Apoptosis: usually no inflammation

    • Physiologic or pathologic role:

    • Necrosis: invariably pathologic (end result of irreversible injury)

    • Apoptosis: often physiologic, but can be pathologic in some contexts (e.g., extensive DNA/protein damage)