Cell Injury, Cell Death, and Adaptations

Introduction to Pathology

  • Definition: Pathology is the study of the structural, biochemical, and functional changes in cells, tissues, and organs that underlie disease.

  • Methodology: It utilizes morphologic, microbiologic, immunologic, and molecular techniques to explain the "whys and wherefores" of signs and symptoms while providing a rational basis for clinical care.

  • Role in Medicine: It serves as the bridge between basic sciences and clinical medicine, acting as the scientific foundation for all of medicine.

  • Divisions of Pathology:

    • General Pathology: Concerned with the common reactions of cells and tissues to injurious stimuli. These reactions are often not tissue-specific (e.g., acute inflammation in response to bacteria produces similar reactions in most tissues).

    • Systemic Pathology: Examines alterations and mechanisms in diseases of particular organ systems.

  • The Four Aspects of Disease Process:

    • Etiology (Causation): The initiating cause. Groups include:

      • Genetic: Inherited or acquired mutations, gene variants, or polymorphisms.

      • Environmental: Infectious, nutritional, chemical, or physical factors.

      • Multifactorial: Most common afflictions (e.g., atherosclerosis and cancer) arise from environmental insults on a genetically susceptible individual.

    • Pathogenesis: The sequence of molecular, biochemical, and cellular events leading to disease development. It refers to the mechanism of disease.

    • Morphologic Changes: Structural alterations in cells or tissues characteristic of a disease and diagnostic of an etiologic process. Now supplemented by analysis of protein expression and molecular alterations (e.g., next-generation sequencing in tumor analysis).

    • Clinical Manifestations: The end results of genetic, biochemical, and structural changes leading to functional abnormalities (symptoms and signs), clinical course, and outcome.

  • Rudolf Virchow: Known as the father of modern pathology, he proposed the cellular basis of disease in the 19th19^{th} century, emphasizing that individuals are sick because their cells are sick.

Overview of Cellular Responses to Stress and Noxious Stimuli

  • Homeostasis: A healthy steady state where the normal cell handles physiologic demands within a narrow range of function and structure.

  • Adaptations: Reversible functional and structural responses to changes in physiologic states (e.g., pregnancy) and some pathologic stimuli. New but altered steady states are achieved to allow survival.

    • Hypertrophy: Increase in cell size and functional activity.

    • Hyperplasia: Increase in cell number.

    • Atrophy: Decrease in cell size and metabolic activity.

    • Metaplasia: Change in the phenotype of cells.

  • Cell Injury: Occurs if limits of adaptive response are exceeded or if cells are exposed to damaging insults, nutrient deprivation, or essential mutations.

    • Reversible Injury: Correctable if the stimulus is removed.

    • Irreversible Injury/Cell Death: Result of persistent or severe injury. Principal pathways are necrosis and apoptosis.

  • Sequential vs. Non-sequential progression: Although they often progress in stages, a sudden complete occlusion (e.g., coronary artery) can lead to irreversible injury without preceding adaptation.

Causes of Cell Injury

  • Oxygen Deprivation (Hypoxia): Deficiency of oxygen reducing aerobic oxidative respiration.

    • Causes: Ischemia (reduced blood flow), cardiorespiratory failure (inadequate oxygenation), anemia, carbon monoxide poisoning, or severe blood loss (decreased carrying capacity).

  • Physical Agents: Mechanical trauma, temperature extremes (burns and deep cold), atmospheric pressure changes, radiation, and electric shock.

  • Chemical Agents and Drugs: Includes glucose or salt (in hypertonic concentrations), oxygen (toxic at high concentrations), poisons (arsenic, cyanide, mercury), environmental pollutants, pesticides, industrial hazards (asbestos), alcohol, cocaine, and therapeutic drugs.

  • Infectious Agents: Range from submicroscopic viruses to large tapeworms, including rickettsiae, bacteria, fungi, and parasites.

  • Immunologic Reactions: Defense against pathogens, but also includes injurious reactions to self-antigens (autoimmune diseases) or external agents (viruses/environment).

  • Genetic Abnormalities: Can range from an extra chromosome (Down syndrome) to single base pair substitutions (sickle cell anemia). Defects cause injury through deficient protein function (metabolic errors) or accumulation of damaged DNA/misfolded proteins.

  • Nutritional Imbalances: Protein-calorie deficiencies, vitamin deficiencies, and nutritional excess (obesity, associated with diabetes and cancer). High-lipid diets predispose to atherosclerosis.

Pathways of Cellular Response to Stress and Injury

  • Integrated Stress Response (ISR):

    • An evolutionarily conserved network of signaling pathways modulating gene expression to adapt to injury.

    • Activated by four stress-sensing kinases: protein kinase R (PKR), PKR-like ER kinase (PERK), general control nonderepressible 22 kinase (GCN2), and heme-regulated eIF2α\alpha kinase (HRI).

    • Mechanism: These kinases phosphorylate a serine residue on eukaryotic translation initiation factor eIF2α\alpha.

    • Result: Global repression of protein translation (to conserve energy) but promotion of select mRNAs like ATF4 (controls antioxidant response and protein folding).

  • Unfolded Protein Response (UPR) and ER Stress:

    • Triggered by accumulation of unfolded/misfolded proteins in the ER.

    • Adaptive UPR: Increases chaperone production, enhances proteasomal degradation, and slows translation.

    • Terminal UPR (ER Stress): If the response is inadequate, the cell activates caspases to trigger apoptosis.

    • Causes of Misfolding: Mutations, aging, viral infections, increased demand for secretory proteins (insulin resistance), changed pH/redox state, and oxygen/glucose deprivation (low ATP).

  • Autophagy (Self-Eating):

    • Delivery of cytoplasmic materials to the lysosome for degradation.

    • Steps:

      1. Nucleation: Formation of an isolation membrane (phagophore), often from the ER.

      2. Autophagosome Formation: Vesicle sequesters organelles/cytosol.

      3. Fusion: Autophagosome fuses with lysosomes to form an autophagolysosome.

      4. Degradation: Lysosomal enzymes digest contents.

    • Regulation: Mediated by over 3030 "autophagy-related genes" (Atgs). Elongation requires linkage of phosphatidylethanolamine (PE) to microtubule-associated protein light chain 33 (LC3). Lipidated LC3 is a marker for autophagy.

    • Role in Disease: Linked to cancer, neurodegeneration (Alzheimer, Huntington), infectious diseases (mycobacteria, HSV-11), and inflammatory bowel disease (linked to SNPs in ATG16L1).

Reversible Cell Injury

  • Characteristics: Early functional/structural changes correctable if stimulus is removed.

  • Key Features:

    • Cellular Swelling (Hydropic Change/Vacuolar Degeneration): Failure of energy-dependent Na$^{+}$-K$^{+}$ pumps due to ATP depletion. Results in influx of water. Small clear vacuoles appear (distended segments of ER).

    • Fatty Change: Accumulation of triglyceride-filled lipid vacuoles in organs involved in lipid metabolism (e.g., liver).

  • Eosinophilia: Cytoplasm appears redder in H&E stains due to loss of RNA (which binds blue hematoxylin).

  • Ultrastructural Changes:

    1. Plasma membrane blebbing, blunting, or loss of microvilli.

    2. Mitochondrial swelling and appearance of small amorphous densities.

    3. Dilation of ER with ribosome detachment.

    4. Myelin figures (phospholipid precipitates from damaged membranes).

    5. Nuclear disaggregation of granular/fibrillar elements.

Cell Death: Necrosis

  • Nature: Usually "accidental" and pathologic; consequence of severe injury (ischemia, toxins, infections, trauma).

  • Mechanism: Denaturation of proteins, leakage of contents through damaged membranes, and enzymatic digestion.

  • Inflammation: Necrotic cells release "damage-associated molecular patterns" (DAMPs) such as ATP from mitochondria and uric acid from DNA. These trigger macrophage receptors and cytokine production, leading to local inflammation.

  • Biomarkers: Leakage of tissue-specific proteins into circulation (e.g., cardiac troponins detect myocardial infarction as early as 22 hours post-necrosis; alkaline phosphatase for bile ducts; transaminases for hepatocytes).

  • Morphology:

    • Cytoplasm: Increased eosinophilia, glassy appearance (loss of glycogen), vacuolated/moth-eaten (digested organelles).

    • Nuclear Changes:

      • Karyolysis: Fading of basophilia due to DNA degradation by endonucleases.

      • Pyknosis: Nuclear shrinkage and increased basophilia; chromatin condenses into a shrunken mass.

      • Karyorrhexis: Fragmentation of the pyknotic nucleus.

  • Tissue Patterns:

    • Coagulative Necrosis: Architecture is preserved for days; firm texture. Injury denatures enzymes, blocking proteolysis. Characteristic of infarcts in solid organs (except brain).

    • Liquefactive Necrosis: Digestion of dead cells results in viscous liquid. Seen in focal bacterial/fungal infections (pus) and hypoxic death in the central nervous system.

    • Gangrenous Necrosis: Clinical term for a limb that lost blood supply (usually coagulative). "Wet gangrene" occurs if bacterial infection (liquefaction) is superimposed.

    • Caseous Necrosis: Cheeselike friable white appearance; characteristic of tuberculosis. Structureless collection of fragmented cells enclosed in a granuloma (epithelioid macrophages).

    • Fat Necrosis: Focal fat destruction from pancreatic lipases (acute pancreatitis). Saponification occurs when fatty acids combine with calcium (chalky-white deposits).

    • Fibrinoid Necrosis: Special vascular damage in immune reactions. Antigen-antibody complexes and plasma proteins leak into vessel walls, appearing bright pink (fibrin-like).

Cell Death: Apoptosis

  • Definition: "Regulated" suction program where cells destined to die activate intrinsic enzymes (caspases) to degrade DNA and proteins.

  • General Contexts:

    • Physiologic: Removal of supernumerary cells in development; hormone-dependent tissue involution (menstrual cycle, weaning); cell turnover (intestinal crypts); elimination of self-reactive or useless lymphocytes.

    • Pathologic: DNA damage (radiation/drugs); accumulation of misfolded proteins (ER stress); viral infections (adenovirus, HIV) or host immune response (viral hepatitis); pathologic atrophy after duct obstruction.

  • Morphology:

    • Cell shrinkage and dense cytoplasm (vs. swelling in necrosis).

    • Chromatin condensation (most characteristic) and aggregation under nuclear membrane.

    • Cytoplasmic blebs and apoptotic bodies (membrane-bound fragments).

    • Phagocytosis of apoptotic bodies by macrophages (efferocytosis).

    • Absence of inflammation: Contents do not leak out; "eat me" signals (e.g., phosphatidylserine flipping to the outer leaflet) facilitate rapid clearance.

  • Mechanisms of Apoptosis:

    • Initiation Phase: Activation of caspases (proteases with cysteine in active site, cleave after aspartic residues).

    • Mitochondrial (Intrinsic) Pathway:

      • Major pathway for most situations. Controlled by the BCL2 family.

      • Anti-apoptotic: BCL2, BCL-XL, MCL1 (44 BH domains). Keep mitochondrial membrane impermeable.

      • Pro-apoptotic: BAX, BAK (33 BH domains). Form oligomeric channels for cytochrome c leakage.

      • Sensors (BH3-only): BAD, BIM, BID, Puma, Noxa. Inhibit anti-apoptotics and activate BAX/BAK.

      • Apoptosome: Released cytochrome c binds APAF-1, then binds and activates initiator caspase-99.

    • Death Receptor (Extrinsic) Pathway:

      • Engagement of TNF receptor family (e.g., TNFR1, Fas/CD95).

      • Fas ligand (FasL) on T-cells engages Fas; death domains recruit FADD adaptor protein.

      • FADD binds and activates initiator caspase-88 (or caspase-1010).

      • Inhibitor: FLIP protein can block caspase-88 activation.

    • Execution Phase: Converge on executioner caspases (caspase-33, caspase-66) which activate DNases and proteolyze the cytoskeleton.

Other Mechanisms of Cell Death

  • Necroptosis: "Programmed necrosis." Mechanism shares features of both pathways. Caspase-independent. Triggered by TNFR1 and viral DNA/RNA sensors.

    • Pathway: RIPK1 and RIPK3 kinases complex; RIPK3 phosphorylates MLKL.

    • Result: MLKL oligomerizes and disrupts plasma membrane, causing lytic death and inflammation.

  • Pyroptosis: "Fiery death." Lytic death accompanied by fever-inducing IL-1 release.

    • Pathway: Canonical (inflammasome activates caspase-11) or Noncanonical (LPS activates caspases-44/55).

    • Execution: Caspases cleave gasdermin D (GSDMD), which forms plasma membrane pores.

  • Ferroptosis: Iron-dependent death triggered by excessive ROS or iron levels overwhelming glutathione defenses.

    • Result: Unchecked membrane lipid peroxidation and loss of permeability.

Principal Mechanisms of Cell Injury

  • Mitochondrial Damage:

    • ATP Depletion: Result of hypoxia/ischemia or toxins (cyanide). Depletion to 5%5\%–10%10\% of normal levels leads to Na$^{+}$-K$^{+}$ pump failure (swelling), increased glycolysis (lactic acid accumulation, low pH), and ribosome detachment (low protein synthesis).

    • Mitochondrial Permeability Transition Pore (MPTP): Opening leads to loss of membrane potential and failure of oxidative phosphorylation.

  • Calcium Homeostasis Disturbance: Normal cytosolic free Ca$^{2+}$ is ≈0.1 μmol\approx 0.1\,\mu mol (vs. 1.3 mmol1.3\,mmol extracellular). Injury releases Ca$^{2+}$ from ER/mitochondria or allows influx across plasma membrane.

    • Consequences: Activates phospholipases (membrane damage), proteases (cytoskeletal damage), endonucleases (DNA fragmentation), and ATPases (hastens ATP depletion).

  • Oxidative Stress (Reactive Oxygen Species - ROS):

    • Generation: Normal respiration (reduction of O2O_2 yields superoxide O2∙O_2^{\bullet}, H2O2H_2O_2, and hydroxyl radical ∙OH^{\bullet}OH), radiant energy, inflammation (NADPH oxidase), and Fenton reaction (H2O2+Fe2+→Fe3++∙OH+OH−H_2O_2 + Fe^{2+} \rightarrow Fe^{3+} + ^{\bullet}OH + OH^{-}).

    • Removal: Antioxidants (Vitamins E/AVitamins\,E/A, glutathione, Vitamin CVitamin\,C), metal carrier proteins (ferritin, transferrin), and enzymes (Catalase, SOD, Glutathione peroxidase).

    • Pathologic Effects: Lipid peroxidation of membranes, oxidative protein modification (cross-linking/degradation), and DNA lesions (breaks/adducts).

  • Membrane Damage: Mechanisms include ROS (lipid peroxidation), decreased phospholipid synthesis (low ATP), increased phospholipid breakdown (calcium-activated phospholipases), and cytoskeletal abnormalities (calcium-activated proteases).

  • DNA Damage: Activates sensors triggering p53-dependent pathways. p53 arrests the cell cycle (G1 phase) for repair. If repair fails, p53 triggers apoptosis.


Clinicopathologic Correlations of Injury

  • Hypoxia vs. Ischemia: Ischemia is more severe because it compromises delivery of glycolytic substrates and removal of metabolites.

  • HIF-1: Hypoxia-inducible factor-11 stimulates survival and vessel formation.

  • Therapeutic Hypothermia: Reducing body temperature to 92extF92^ ext{F} can reduce metabolic demand and injury in brain/spinal cord cases.

  • Ischemia-Reperfusion Injury: Restoration of flow paradoxically exacerbates injury via ROS generation, calcium overload (opening MPTP), inflammation (neutrophil recruitment), and complement activation (IgM deposition).

  • Chemical Injury:

    • Direct Toxicity: Mercuric chloride (binds sulfhydryl groups), Cyanide (poisons cytochrome oxidase).

    • Toxic Metabolites: Often converted by cytochrome P-450450 in the smooth ER. Example: CCl<em>4CCl<em>4 is converted to ∙CCl</em>3\bullet CCl</em>3, causing lipid peroxidation. Acetaminophen is also converted to a toxic product.

Cellular Adaptations

  • Hypertrophy: Increased protein production and cell size.

    • Mechanism: Mechanical sensors (integrins) and growth factors (TGF-β\beta, IGF-11) activate signaling (PI3K/AKT, G-protein pathways) and transcription factors (GATA4, NFAT, MEF2). Can involve fetal gene re-expression (e.g., ANFANF, fetal myosin isoforms).

  • Hyperplasia: Growth factor-driven proliferation of mature cells or stem cells.

    • Physiologic: Breast development at puberty; compensatory liver regeneration after partial hepatectomy.

    • Pathologic: Endometrial hyperplasia (excess estrogen); skin warts (papillomaviruses).

  • Atrophy: Decreased protein synthesis (reduced trophic signals) and increased degradation (ubiquitin-proteasome pathway). Often involves increased autophagy and residual bodies (lipofuscin).

  • Metaplasia: Reprogramming of local tissue stem cells or colonization by adjacent cells.

    • Columnar to Squamous: Trachea in smokers; vitamin A deficiency.

    • Squamous to Columnar: Barrett esophagus (gastric acid reflux).

    • Connective Tissue: Myositis ossificans (bone formation in muscle).

Intracellular Accumulations

  • Lipids: Steatosis (triglycerides, common in liver/heart); Cholesterol/Cholesterol Esters (foam cells in atherosclerosis, xanthomas in hyperlipidemia, gallbladder cholesterolosis, and Niemann-Pick disease type C).

  • Proteins: Reabsorption droplets in renal tubules (proteinuria); Russell bodies (immunoglobulins in plasma cells); Misfolded protein aggregates ($ ext{α 1-antitrypsin}$ deficiency, amyloidosis).

  • Glycogen: Seen in Diabetes Mellitus (renal tubules, heart, liver) and Glycogen Storage Diseases.

  • Pigments:

    • Exogenous: Carbon (anthracosis/coal worker's pneumoconiosis); Tattoo pigments.

    • Endogenous: Lipofuscin ("wear-and-tear"); Melanin; Hemosiderin (iron storage granules, seen in bruising or systemic hemosiderosis/hemochromatosis).

Pathologic Calcification

  • Dystrophic Calcification: Deposition in dying/necrotic tissues (e.g., aged heart valves, atheromas). Serum calcium is normal.

  • Psammoma Bodies: Lamellated configurations of mineral deposits (found in papillary thyroid cancer).

  • Metastatic Calcification: Deposition in normal tissues due to hypercalcemia.

    • Causes: PTH excess, bone resorption (tumors, immobilization), vitamin D disorders (sarcoidosis, intoxication), and renal failure (phosphate retention).

Cellular Aging

  • DNA Damage: Accumulation of mutations (e.g., 1414/year in hematopoietic stem cells). Werner syndrome/Bloom syndrome involve defective DNA repair.

  • Cellular Senescence:

    • Telomere Attrition: Somatic cells lack telomerase; telomeres shorten with every division until arrest. Germ cells and cancer cells have active telomerase.

    • Tumor Suppressors: p16 (INK4a) expression correlates with chronological age and controls G1-S progression.

    • Defective Protein Homeostasis: Impaired chaperones and autophagy-lysosome systems. Rapamycin (mTOR inhibitor) promotes autophagy and increases lifespan in mice.

    • Dysregulated Nutrient Sensing:

    • Insulin/IGF-1 Pathway: Lowered signaling (via caloric restriction) reduces growth/metabolism and damage.

    • Sirtuins: NAD-dependent protein deacetylases (e.g., Sirtuin-66) adapt body to stress, activate DNA repair, and improve glucose metabolism.