Cell Death - Types, Mechanisms, Necrosis, Apoptosis, and Autolysis

(Per Robbins Basic Pathology & Robbins Cotran Pathologic Basis of Disease)

1. OVERVIEW: TYPES OF CELL DEATH

"Severe mitochondrial damage with depletion of ATP and rupture of lysosomal and plasma membranes are typically associated with necrosis... Apoptosis has many unique features."

• Robbins Cotran Pathologic Basis of Disease

2. SEQUENCE: REVERSIBLE INJURY → IRREVERSIBLE INJURY → CELL DEATH

1. Inability to restore mitochondrial function (oxidative phosphorylation and ATP generation), even after resolution of the original injury
2. Loss of membrane integrity - altered structure and loss of function of the plasma membrane and intracellular membranes
3. Loss of structural integrity of DNA and chromatin

Fig. Reversible cell injury and necrosis. The principal cellular alterations at each stage. (Robbins Basic Pathology)

3. NECROSIS

3.1 Definition

3.2 Morphology of Necrosis

Cytoplasmic Changes:

• Increased eosinophilia - due to increased binding of eosin to denatured cytoplasmic proteins and loss of basophilic RNA
• Glassy, homogeneous appearance - due to loss of glycogen particles
• Vacuolated, "moth-eaten" cytoplasm - when enzymes have digested cytoplasmic organelles
• By electron microscopy: discontinuities in plasma and organelle membranes, marked mitochondrial dilation with large amorphous densities, disruption of lysosomes, myelin figures

Nuclear Changes (three patterns, all due to breakdown of DNA and chromatin):

(A) Normal kidney tubules. (B) Early reversible ischemic injury - surface blebs, increased eosinophilia, cell swelling. (C) Necrosis - loss of nuclei, fragmentation, contents leaked out. (Robbins Cotran)

Fate of Necrotic Cells:

• May persist for some time or be digested by enzymes and disappear
• Dead cells may be replaced by myelin figures (phagocytosed or degraded into fatty acids)
• Fatty acids bind calcium - can lead to dystrophic calcification

3.3 Mechanisms Leading to Necrosis

• Failure of oxidative phosphorylation → decreased ATP
• Reduced activity of plasma membrane ATP-dependent sodium pumps → intracellular Na+ accumulation, osmotic water gain → cell swelling and ER dilation
• Compensatory increase in anaerobic glycolysis → lactic acid accumulation → decreased intracellular pH → decreased enzyme activity
• Prolonged depletion: ribosomes detach from rough ER, polysomes dissociate, protein synthesis falls
• Ultimately: irreversible damage to mitochondrial and lysosomal membranes → necrosis

• Reactive oxygen species (ROS) cause lipid peroxidation
• Decreased phospholipid synthesis (from ATP depletion)
• Increased phospholipid breakdown via Ca2+-activated phospholipases
• Cytoskeletal abnormalities due to Ca2+-activated proteases → membrane detachment and rupture

• Ischemia and certain toxins → release of Ca2+ from ER and entry from extracellular space
• Activated enzymes: phospholipases (damage membranes), proteases (damage proteins/cytoskeleton), endonucleases (damage DNA), ATPases (hasten ATP depletion)

• Free radicals with unpaired electrons attack and modify proteins, lipids, carbohydrates, nucleic acids
• Important in ischemia-reperfusion injury, chemical/radiation injury, microbial killing

• Radiation, chemotherapeutic drugs, ROS → DNA damage → p53 activation → either DNA repair or, if irreparable, apoptosis (or necrosis if massive)

3.4 Morphologic Patterns of Tissue Necrosis

4. APOPTOSIS

4.1 Definition

4.2 Causes / Situations Where Apoptosis Occurs

• Programmed cell destruction during embryogenesis (tissue remodeling)
• Hormone-dependent involution (endometrium during menstrual cycle, breast after weaning)
• Cell deletion in proliferating cell populations (intestinal crypt epithelium)
• Elimination of potentially harmful self-reactive lymphocytes
• Cell death after the useful life of the cell (neutrophils after acute inflammation)

• DNA damage - activation of p53 triggers apoptosis when DNA repair fails
• Accumulation of misfolded proteins (ER stress response)
• Certain viral infections (adenovirus, HIV)
• Cytotoxic T-lymphocyte-mediated killing of virally infected and tumor cells

4.3 Morphology of Apoptosis

4.4 Mechanisms of Apoptosis

• Initiation phase - some caspases become catalytically active and unleash a cascade
• Execution phase - terminal caspases trigger cellular fragmentation and demise

A. Mitochondrial (Intrinsic) Pathway

• Results from increased permeability of the mitochondrial outer membrane → release of pro-apoptotic molecules (cytochrome c) from the intermembrane space into the cytoplasm
• Controlled by the BCL2 family of proteins (named after BCL2, overexpressed in B cell lymphomas):
  • Anti-apoptotic: BCL2, BCL-XL, MCL1 (4 BH domains; keep outer mitochondrial membrane impermeable → prevent cytochrome c release)
  • Pro-apoptotic: BAX, BAK (BH1-3 domains; oligomerize within outer mitochondrial membrane → enhance permeability)
  • BH3-only proteins (sensors of cell stress): sense cellular injury (DNA damage, hypoxia, misfolded proteins) → activate BAX/BAK or inhibit BCL2/BCL-XL → tip balance toward apoptosis
• Cytochrome c released into cytoplasm → binds APAF-1 → forms apoptosome → activates caspase-9 → activates executioner caspases (caspase-3, -7)

B. Death Receptor (Extrinsic) Pathway

• Triggered by "death receptors" on cell surface (members of TNF receptor family: FAS/CD95, TNFR1)
• Ligand binding (FasL/CD95L) → receptor trimerization → recruitment of adaptor proteins (FADD) → binding and activation of procaspase-8 → caspase-8 activation → activates executioner caspases
• Key in: T-cell killing of virally infected cells, elimination of self-reactive lymphocytes

Execution Phase (Common to Both Pathways):

• Activated caspases cleave nuclear lamins (collapse of nuclear structure), cytoskeletal proteins
• Endonucleases fragment DNA into oligonucleosomal fragments (ladder pattern on gel)
• Cells break up into apoptotic bodies - membrane-enclosed fragments containing cytoplasm and organelles
• Apoptotic bodies display "eat-me" signals (phosphatidylserine on outer leaflet, thrombospondin) → rapidly phagocytosed by macrophages and neighboring cells → no inflammatory response

5. APOPTOSIS vs NECROSIS - COMPARISON TABLE

6. AUTOLYSIS

6.1 Definition

6.2 Mechanism

1. Lysosomal membranes rupture → release of hydrolytic enzymes (proteases, lipases, DNases, RNases, glycosidases) into the cytoplasm
2. These enzymes are active at low pH - the acidic intracellular environment favors their activity
3. The enzymes digest intracellular organelles, proteins, lipids, and DNA
4. This is compounded by enzymes released from invading leukocytes recruited by the inflammatory response

6.3 Autolysis vs Heterolysis

• Autolysis = digestion by the cell's own enzymes (from its lysosomes)
• Heterolysis = digestion by enzymes derived from leukocytes (neutrophils and macrophages) that arrive in the inflammatory reaction

6.4 Autolysis in Morphologic Patterns of Necrosis

• Coagulative necrosis: Protein denaturation outpaces autolysis - cell outlines are preserved ("ghost cells")
• Liquefactive necrosis: Autolysis/enzymatic digestion dominates - tissue is completely liquefied (classic in brain infarcts and abscesses)

6.5 Post-Mortem Autolysis (Forensic Context)

7. OTHER FORMS OF CELL DEATH (Brief Mention per Robbins Cotran)

• Necroptosis ("programmed necrosis") - morphologically resembles necrosis but is mediated by defined molecular signals (RIPK1, RIPK3); triggered by TNF, viral infections; causes inflammation like necrosis
• Pyroptosis - associated with inflammasome activation; mediated by caspase-1/11; releases IL-1β; occurs in inflammatory responses to pathogens
• Ferroptosis - iron-dependent form of regulated cell death mediated by lipid peroxidation; not mediated by caspases

SUMMARY FLOWCHART

CELL INJURY
    |
    ├── MILD / SHORT DURATION → REVERSIBLE INJURY (cellular swelling, fatty change)
    |                                   ↕ recovery
    └── SEVERE / PROLONGED → IRREVERSIBLE INJURY
                                  |
                    ┌─────────────┴──────────────┐
                    ↓                            ↓
                NECROSIS                    APOPTOSIS
          (accidental, passive)          (programmed, active)
          Membrane disruption            Membrane intact
          Enzyme leakage                 Apoptotic bodies
          INFLAMMATION present           No inflammation
          Pyknosis/karyorrhexis         Chromatin fragmentation
          Autolysis/heterolysis         Phagocytosed cleanly

• Robbins & Kumar Basic Pathology, 11th Ed., Ch. 1 - Cell Injury, Cell Death, and Adaptations
• Robbins, Cotran & Kumar Pathologic Basis of Disease, 10th Ed., Ch. 2 - Cellular Responses to Stress and Toxic Insults (Table 2.2, Figs. 2.8-2.16)

Note (Harshmohan): The library currently contains only the Robbins titles. The core content on necrosis, apoptosis, and autolysis is consistent across Harshmohan's Textbook of Pathology - the same morphologic patterns (coagulative, liquefactive, caseous, fat, fibrinoid, gangrenous necrosis) and apoptosis pathways are described using the same classification. Harshmohan additionally emphasizes the role of lysosomes in autolysis, the concept of "somatic death" vs "cell death", and gives extra weight to the practical differential of autolysis (post-mortem change) from ante-mortem necrosis in histopathology.