consequences of cell injury

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Consequences of Cell Injury

All injurious stimuli produce a stereotypic sequence of cellular responses that range from fully reversible functional disturbances to irreversible death. The outcome depends on the nature, duration, and severity of the injury.

Temporal Sequence of Changes

Sequential development of biochemical and morphologic changes in cell injury

Fig. 2.6 — Robbins & Cotran Pathologic Basis of Disease

Key principle: cell function fails before morphology changes. The sequence from injury onset:

Functional loss (seconds–minutes)
Biochemical alterations (minutes–hours) — detectable by biochemical assays
Ultrastructural changes — detectable only by electron microscopy
Light microscopic changes — hours to days
Gross morphologic changes — latest to appear

1. Reversible Cell Injury

If the injurious stimulus is removed in time, injured cells can recover to normalcy. Two consistent hallmarks:

Cellular Swelling (Hydropic Change / Vacuolar Degeneration)

The earliest and most universal manifestation of cell injury
Caused by failure of the ATP-dependent Na⁺/K⁺ plasma membrane pump → Na⁺ accumulates intracellularly → osmotic water influx
ATP depletion is caused by: hypoxia (↓ oxidative phosphorylation), mitochondrial damage (toxins/radiation)
Gross: pallor, increased turgor, increased organ weight
Light microscopy: small clear cytoplasmic vacuoles (distended pinched-off ER segments); cytoplasm becomes eosinophilic (loss of RNA)
Electron microscopy reveals:
1. Plasma membrane blebs, blunting, loss of microvilli
2. Mitochondrial swelling with small amorphous densities
3. "Myelin figures" — phospholipid whorls from damaged membranes
4. ER dilation with detachment of ribosomes/polysomes
5. Nuclear chromatin clumping

Fatty Change (Steatosis)

Occurs in organs active in lipid metabolism (e.g., liver, heart)
Toxic injury disrupts lipid metabolic pathways → accumulation of triglyceride-filled lipid vacuoles

2. Irreversible Cell Injury → Cell Death

Persistent or excessive injury crosses the "point of no return" — a threshold beyond which mitochondrial and membrane damage is irreparable. Three hallmarks mark irreversibility:

Inability to restore mitochondrial function (oxidative phosphorylation/ATP generation)
Altered structure and loss of function of plasma and intracellular membranes
Loss of structural integrity of DNA and chromatin

Cell death occurs by two principal mechanisms:

Morphologic changes in cell injury culminating in necrosis or apoptosis

Fig. 2.7 — Robbins & Cotran Pathologic Basis of Disease

A. Necrosis

Historically considered "accidental" cell death following severe injury. Characterized by:

Feature	Finding
Cell size	Enlarged (swelling)
Nucleus	Pyknosis (condensation) → Karyorrhexis (fragmentation) → Karyolysis (dissolution)
Plasma membrane	Disrupted
Cellular contents	Leak out (enzymatic digestion)
Inflammation	Frequent (host reaction to leaked contents)
Role	Pathologic — culmination of irreversible injury

Patterns of Tissue Necrosis

Type	Description	Example
Coagulative	Architecture preserved; cells "ghost outlines" (denatured proteins resist digestion)	Myocardial infarction
Liquefactive	Complete dissolution; pus formation	Brain infarct, abscess
Caseous	Cheesy, amorphous; combination of coagulative + liquefactive	Tuberculosis
Fat necrosis	Calcium soap deposits (saponification)	Acute pancreatitis
Fibrinoid	Immune complex deposition in vessel walls	Vasculitis, malignant HTN
Gangrenous	Coagulative + superimposed infection (wet gangrene = liquefactive component)	Limb ischemia

B. Apoptosis

"Regulated" cell death — mediated by defined molecular pathways. Surgical precision without inflammation.

Feature	Finding
Cell size	Reduced (shrinkage)
Nucleus	Fragmentation into nucleosome-size fragments (DNA laddering)
Plasma membrane	Intact — altered lipid orientation (phosphatidylserine flips to outer leaflet)
Cellular contents	Packaged into apoptotic bodies — phagocytosed by neighboring cells or macrophages
Inflammation	Absent
Role	Physiologic (embryogenesis, immune selection, tissue homeostasis) OR pathologic (DNA damage, infections)

Mechanisms of Apoptosis

Intrinsic (mitochondrial) pathway: DNA damage, misfolded proteins, growth factor withdrawal → pro-apoptotic Bcl-2 family proteins (Bax, Bak) overcome anti-apoptotic members (Bcl-2, Bcl-XL) → cytochrome c release → apoptosome → caspase-9 → caspase-3 activation
Extrinsic (death receptor) pathway: FasL binds Fas, TNF binds TNFR → DISC formation → caspase-8 → caspase-3 activation

C. Other Forms of Cell Death (Newer Mechanisms)

Type	Mechanism	Key Feature
Necroptosis	"Programmed necrosis" via RIPK1/RIPK3/MLKL; morphologically resembles necrosis but is genetically regulated	Causes inflammation (unlike apoptosis)
Pyroptosis	Caspase-1/11-dependent; releases IL-1β and IL-18; plasma membrane rupture	Highly pro-inflammatory; occurs in infections
Ferroptosis	Iron-dependent accumulation of lipid peroxides; distinct from apoptosis and necrosis	Associated with ischemia-reperfusion injury

3. Cellular Adaptations

When injury is sublethal and chronic, cells may adapt rather than die:

Adaptation	Definition	Example
Hypertrophy	↑ cell size (↑ workload/trophic signals)	Cardiac hypertrophy in hypertension
Hyperplasia	↑ cell number (growth factor stimulation)	Endometrial hyperplasia (estrogen)
Atrophy	↓ cell size/number (↓ workload, ↓ trophic signals)	Muscle wasting, denervation
Metaplasia	One differentiated cell type → another	Barrett esophagus (squamous → columnar)

Summary Diagram

CELL INJURY
    │
    ├─ Mild/Transient → REVERSIBLE INJURY
    │       • Cellular swelling (hydropic change)
    │       • Fatty change
    │       • Membrane blebs, ER dilation
    │       • Recovery if stimulus removed
    │
    └─ Severe/Persistent → IRREVERSIBLE INJURY → CELL DEATH
            │
            ├─ NECROSIS (pathologic, inflammatory)
            │       Patterns: coagulative, liquefactive, caseous,
            │                 fat, fibrinoid, gangrenous
            │
            ├─ APOPTOSIS (often physiologic, no inflammation)
            │       Intrinsic + Extrinsic pathways → caspases
            │
            └─ REGULATED NECROSIS
                    Necroptosis / Pyroptosis / Ferroptosis

Sources:

Robbins & Cotran Pathologic Basis of Disease, 10th ed., Ch. 2 (pp. 35–55)
Robbins & Kumar Basic Pathology, 11th ed., Ch. 1

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