Pathology: Cell Injury, Inflammation, and Neoplasia

General pathology ties together how cells respond to stress: they adapt, they get hurt reversibly, and eventually they die by necrosis or apoptosis. Layer on how the body reacts with inflammation and healing, then how growth control fails in neoplasia, and you have a framework that reappears across every organ system on Step 1.

Core Idea

• Reversible injury (cellular swelling, membrane blebbing, ribosomal detachment) becomes irreversible once mitochondria fail and membranes rupture — the point of no return is severe membrane damage.
• Necrosis is uncontrolled death with inflammation and enzyme leakage; apoptosis is programmed, ATP-dependent, and does not trigger inflammation.
• Neoplasia is a stepwise loss of growth control; malignancy is defined by invasion through the basement membrane and the capacity to metastasize.

Reversible vs. Irreversible Cell Injury

The most common initial injury is hypoxia/ischemia, which drops ATP. Low ATP shuts down the Na+/K+ pump, so sodium and water rush in — this is cellular (hydropic) swelling, the hallmark of reversible injury. Anaerobic glycolysis lowers pH, ribosomes detach (decreased protein synthesis), and membranes bleb. Everything so far can recover if perfusion returns.

The transition to irreversible injury centers on the mitochondria and cell membranes. Persistent low ATP and rising cytosolic calcium activate phospholipases, proteases, and endonucleases. The mitochondrial permeability transition pore opens, and plasma-membrane damage lets enzymes leak out (measurable in serum) and calcium flood in. Reperfusion can paradoxically worsen injury by generating reactive oxygen species.

Necrosis vs. Apoptosis

Necrosis is death of groups of cells with an inflammatory response. Recognize the patterns conceptually:

• Coagulative — preserved tissue architecture with loss of nuclei; classic for ischemic infarcts of most solid organs (except brain).
• Liquefactive — enzymatic digestion into liquid; seen in the brain (infarct) and in abscesses (bacterial).
• Caseous — soft, "cheesy," granulomatous; classic for tuberculosis and fungal infection.
• Fat — saponification with calcium; seen in acute pancreatitis and breast trauma.
• Fibrinoid — pink, proteinaceous deposits in vessel walls; seen in vasculitis and malignant hypertension.

Apoptosis is single-cell, programmed, ATP-requiring death with cell shrinkage and nuclear fragmentation, producing apoptotic bodies that are cleared without inflammation. The intrinsic (mitochondrial) pathway responds to internal stress/DNA damage: pro-apoptotic factors (Bax/Bak) overcome anti-apoptotic Bcl-2, releasing cytochrome c to activate caspases. The extrinsic (death receptor) pathway is triggered externally through Fas–FasL or TNF receptor. Both converge on executioner caspases.

Cellular Adaptations

Cells adapt to chronic stress before they die:

• Hypertrophy — larger cells (more proteins/organelles), e.g., cardiac muscle under pressure.
• Hyperplasia — more cells, e.g., endometrium under estrogen; requires cells able to divide.
• Atrophy — smaller/fewer cells via decreased synthesis and increased ubiquitin-proteasome and autophagic degradation.
• Metaplasia — reversible switch of one differentiated type to another, e.g., squamous-to-columnar in Barrett esophagus.
• Dysplasia — disordered, atypical growth that is pre-neoplastic but still confined; it is not yet cancer but can progress.

Acute vs. Chronic Inflammation

Acute inflammation is rapid and dominated by neutrophils, driven by vasodilation and increased vascular permeability. The five cardinal signs — rubor (redness), calor (heat), tumor (swelling), dolor (pain), and functio laesa (loss of function) — reflect these vascular changes. Key mediators: histamine (vasodilation, permeability), prostaglandins (pain, fever, vasodilation), leukotrienes (chemotaxis, bronchoconstriction), bradykinin (pain), and complement (C3a, C5a).

Chronic inflammation features macrophages, lymphocytes, and plasma cells, with tissue destruction and simultaneous repair (fibrosis, angiogenesis). Granulomatous inflammation — organized epithelioid macrophages and giant cells — is a special chronic pattern from persistent antigens like TB.

Wound Healing and Neoplasia

Healing proceeds through inflammation, then granulation tissue (new capillaries, fibroblasts, loose collagen), then remodeling. Labile tissues (gut, skin) regenerate well; permanent tissues (neurons, cardiac muscle) heal by scar. In neoplasia, benign tumors are well-differentiated, slow, encapsulated, and non-invasive; malignant tumors are poorly differentiated (anaplastic), invasive, and metastatic. Carcinomas arise from epithelium; sarcomas from mesenchyme. Grade describes how differentiated the tumor looks; stage describes how far it has spread (TNM) — stage matters more for prognosis.

High-Yield Exam Patterns

• Ischemia → coagulative necrosis everywhere except the brain, which undergoes liquefactive necrosis.
• Caseous necrosis + granuloma = think TB; abscess with pus = liquefactive.
• Rising serum enzymes (e.g., troponin, transaminases) signal membrane rupture = irreversible injury.
• Apoptosis is ATP-dependent and non-inflammatory; necrosis is ATP-depleted and inflammatory.
• Barrett esophagus = metaplasia (adaptive), a risk factor that can progress to dysplasia then adenocarcinoma.
• Invasion through the basement membrane distinguishes invasive carcinoma from carcinoma in situ; ability to metastasize defines malignancy.

Common Traps to Avoid

• Do not equate dysplasia with cancer — it is pre-malignant and, if confined, potentially reversible.
• Do not assume all necrosis looks the same; the pattern points to the etiology and location.
• Do not confuse grade (differentiation) with stage (spread) — stage is the stronger prognostic factor.
• Do not label apoptosis as inflammatory; apoptotic bodies are cleared cleanly.
• Do not forget that hyperplasia requires dividing cells, so permanent tissues like cardiac muscle respond only with hypertrophy.