🧬 Morphologic pathology
- Pathology = study of disease through structural, functional & molecular alterations
- Morphologic changes assessed at two levels:
- Gross (macroscopic) – organ size, shape, consistency, color
- Microscopic – tissue sections, cellular architecture
- Etiology: genetic or environmental (or combined)
- Pathogenesis: temporal sequence of cellular injury leading to disease
- Clinical significance: signs/symptoms, course, complications, prognosis
Etiology→
Pathogenesis→
Morphology (gross/micro)→
Clinical manifestation
- Gross examination: identify organ → identify abnormality (e.g., nodules, necrosis)
🎨 Histochemical stains
- H&E gold standard; hematoxylin (blue-purple) binds nucleic acids & calcium; eosin (pink-red) stains cytoplasmic & extracellular proteins
- Prussian blue – iron (hemosiderin, hemochromatosis)
- Congo red – amyloid (apple-green birefringence)
- Acid-fast (Ziehl-Neelsen, Fite) – mycobacteria (M. tuberculosis, MAC)
- PAS – high-carbohydrate molecules: Candida, α1-antitrypsin globules, glycogen storage, diabetic nephropathy (Kimmelstiel-Wilson)
- Trichrome – connective tissue, type IV collagen; cirrhosis, crescentic GN
- Reticulin – type III collagen; cirrhosis, myelofibrosis
| Stain | Target | Associated diseases |
|---|---|---|
| Prussian blue | Iron | Hemochromatosis, hemosiderosis |
| Congo red | Amyloid | Amyloidosis |
| Acid-fast | Mycobacteria | Tuberculosis, MAC |
| PAS | Glycoproteins | Candida, α1-AT, GSD, diabetic nephropathy |
| Trichrome | Collagen IV | Cirrhosis, crescentic GN |
| Reticulin | Collagen III | Cirrhosis, myelofibrosis |
- “Eosinophilic” = intensely pink (cytoplasmic proteins, RBCs, collagen)
🔬 Ancillary diagnostic techniques
- Immunofluorescence (IF) – renal & autoimmune diseases
- Transmission EM – renal, neoplasms, infections, genetic disorders
- Molecular & cytogenetic:
- Protein electrophoresis (serum/urine)
- Southern / Western blot
- PCR (fresh or formalin-fixed tissue)
- Next-generation sequencing (NGS)
- Karyotyping, FISH (in situ hybridization)
- Immunohistochemistry (IHC) – antibody-based, brown chromogen = positive
- Cytokeratin → epithelial cells
- Vimentin → mesenchymal (except muscle); sarcomas
- Desmin → smooth, cardiac, skeletal muscle
- Prostate-specific antigen (PSA), etc.
- IFM: linear vs. granular patterns (anti-GBM, IgA)
- EM can identify dense deposits, fibrils, viral particles
🩺 Clinical integration
- Gross features guide differential:
- Size (enlarged/shrunken)
- Shape (contour, symmetry)
- Consistency (soft, firm, cystic)
- Color (pale, hemorrhagic, bile-stained)
- Microscopic diagnosis is definitive; multiple stains often needed
- Pathology report integrates:
- Morphologic description
- Special stain results
- IHC / molecular markers
- Clinical correlation
Gross exam→
Tissue processing→
H&E + special stains→
IHC / molecular→
Final diagnosis
💎 Pearls & examination traps
- high yield Prussian blue = iron (hemosiderin) – not bilirubin
- high yield Congo red requires polarized light for apple-green birefringence
- high yield PAS highlights basement membranes in diabetic nephropathy (Kimmelstiel-Wilson nodules)
- high yield Trichrome stains collagen blue/green – used in liver fibrosis (cirrhosis) and kidney (crescents)
- high yield Reticulin (silver) stains type III collagen – highlights architecture in myelofibrosis
- IHC: cytokeratin (+) = carcinoma; vimentin (+) = sarcoma (except muscle); desmin (+) = muscle tumors
- Do not confuse “eosinophilic” with “basophilic” – H&E color is diagnostic clue
- Multiple stains may be needed for a single specimen – each highlights a different structure