Introduction to Histopathology and Cytology
Key concepts, terminology, and practical workflow for students and trainees
1. What Is Histopathology?
| Aspect |
Description |
| Definition |
Microscopic examination of whole tissue sections to study disease‑related structural changes. |
| Specimen |
Fixed, processed, embedded in paraffin, cut into 3–5 µm sections, stained (e.g., H&E). |
| Primary Uses |
Cancer diagnosis & grading, inflammatory disorders, infectious disease, degenerative changes, transplant rejection. |
| Typical Workflow |
1. Gross examination → 2. Fixation → 3. Tissue processing → 4. Embedding → 5. Sectioning → 6. Staining → 7. Microscopic interpretation. |
| Key Tools |
Microtome, cryostat, automated stainers, digital slide scanners, light microscope (bright‑field, polarized, fluorescence). |
1.1 Core Histological Stains
| Stain |
Main Targets |
Typical Applications |
| Hematoxylin & Eosin (H&E) |
Nuclei (hematoxylin) + cytoplasm/extracellular matrix (eosin) |
Routine diagnostic work‑up |
| Periodic Acid‑Schiff (PAS) |
Glycogen, mucopolysaccharides, basement membranes |
Fungal organisms, glycogen storage disease |
| Masson’s Trichrome |
Collagen (blue), muscle (red), nuclei (black) |
Fibrosis, muscle pathology |
| Silver stains (e.g., GMS, reticulin) |
Fungi, reticular fibers |
Opportunistic infections, liver architecture |
| Immunohistochemistry (IHC) |
Specific antigens using antibodies |
Tumor typing, prognostic markers, infectious agents |
2. What Is Cytology?
| Aspect |
Description |
| Definition |
Study of individual cells or small clusters obtained without preserving tissue architecture. |
| Specimen Types |
Fine‑needle aspiration (FNA), body fluids (pleural, peritoneal, cerebrospinal), brushings, exfoliative smears, liquid‑based cytology (LBC). |
| Primary Uses |
Rapid triage of lesions, screening (Pap test), evaluation of metastatic disease, infectious disease detection. |
| Typical Workflow |
1. Sample collection → 2. Immediate fixation or air‑drying → 3. Smear preparation or cell block → 4. Staining (Diff‑Quik, Papanicolaou, Giemsa) → 5. Microscopic review → 6. Ancillary studies (IHC, molecular). |
| Key Tools |
Cytocentrifuge, slide printer, automated LBC processor, fluorescence microscope, digital imaging. |
2.2 Common Cytology Stains
| Stain |
Main Targets |
Typical Applications |
| Papanicolaou (Pap) |
Nuclear detail (hematoxylin) + cytoplasmic transparency (orange‑green counterstains) |
Cervical screening, body‑fluid cytology |
| Diff‑Quik (Romanowsky) |
Granular cytoplasmic components, metachromasia |
Rapid on‑site evaluation (ROSE) of FNA |
| Giemsa |
Nuclear chromatin, cytoplasmic granules |
Hematologic smears, parasite detection |
| May‑Grünwald‑Giemsa (MGG) |
Similar to Diff‑Qu |
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