Introduction to Histopathology and Cytology
What this domain tests
This domain introduces tissue handling from fixation to staining, plus cytology collection and basic report interpretation.
Core rules
- Fix tissue in 10% neutral buffered formalin, usually at about 10 times the tissue volume, to prevent autolysis and preserve morphology.
- Process through graded alcohols for dehydration, xylene for clearing, and paraffin for embedding. Section at 3-5 µm on a microtome.
- Routine staining is hematoxylin and eosin: hematoxylin stains nuclei blue-purple, eosin stains cytoplasm and collagen pink.
- Special stains answer specific questions: Ziehl-Neelsen for acid-fast bacilli, periodic acid-Schiff for glycogen and fungi, and Prussian blue for iron.
- Frozen sections give rapid intraoperative answers but have poorer morphology. Cytology includes Pap smears, fluid cytology and fine-needle aspiration.
- Label specimens, record fixation time and avoid crushing or drying.
Worked example
A large surgical specimen is placed in a small volume of formalin and the centre is underfixed. Reasoning: formalin penetrates slowly, so the centre undergoes autolysis and stains poorly. Slice the specimen to allow penetration and use an adequate volume of fixative; underfixation cannot be corrected later.
Traps
- Using normal saline instead of formalin for routine fixation.
- Over-dehydrating small biopsies.
- Cutting thick sections that float off the slide.
- Reporting cytology as histology without noting the limitation.