SN4 SHORT NOTE 5M ★ High-yield

Describe the preparation of a histological section and the common stains used.

Anatomy · 1st Professional — Pre-Clinical

Stain	What it shows	Colour
Haematoxylin	Basophilic structures — nuclei, ribosomes, rough ER	Blue-purple
Eosin	Acidophilic structures — cytoplasm, collagen, muscle	Pink
PAS (periodic acid–Schiff)	Carbohydrate — glycogen, mucin, basement membrane, fungal walls	Magenta
Masson trichrome	Collagen against muscle	Collagen blue or green, muscle red
Verhoeff / orcein	Elastic fibres	Black or brown
Silver impregnation	Reticular fibres; neurones and neuroglia	Black
Osmium tetroxide / Sudan black	Lipid (needs a frozen section)	Black
Toluidine blue	Mast cell granules — metachromasia	Purple against a blue background
Prussian blue (Perls)	Iron (haemosiderin)	Blue
Congo red	Amyloid — apple-green birefringence in polarised light	Red

Technique	Principle	Use
Frozen section	Tissue frozen and cut on a cryostat	Rapid diagnosis during an operation; lipid stains
Immunohistochemistry	Labelled antibody to a specific antigen	Identifying tumour origin and receptor status
Electron microscopy	Electron beam; ultrathin sections on grids	Ultrastructure — cilia, basement membrane, podocytes
Histochemistry	Chemical reaction in situ	Enzymes and specific chemical groups
In situ hybridisation	Labelled nucleic acid probe	Viral DNA, gene amplification

Resolving power — the smallest distance at which two points are seen as separate: about 0.2 micrometres for the light microscope and 0.2 nanometres for the electron microscope
Magnification is the product of the objective and the eyepiece
Oil immersion raises the numerical aperture and so the resolution
Phase contrast allows unstained living cells to be examined; polarising microscopy demonstrates birefringent material such as amyloid and crystals

Frozen section during surgery answers whether a lesion is malignant and whether the resection margins are clear, while the patient is still on the table
Immunohistochemistry decides treatment — oestrogen, progesterone and HER2 receptor status in breast carcinoma determine whether hormonal or targeted therapy is used
Congo red with polarised light is the definitive test for amyloid; the apple-green birefringence is diagnostic
Artefacts must be recognised — folds, knife marks, shrinkage from fixation, and post-mortem autolysis, or they are mistaken for disease
Fine needle aspiration cytology examines cells rather than architecture; it is quick and cheap but cannot distinguish follicular adenoma from carcinoma, which needs the capsule to be assessed

All questions in General Histology