Histological Techniques for BDS
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Direct answer
Ten per cent neutral buffered formalin is the workhorse fixative of every dental histology laboratory, and the pipeline it feeds runs fixation, dehydration through ascending alcohols, clearing in xylene, infiltration with paraffin, embedding, sectioning at about 4-6 micrometres on a rotary microtome, staining (haematoxylin and eosin as default) and mounting. Teeth impose two special problems: their mineral must be removed for soft-tissue detail, or preserved at the cost of grinding. Decalcification uses acids (5 per cent nitric or formic acid) or the chelator EDTA, which is slower but kinder to morphology; decalcified sections then show pulp, odontoblasts and epithelium beautifully but lose the enamel entirely. Ground sections — tooth slices thinned on carborundum to roughly 25-100 micrometres — keep enamel, dentine and cementum intact and are viewed unstained by transmitted or polarised light, which is why striae of Retzius and Hunter-Schreger bands are demonstrated on them. Frozen sections complete the trio for fat, enzymes and rapid diagnosis.
What you must remember
- Fixation facts: 10 per cent neutral buffered formalin (about 4 per cent formaldehyde from 37-40 per cent stock), penetrating tissue at roughly 1 mm per hour as commonly quoted; glutaraldehyde gives superior ultrastructure and is the fixative for electron microscopy; the goal is to prevent autolysis and putrefaction while preserving structure.
- Processing sequence in order: fixation, dehydration (ascending ethanol), clearing (xylene or chloroform, making tissue transparent and paraffin-miscible), infiltration/impregnation, embedding, sectioning, staining, mounting — the order is itself an exam question.
- Section thickness: routine paraffin sections at about 4-6 micrometres; ultrathin electron microscopy sections at 50-100 nanometres (a favourite comparison pair).
- Decalcification menu: acids (nitric acid fastest but damages morphology; formic acid a balanced compromise) versus EDTA chelation (slowest, preserves cells and enzyme activity best); endpoint checked radiographically or chemically.
- Ground section essentials: undecalcified slices of teeth ground on abrasive (carborundum, pumice) to about 25-100 micrometres, examined unstained; the only technique that displays enamel rods, striae of Retzius, Hunter-Schreger bands, tufts and spindles.
- Decalcified section essentials: enamel dissolves away during acid treatment — the classic explanation for why decalcified tooth sections show dentine, pulp and periodontal ligament but no enamel; Schmorl-type and H&E staining on decalcified dentine; pulp detail is best here.
- Stain selection payload: H&E routine; Masson's trichrome and Mallory's for collagen; periodic acid-Schiff for glycogen, basement membranes and fungi; toluidine blue for mast cell metachromasia; von Kossa and alizarin red for calcium; Verhoeff's for elastic fibres; silver impregnation for reticulin.
Choosing between ground and decalcified sections
The laboratory decision is dictated by the question being asked. To study enamel — rods, cross-striations, striae, tufts, lamellae, spindles, the neonatal line — nothing but a ground section will do, because acid that decalcifies bone and dentine strips the 96-per-cent-mineral enamel away completely; the section is viewed dry or mounted unstained, with transmitted or polarised light, and the examiner's practical identifying features live on the brown incremental lines. To study the pulp — its zones, cell-rich and cell-free layers, odontoblast palisade, inflammation — the decalcified paraffin section is mandatory, since ground sections of that thickness cannot pass light through a tooth and pulp soft tissue cannot survive grinding. For dentine both techniques contribute: tubules and peritubular detail in ground sections; the odontoblast layer, predentine and tertiary responses in decalcified ones. Periodontal ligament, cementum and bone are studied decalcified, with trichrome or haematoxylin combinations displaying Sharpey fibres. Frozen sections complete the toolkit where speed (intraoperative consultation) or chemistry (lipids, enzymes, immunofluorescence that paraffin heat would destroy) decides — a triad of technique, purpose and sacrifice worth writing exactly that way in the practical record.
Practical exam questions
University practicals in Indian dental colleges revolve around three demonstrations. The unidentified slide of a tooth will nearly always be a ground section for enamel or a decalcified section for pulp and periodontium, and the first marking sentence is naming the technique from the evidence: enamel present means ground; enamel absent with pulp detail means decalcified. The second staple is the processing schedule as a long answer — write it as a flow with the purpose of each step named (dehydration removes water because paraffin is immiscible with it; clearing replaces alcohol because paraffin is immiscible with alcohol and xylene bridges them), since purpose-carrying sentences score where bare lists do not. The third is decalcifying-agent choice: formic acid or EDTA when morphology or histochemistry must survive, with nitric acid acceptable when speed matters — and note that over-decalcification makes sections stain poorly with haematoxylin, a real artefact students see in their own sections. Expect one viva question on fixation volume (roughly ten to twenty times the tissue volume, the conventional teaching) and one on why decalcified bone sections sometimes show swollen osteocyte lacunae — an acid artefact, not pathology.
Frequently asked questions
Why is 10 per cent neutral buffered formalin the routine fixative?
It penetrates reliably, preserves morphology adequately for light microscopy and is compatible with most downstream stains, while buffering prevents the acid haematin pigment artefact of unbuffered formalin.
Why can enamel not be studied in decalcified sections?
Decalcifying acids dissolve the 96 per cent mineral content of enamel completely, so only ground sections of intact teeth — thinned to roughly 25-100 micrometres and viewed unstained — can display enamel structures.
What are the steps of routine tissue processing?
Fixation, dehydration in ascending alcohols, clearing in xylene, paraffin infiltration, embedding, sectioning at 4-6 micrometres, staining and mounting — each step preparing the tissue for the next.
How do acid and EDTA decalcification differ?
Acids such as nitric or formic remove mineral quickly but can distort morphology, whereas EDTA chelates calcium slowly while preserving cellular detail and enzyme activity, which is why it is preferred for fine histology.
When are frozen sections preferred over paraffin sections?
For rapid intraoperative diagnosis, fat and enzyme histochemistry, and immunofluorescence, because freezing avoids the solvent and heat steps that dissolve lipids and denature proteins.