# Tissue Processing and Histotechnology > FMGE Pathology notes on tissue processing: formalin fixation rates, dehydration, clearing, paraffin embedding, section thickness and frozen sections. - Canonical URL: https://prepelephant.com/topics/fmge/pathology/tissue-processing-histotechnology - Exam / course: FMGE · Subject: Pathology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Tissue Processing and Histotechnology", PrepElephant, https://prepelephant.com/topics/fmge/pathology/tissue-processing-histotechnology ## Direct answer Tissue processing converts a fresh surgical specimen into a glass slide by a fixed sequence — fixation, grossing, dehydration, clearing, impregnation, embedding, sectioning and staining — with 10 per cent neutral buffered formalin as the universal fixative. Formalin (4 per cent formaldehyde) cross-links proteins through methylene bridges, penetrates tissue at roughly 1 mm per hour, and needs about ten times its own volume of tissue, numbers that recur in every FMGE and viva paper. Paraffin with a melting point of 56-62°C infiltrates xylene-cleared tissue, sections are cut at 3-5 micrometres, and haematoxylin-eosin stains nuclei blue and cytoplasm pink. Frozen sections, cut on a cryostat within minutes, serve intraoperative margin and node decisions and remain the only route for fat stains like Oil Red O. ## What you must remember - **Fixative arithmetic:** 10 per cent neutral buffered formalin equals 4 per cent formaldehyde; penetration averages 1 mm per hour and the fixative volume should be 10-20 times tissue volume. - **Buffering rationale:** neutral pH prevents acid artefact — formalin pigment, a dark brown birefringent haematin deposit in blood-rich tissues. - **Fixative alternatives:** glutaraldehyde for electron microscopy, absolute alcohol for cytological smears, Bouin (picric acid) for testicular biopsies, Carnoy for glycogen, and Hollande or Zenker as heavy-metal mixtures. - **Dehydration to embedding:** ascending grades of ethyl alcohol dehydrate, xylene clears (its refractive index makes tissue transparent), and paraffin at 56-62°C impregnates before embedding. - **Section numbers:** routine sections are 3-5 micrometres thick; semithin resin sections approach 1 micrometre for kidney and nerve biopsies. - **Frozen section logic:** cryostat sections in minutes for margins, lymph node involvement and tissue identification — and the only way to demonstrate lipid with Oil Red O or Sudan Black. - **Decalcification:** acid (formic or hydrochloric) is fast but damages antigens, while EDTA is slow but preserves immunoreactivity — the reason IHC on bone marrow trephines prefers EDTA. ## One specimen's journey through the laboratory Follow a gallbladder received fresh from theatre. The surgeon wants a margin assessment, so a frozen section on the cystic duct margin takes ten minutes; the rest of the specimen immerses in ten volumes of buffered formalin. A 4 mm thick wall fixed overnight at 1 mm per hour is adequately penetrated by morning. Grossing documents stones, wall thickness and mucosal colour, and representative blocks — the number chosen by size and suspicion — travel overnight through the automated processor: alcohol gradients that pull water out, xylene that pulls alcohol out and renders tissue translucent, then molten paraffin that fills every space alcohol once occupied. Embedded and cut at 4 micrometres, stained with haematoxylin-eosin, the slide shows what the frozen section only hinted at. Underfixation at any point — thick slices, dilute formalin, a weekend on the bench — shows up later as smudged chromatin and immunohistochemistry that fails to stain, a practical chain of consequence that examiners reward being narrated. ## Where students slip Numbers are where marks leak: candidates quote "10 per cent formalin" without knowing it means 4 per cent formaldehyde, or give section thickness in millimetres. The clearing step is misnamed in memory — xylene "clears" because tissue turns translucent, not because it removes something, and it replaces alcohol because paraffin is immiscible with water. Formalin pigment gets confused with haemosiderin; the quick discriminator is that formalin pigment is birefringent, dissolves in saturated picric acid or dilute alkali, and lacks the Prussian blue reaction. Finally, remember that artifacts have owners: knife lines, chatter and freeze artefact are processing faults, not disease. ## Frequently asked questions ### Why is neutral buffered formalin the standard fixative? Buffering to neutral pH prevents acidic formalin pigment formation while methylene-bridge cross-linking preserves morphology and most antigens at modest cost and toxicity. ### How fast does formalin penetrate tissue and in what ratio? Roughly 1 mm per hour from each surface, with the fixative volume kept at least 10 times the tissue volume — the numbers behind overnight fixation of standard blocks. ### What does the clearing step in tissue processing achieve? Xylene replaces dehydrating alcohol and renders tissue translucent — "clear" — while making it receptive to paraffin, which cannot mix with water or alcohol. ### When is a frozen section indicated? For intraoperative decisions on margins, lymph node involvement or tissue identity, and for demonstrating lipids with Oil Red O, since paraffin processing dissolves fat. ### How is calcified bone made sectionable? Decalcification with formic or hydrochloric acid dissolves mineral quickly, while EDTA does so slowly but preserves antigenicity for immunohistochemistry.