Histopathology is the microscopic examination of diseased tissue. Before a pathologist can evaluate a specimen, the tissue must undergo a series of preparative steps that preserve structure, harden the sample for cutting, and produce sections thin enough to transmit light. The quality of the final diagnosis depends directly on the quality of processing and sectioning.
Fixation
Fixation preserves tissue architecture by cross-linking proteins and inactivating degradative enzymes. The most common fixative is 10% neutral buffered formalin (NBF), a 3.7% formaldehyde solution that forms methylene bridges between amino groups, stabilizing protein structure without excessive shrinkage. Fixation time depends on tissue size — a typical biopsy requires 6-24 hours, while larger specimens may need 24-48 hours. Under-fixation leaves the tissue vulnerable to autolysis and produces poorly stained sections; over-fixation causes excessive cross-linking that masks antigens for immunohistochemistry. Other fixatives include glutaraldehyde (for electron microscopy), Bouin’s solution (for endocrine tissues), and alcohol-based fixatives (for rapid processing in frozen sections).
Processing
After fixation, tissue must be infiltrated with a solid support medium — typically paraffin wax — before sectioning. Processing occurs in an automated tissue processor over 12-24 hours through sequential stages:
Dehydration removes water from the tissue using graded ethanols (70%, 80%, 95%, 100%) to prevent distortion. Incomplete dehydration prevents wax penetration and causes sectioning artifacts.
Clearing replaces ethanol with an organic solvent miscible with both alcohol and paraffin. Xylene is the most common clearing agent; it renders the tissue translucent and prepares it for infiltration.
Infiltration replaces the clearing agent with molten paraffin wax in a vacuum oven, ensuring complete penetration throughout the tissue block. The vacuum removes air bubbles that would otherwise cause holes in sections.
Embedding
Embedding orients the infiltrated tissue in a paraffin block for sectioning. The tissue is placed in a metal mold, covered with molten wax, and cooled on a cold plate until solid. Correct orientation is critical: most biopsies are embedded to present the maximal cross-sectional area to the microtome blade. Small or fragmented specimens (needle biopsies, cell blocks from cytology) are often wrapped in lens paper or embedded in agar before processing to prevent loss.
Microtomy
Sectioning is performed on a rotary microtome that advances the paraffin block by precise increments — typically 3-5 µm for routine histology. The ribbon of sections floats on a heated water bath (40-45°C) to flatten wrinkles, then is collected onto glass slides. Factors affecting section quality include blade condition (disposable blades are changed regularly), block temperature (cooling blocks on ice improves sectioning of fatty tissues), and cutting speed (slow, even strokes produce the best ribbons).
After mounting, sections are dried on a hot plate or in an oven to ensure adhesion. Subsequent staining (most commonly H&E — hematoxylin and eosin) reveals nuclear and cytoplasmic detail for microscopic evaluation.
Common Artifacts
Knife marks appear as parallel score lines from nicks in the microtome blade; they can be minimized by using fresh blade edges for hard tissues.
Compression occurs when the block is too warm or the blade is dull, causing the section to be shorter than the block face. Cooling the block or replacing the blade resolves this.
Chatter (alternating thick and thin bands) results from vibration in the microtome or a loose block — the block must be firmly secured in the chuck.
Folds and wrinkles in the section are caused by improper floating on the water bath or excessive heat; adding a drop of detergent to the water reduces surface tension.
Floating artifacts are fragments of tissue from other cases that contaminate the water bath — filtering the water bath and changing it daily prevents cross-contamination.
Bubbles arise from incomplete paraffin infiltration during processing, leaving air trapped in the tissue. Re-processing under vacuum may be necessary.
Quality Control
Every stained slide should be checked for section thickness (uniform 3-5 µm), absence of folds and tears, complete representation of the tissue, and correct labeling. Inadequate sections must be re-cut before the pathologist reviews the case. Proper processing and sectioning are prerequisites for accurate histopathological diagnosis and for downstream techniques such as immunohistochemistry, in situ hybridization, and digital pathology.
