ASCP H key terms

A fixative made by diluting concentrated formaldehyde in water and buffering it with phosphate salts to pH 7.2–7.4, giving roughly 4% formaldehyde; it is the standard fixative for routine surgical tissue.

A fixative, typically formalin, that joins protein molecules with covalent bonds and thereby locks tissue structure in place; cross-linking preserves morphology well but can hide antigen sites.

A fixative such as ethanol, methanol, or Bouin fluid that precipitates proteins instead of chemically linking them; it preserves some antigens and nucleic acids but often shrinks and hardens tissue.

The time from when a tissue loses its blood supply in the operating room until it is placed in fixative; longer times degrade DNA, RNA, and labile proteins.

Passing fixed tissue through increasing concentrations of alcohol to remove water, preparing it for a clearing agent that is not miscible with water.

Replacing alcohol in dehydrated tissue with a fluid such as xylene that mixes with both alcohol and paraffin; the tissue becomes translucent and ready for wax.

Allowing molten paraffin to soak into cleared tissue so that spaces around cells fill with wax; the block then holds together and can be cut into thin sections.

Removing calcium from bone or other hard tissue with acid or a chelating agent so it can be cut; excessive treatment harms nuclear detail and antigen staining.

A microtome housed inside a freezing cabinet, usually near −20 °C, used to cut unfixed frozen tissue for rapid diagnosis or for substances lost during processing.

A small perforated plastic basket that holds tissue during processing and embedding; it carries the specimen label and lets processing fluids move freely around the sample.

The micrometer setting on a microtome that controls how thick each slice is; routine paraffin sections for H&E are usually 3–5 micrometers.

Evenly spaced parallel scratches, wrinkles, or thick-thin bands in a section, caused by vibration of the block or blade during cutting.

A shallow water bath warmed to about 40–45 °C where cut paraffin ribbons spread and flatten before being lifted onto a glass slide.

Roughly cutting away excess wax and superficial tissue from a block until a full, representative face of the specimen is exposed for thin sectioning.

A basic dye obtained from logwood that, once oxidized and bound to a metal mordant, stains chromatin and other acidic cell components blue to purple.

An acidic pink-red dye that binds basic proteins, staining cytoplasm, collagen, and red blood cells in shades of pink to orange, giving contrast to blue nuclei.

A metal ion, usually aluminum in hematoxylin solutions, that chemically bridges dye and tissue so the dye binds strongly enough to color nuclei.

A step that raises the pH of hematoxylin-stained slides, usually in running tap water or a weak alkaline solution, turning the reddish dye complex into a stable blue.

Deliberately washing stained slides in acid alcohol or dilute acid to pull excess dye out of the tissue, leaving crisp nuclear color and a clean background.

A method in which hematoxylin is left on the tissue just long enough to reach the wanted intensity, so no separate destaining step is needed afterward.

A method in which slides are overstained with hematoxylin and then partly destained with acid alcohol, letting the technologist control final nuclear intensity by eye.

A method in which periodic acid turns certain sugars into aldehydes that then react with Schiff reagent, coloring glycogen, basement membranes, mucin, and fungi magenta.

A stain that applies several acid dyes in sequence so collagen appears blue or green, muscle and cytoplasm red, and nuclei dark, useful for showing fibrosis.

A silver-based method in which tissue reduces silver salts onto reticulin fibers, showing them as black lines against a pale background; used to assess liver and marrow architecture.

A method that uses acid to free iron from storage proteins, which then reacts with ferrocyanide to form a blue pigment, demonstrating hemosiderin in tissue.

A dye that attaches to proteins folded as beta-pleated sheets and shows apple-green birefringence under polarized light, the standard stain for amyloid.

The property of some dyes to stain certain tissue materials a different hue from the dye solution itself, such as purple-red mast cell granules with toluidine blue.

The small specific site on a molecule that an antibody recognizes and binds; fixation and processing can alter or hide epitopes, so retrieval steps are often required.

A pretreatment that heats slides in buffer, often citrate at pH 6 or EDTA at pH 9, to reverse fixation-related masking so antibodies can reach their targets.

An antibody made by one cloned cell line that binds a single epitope, giving consistent specificity and low background between lots but sometimes less robust staining.

Naturally occurring peroxidase activity, mainly in red cells and neutrophils, that can react with the detection system and cause false staining; it is blocked with hydrogen peroxide.

A detection method where many enzyme molecules and antibodies are attached to a polymer chain, producing strong signal without biotin and avoiding biotin-related false staining.

A chromogen that peroxidase converts into a brown, alcohol-insoluble precipitate at the site of antibody binding, the most common color in immunohistochemistry.

A technique that shows where an enzyme is active by incubating tissue with a substrate that forms an insoluble colored product at that site, such as esterase or phosphatase methods.

Planned checks, such as staining known positive and negative tissue alongside patient slides, that show a procedure is working and results can be released.

A program that sends unknown samples to a laboratory and compares its results with those of other laboratories; required for accreditation and useful for finding systematic errors.

A standard document from a chemical supplier that lists hazards, safe handling, exposure limits, and first aid; it must be available to everyone using the chemical.

The OSHA limit for formaldehyde vapor: 0.75 ppm averaged over eight hours and 2 ppm for any 15-minute period, monitored where formalin is handled.

Scheduled cleaning, inspection, and servicing of instruments such as microtomes, processors, and stainers to prevent failures and keep sections and stains consistent.