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Short answer: a trichrome stain is a multi-dye stain that colours different components of a specimen in three contrasting colours. In tissue sections, Masson’s trichrome is used to highlight connective tissue, with collagen blue and nuclei blue-black. In stool smears, a different trichrome formulation is used to show intestinal protozoa. The two share a name and a three-colour logic, but their colours and purposes differ.
What are trichrome stains?
The name describes the outcome: three colours on one slide. Two quite separate laboratory uses carry the name, and they are often confused because search results mix them.
- Histology (tissue sections): Masson’s trichrome, also written Masson trichrome, Masson’s stain or, on some pages, “Mason’s” trichrome, is used to highlight connective tissue fibres in a tissue section (UNC Histology Research Core, Special Stains page, read 2026-11-07). A published 2026 paper describes it as a technique that differentiates collagen from other tissue elements (Husain et al., Health Science Reports 9(3):e71998, open access, read 2026-11-07).
- Parasitology (stool smears): the CDC DPDx stool staining page describes trichrome as producing a permanent stained smear that facilitates detection and identification of cysts and trophozoites of intestinal protozoa (CDC DPDx, read 2026-11-07).
Masson’s trichrome belongs to the special stains, which are applied when routine hematoxylin and eosin staining does not answer the question. For the wider family see histological staining and the histopathology workflow.
Masson’s trichrome: principle and colours
Masson’s trichrome combines an iron hematoxylin, a red dye mixture and a blue collagen dye on one section. The published protocol in Husain et al. (2026) uses formalin-fixed, paraffin-embedded rat skin and this reagent sequence, which is one laboratory’s protocol rather than a universal one:
| Reagent in the protocol | Stated purpose or role |
|---|---|
| Bouin’s solution (1 h at 60 °C) | Pre-treatment step listed first in the protocol |
| Weigert’s iron hematoxylin (10 min) | Hematoxylin step (nuclei end up blue-black per UNC) |
| Biebrich scarlet-acid fuchsin (5 min) | Red dye step (cytoplasm and muscle end up pink to red per UNC) |
| Phosphotungstic/phosphomolybdic acid (10 min) | Described by the authors as enhancing collagen differentiation |
| Aniline blue (5 min) | Stains collagen fibres |
| 1% acetic acid (1 min), then ethanol and xylene | Rinse, dehydration and clearing |
The expected result described by the UNC core facility is collagen blue, nuclei blue-black, and cytoplasm, keratin and muscle fibres pink to red. Manufacturer kits and modified versions such as Gomori’s trichrome vary in reagents and timings, so the instructions supplied with a given kit govern that kit.
What is the basic procedure of staining?
In general terms, tissue staining follows a fixed order: the section is fixed and processed to a slide, deparaffinised and rehydrated, stained with the dye sequence, differentiated or rinsed, dehydrated, cleared and coverslipped. That general order is visible in the reagent table above. This page does not give timings as a universal recipe, because they differ between kits, tissues and laboratories; use the instructions for use of the kit or the laboratory’s approved SOP. Fixation affects every later step, as covered in fixing cells.
Which stain is used for parasites?
For intestinal protozoa in stool, CDC DPDx names the trichrome stain as a permanent stain for the purpose, because small protozoa are often missed on wet mounts. The same page notes that it stains intestinal protozoa, human cells, yeast and artifact material, so it is a stain for the smear as a whole and not specific to one organism. For other parasitology stains and blood-film work, see Giemsa staining and the overview what is parasitology.
What is a protozoa smear with trichrome stain?
It is a fixed stool smear stained so that protozoan cysts and trophozoites can be examined under high-power oil immersion. According to the CDC DPDx overview, the workflow runs from the fixed smear through alcohol rinses, the trichrome solution, an acid-alcohol destain, dehydration and a clearing agent before examination; this is a summary, not a bench procedure. The same page states that a control slide of a known protozoan, for example Giardia from a preserved specimen, should be run each time. Reported appearances:
- Trophozoite cytoplasm: blue-green, sometimes with a purple tinge; cysts slightly more purple.
- Nuclei, inclusions, red blood cells, bacteria and Charcot-Leyden crystals: red, sometimes with a purple tinge.
- Glycogen dissolves and leaves a clear area; the background is usually green.
Tissue trichrome versus stool trichrome
This side-by-side comparison, built from the two sources above, is the quickest way to avoid mixing them up.
| Feature | Masson’s trichrome (tissue) | Trichrome (stool smear) |
|---|---|---|
| Specimen | Tissue section | Fixed stool smear |
| What it is used to show | Connective tissue fibres, collagen | Cysts and trophozoites of intestinal protozoa |
| Nuclei | Blue-black | Red |
| Cytoplasm | Pink to red (with muscle and keratin) | Blue-green, sometimes purple tinged |
| Background | Collagen is blue | Usually green |
| Source | UNC Histology Research Core | CDC DPDx |
Where it matters for labs
Because Masson’s trichrome is qualitative, Husain et al. note that its reproducibility and statistical comparison are limited. In practice that means recording the kit, lot and protocol used, and including a known positive control with each run, as the CDC advises for the stool method. Kit documents and a laboratory’s SOP, not this overview, define acceptance criteria.
Common misunderstandings
- “Trichrome” is one stain. It is a family; tissue and stool versions use different reagents and give different colours.
- Collagen is always blue. That is the expected result for Masson’s trichrome as described by UNC; other trichrome variants may differ, so follow the kit’s stated colours.
- Spelling variants are different stains. Masson, Masson’s and “Mason’s” in the sources refer to the same named technique.
- A stool trichrome identifies the organism by colour alone. The CDC notes the stain also colours human cells, yeast and artifact material, so morphology is still read by a trained examiner.
FAQ
What are trichrome stains?
Three-colour stains that separate components of a specimen by colour. Masson’s trichrome is the tissue version for collagen; a separate trichrome is used on stool smears for intestinal protozoa.
What is the basic procedure of staining?
Fix, section or smear, rehydrate where needed, apply the dyes in the stated order, rinse or differentiate, dehydrate, clear and coverslip. Exact timings come from the kit instructions or laboratory SOP.
Which stain is used for parasites?
For intestinal protozoa in stool, CDC DPDx describes the trichrome stain on a permanent smear.
What is a protozoa smear with trichrome stain?
A fixed stool smear stained with trichrome and examined under oil immersion, with cysts and trophozoites appearing blue-green to purple and nuclei red, alongside a control slide.
Sources
- UNC Histology Research Core, Special Stains: https://histologyresearchcorefacility.web.unc.edu/special-stains/ (read 2026-11-07)
- CDC DPDx, Staining of stool specimens: https://www.cdc.gov/dpdx/diagnosticprocedures/stool/staining.html (read 2026-11-07)
- Husain O, Wargasetia TL, Gunadi JW, Hamdan AR. Health Science Reports 2026;9(3):e71998, doi:10.1002/hsr2.71998, PMC12967519 (read 2026-11-07)
