Trichrome stains, such as the Masson's trichrome, are named after the three dyes used in the staining procedure. These three dyes selectively stain muscle, collagen fibers, fibrin, and erythrocytes using acid-base chemistry.
Tissue sections are first placed in Bouin’s solution. Bouin’s solution is a mordant that links the dye to the targeted tissue components. Nuclei are stained with Weigert’s hematoxylin, an iron hematoxylin resistant to decolorization by the subsequent acidic staining solutions.
Biebrich scarlet-acid fuchsin solution stains all acidophilic tissue elements such as cytoplasm, muscle, and collagen. After applying phosphomolybdic/phosphotungstic acid, it is used as a decolorizer that causes the Biebrich scarlet-acid fuchsin to diffuse out of the collagen fibers, leaving the muscle cells stained red.
The phosphomolybdic/phosphotungstic acid also links the decolorized collagen and aniline blue, which is applied after this differentiation step. Following aniline blue staining, 1% acetic acid differentiates the tissue sections.
