PRINCIPLE:
As the name implies, three dyes are employed selectively
staining muscle, collagen fibers, fibrin, and erythrocytes. The general rule
in trichrome staining is that the less porous tissues are colored by the
smallest dye molecule; whenever a dye of large molecular size is able to
penetrate, it will always do so at the expense of the smaller molecule.
Others suggest that the tissue is stained first with the acid dye, Biebrich
Scarlet, which binds with the acidophilic tissue components. Then when
treated with the phospho acids, the less permeable components retain the
red, while the red is pulled out of the collagen. At the same time causing a
link with the collagen to bind with the aniline blue