Stain uptake is dependent upon the affinity the tissue element has for the dye component of the stain solution. Affinity referes to the tendency of a dye to transfer from a staining solution onto the tissue section.
Factors that affect affinity and ultimately staining include the following:
Concentration of the dye - The greater the concentration of the dye, the more the dye is bound to tissue components.
Temperature - An increase in temperature increases the rate at which the dye diffuses throughout the tissue sample. It can also alter tissue components so that they are more receptive to dye penetration.
pH of the staining solution - Cells and other tissue elements often have an affinity for stains/dyes with specific pH ranges. Therefore, the pH of the staining solution can have a direct impact on the ability of a dye to bind with its intended tissue element.
Tissue fixation - Fixation alters and reorganizes certain molecular structures within the tissue sample such that there is increased permeability for staining. Unfixed tissue elements have limited binding sites for dyes.
Mordants - Mordants are chemicals that may be needed to bind dyes to certain tissue elements. They can also be used to intensify the staining results.
