Toluidine blue

Toluidine blue is also a conventional stain. It is a blue-colored basic dye and, like hematoxylin, has a high affinity for negatively charged components of tissue. This stain is often used with plastic-embedded tissue sections. Peripheral nerve 1000x

Toluidine blue is also a conventional stain. It is a blue-colored basic dye and, like hematoxylin, has a high affinity for negatively charged components of tissue. This stain is often used with plastic-embedded tissue sections. Peripheral nerve 1000x

Toluidine blue is also a conventional stain. It is a blue-colored basic dye and, like hematoxylin, has a high affinity for negatively charged components of tissue. This stain is often used with plastic-embedded tissue sections. Peripheral nerve 1000x

The crisp, dark staining of the myelin sheath is enhanced by osmium tetroxide, which is used to fix lipid. Myelin membranes are uniquely high in lipid content.

Metachromasia results when toluidine blue, a basic dye, binds to structures with abundant polyanions (negatively charged molecules).  Due to the high density of the negative charges, the dye molecules are bound in close proximity and form aggregates with different absorptive properties from the single dye molecule. The secretory granules in these mast cells demonstrate metachromasia, staining magenta, not blue, indicating the high charge density of their contents (proteoglycans).