The stain shown in the image is most likely which dye?

• A. Toluidine blue
• B. Hematoxylin
• C. Sirius red
• D. Van Gieson's

Answer: (A)

Toluidine blue demonstrates metachromasia, a key concept when identifying this stain. It’s a basic dye that binds strongly to acidic tissue components, especially sulfated glycosaminoglycans found in cartilage matrix and mast cell granules. In tissues rich in these substances, the dye molecules aggregate and cause a color shift from blue to purple or reddish-purple. That metachromatic purple color is a distinctive feature of toluidine blue staining. The other dyes produce more straightforward, unshifted colors: hematoxylin stains nuclei blue; Sirius red stains collagen fibers red; Van Gieson gives a pink collagen and yellow/red muscle contrast. The observed purple coloration points to toluidine blue as the dye responsible.