A postmortem heart section from a 62-year-old man (stained with hematoxylin and eosin) is pictured, along with a special stain highlighting the pathology.
The hematoxylin and eosin image (taken at 200x original magnification) shows somewhat shrunken-appearing cardiomyocytes surrounded by lace-like, eosinophilic extracellular material. The differential, based on the H&E image, includes interstitial fibrosis and amyloidosis. This differential can be quite challenging, especially when there’s little material to evaluate.
The classic stain used to confirm amyloid deposition is Congo red, which intercalates into the beta-pleated sheets of amyloid proteins and is held there via hydrogen bonds. Congophilic material that also demonstrates birefringence under cross-polarized light is considered specific for a diagnosis of amyloid (see image).
The downside is that Congo red is a technically challenging stain to perform and read. Overstaining can result in non-specific deposition; under-staining can cause a lack of sensitivity. Moreover, collagen will polarize (adding to the confusion), but its polarization is more of a whitish color rather than the birefringence of amyloid.
Sulfated Alcian blue is the special stain pictured in the question. Alcian blue stains mucopolysaccrides, not amyloid itself. Nevertheless, in organ systems with minimal naturally occurring glycoaminoglycans, it can be a very useful tool to highlight amyloid.
Amyloid is composed of a misfolded extracellular fibrillary protein, but it also contains other material including a non-fibrillary glycoprotein: (serum) amyloid P (SAP) component. SAP binds to glycoaminoglycans and other cellular components, incorporating those into the amyloid matrix. Thus, sulfated Alcian blue stains the mucopolysaccrides that are associated with amyloid deposits, but not amyloid itself.
For this reason, sulfated Alcian blue is a sensitive stain in the heart for amyloid (generally allowing for easy detection in tissue), but not specific. Confirmation via a Congo red stain is recommended, with its telltale birefringence.