The extracellular matrix is critical in guiding cell behavior in normal and pathologic tissues, particularly in cancers and fibrosis. Highlighting the organization (or disorganization) of a collagen-containing matrix can be very useful for understanding or grading pathophysiological conditions. The picrosirius red stain (also called "Sirius red" stain) is one of the best understood histochemical techniques able to selectively highlight collagen networks. Relatively inexpensive, the technique relies on the birefringent properties of collagen molecules. While the picrosirius red stain alone does not selectively bind collagen network, it becomes more specific than the other common collagen stains when combined with polarized light detection. This is why the selective histochemical procedure for collagen detection should be called the picrosirius-polarization method. In this chapter, we will provide essential explanation and detailed protocols and tips to allow collagen researchers not only to better understand how this staining technique works but also to easily apply this technique to their collagen-related research.
Keywords: Birefringence; Collagen; Orientation; Polarization; Sirius red.