Amyloids are proteins that gather to form deposits in organs such as the liver or kidneys but are commonly known for their formation of plaques in the brain that are associated with Alzheimer’s. The amyloid protein has the ability to fold and form what are known as beta-pleated sheets. These sheets give the amyloid protein its stable structure that makes it resistant to proteolysis (breakdown by enzymes). There are several stains that demonstrate amyloids, including Congo red, crystal violet, and Thioflavin T.
Congo Red works by hydrogen bonding to amyloid’s beta pleated sheet, which means that it can only be used for amyloids that have the beta pleated sheet intact and can give false negative results if used in old amyloid deposits that are losing their beta pleated sheet structure. This is also something that you need to keep in mind when it comes to fixation times for Congo Red staining as well, as excessive NBF fixation can diminish staining intensity as it is disrupting the beta pleated sheet structure.
When observed under a light microscope, the amyloid in Congo Red staining will appear pink/orange, nuclei will be blue, and connective tissue will be pale pink. Sections for Congo red staining should be cut at 7-8 microns so that the amyloid presence can be confirmed with apple green birefringence. This refers to the color of the amyloid when looked at it with a polarizing lens. If cut too thin or too thick, that birefringence will be blue or yellow and can give you a false negative.
So where did Congo red get its name? You would assume because the dye originated in the Congo, but that is actually not the case. Congo red dye has its origins in fabric design in Germany in the 1880’s where it was created by chemist Paul Böttinger. His employer, Friedrich Bayer Company, wasn’t interested in his discovery as they were trying to get a purple dye that could be used without a mordant, not a red dye. Böttinger finally found a taker for his dye, Berlin company Agfa, who named it Congo red as part of a marketing ploy.
At the time of the patent in 1885, Africa was the talk of the town in Germany, specifically how the Europeans were going to divvy up colonial Africa at the Berlin West Africa Conference. The Congo basin was a hot, contested commodity, so the marketers at Agfa decided to name their dye “Congo red” to capitalize on the interest in the area at the time. Nothing about the dye has anything to do with the Congo, but its bright red appearance conjured up images of an exotic far away land when compared with the drab garments the Europeans had been wearing.
In case history isn’t your thing and you’re more into German patent law, Congo red actually has an important place there too! Remember Bayer’s rejection of Böttinger’s dye? Well after Agfa’s success with their “Congo” red, Bayer made their own version which led to Agfa suing them for patent infringement. They settled their differences, but the Congo red decision led to the doctrine of technical effect. Essentially, the court decided that the inventiveness of the process of making something wasn’t the only grounds for a patent, it could be based alone on the technical and commercial value of the resulting product. After this case, you start to see a shift away from individual inventors, and it becomes economically feasible for companies to start large scale R&D departments that will hold patents for their resulting products, as long as they have technical and commercial value.
References:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2634529/
https://www.sciencedirect.com/topics/medicine-and-dentistry/amyloid
https://www.wipo.int/export/sites/www/meetings/en/2006/scp_of_ge_06/presentations/scp_of_ge_06_dutfield.pdf
https://bibliodarq.files.wordpress.com/2015/09/bijker-w-the-social-construction-of-technological-systems.pdf
http://www.chm.bris.ac.uk/motm/congo-red/congo-redh.htm