Do old dyes simply fade away? Or do they just move into the histopathology lab?
With thanks to SDC member Richard W Horobin, School of Life Sciences, University of Glasgow, Scotland, for writing this blog.
Old dyes? Just look in the Colour IndexTM and you may be surprised by how many dyes were first synthesised and manufactured in the nineteenth century. And, maybe, surprised again by realising how many of these dyes were never been manufactured on any significant scale, or were soon superseded by superior products, or were for various reasons abandoned as industrial colorants.
For instance, Azur B (CI 52010) was discovered by Bernsthen in 1885, but has probably never been used industrially. Congo red (CI 22120, discovered 1884), was an immediate commercial success, as it was the first direct red cotton dye. However the dye had problems, not least its acid–base indictor properties, and was replaced by alternative products. And consider Hematoxylin (CI 75290), a traditional natural dye, first isolated chemically by Chevreul in 1812. This was a significant textile dye, which however steadily lost market share through the nineteenth century — the ending of the Victorian enthusiasm for black silk being one commercial blow.
However, all three dyes became, and remain, widely used as “biological stains”. That is to say, they are used to selectively colour biological specimens to facilitate microscopic examination. Using such dyes, particular extra-cellular structures, particular types of cells, or particular sub-cellular structures may be distinguished by their being “colour coded” in distinct ways.
Thus, Azur B, as such or present as the key component of “polychromed” Methylene blue, is applied together with Eosin Y as a stain to aid the diagnosis of various haematological pathologies, and the detection of blood-born parasites. And, yes, Methylene blue (CI 52015, discovered 1876) has a number of current biomedical applications, but was never used on an industrial scale. Much the same can be said for Eosin Y (CI 45380, discovered 1881), although this dye has had many commercial applications. Congo red is currently in widespread laboratory use to detect amyloids. These strange protein derivatives result from the pathological rearrangement of normal proteins, and are found in such well-known medical disasters as Alzheimer’s dementia. Finally, Hematoxylin, or more specifically the aluminium or iron coordination complexes of its oxidation-product hematein, is today the dye most widely used as a microscopic stain in biology and medicine, most commonly to selectively stain cell nuclei. Finally, note that Eosin Y is widely applied along with Hematoxylin, as a colorant of the non-nuclear, high-protein components.
But what was that about histopathology? Although these dyes — and others, see below — are used by anatomists through to zoologists, by far the largest biomedical sales of such colorants are to histopathology laboratories. Here the microscopic observation of cells and tissues, obtained from patients, is used to aid diagnosis of disease. To facilitate detailed microscopic observation, the biopsies — whether of solid tissues such as muscles, or of biological fluids such as blood — must be made into the form of optically thin layers. But most human cells and tissues are almost colourless, even single red blood cells appear only faintly orange. Consequently colorants such as dyes are necessary to permit observation.
Current biomedical usage of five dyes — all originating in the nineteenth century, all abandoned as industrial dyes — has now been sketched. But are there more such dyes? Yes indeed. Taking the colorants currently certified by the Biological Stain Commission, an independent quality control organisation, as defining the more widely used dyes in biomedical laboratories, then a list of almost 40, from Acid fuchsine to Toluidine blue, is generated. Of these, one was discovered in the eighteenth century and one in the twentieth. The others were first produced in the nineteenth century, with three quarters of them first introduced in 1860–1890. The only twentieth century dye listed is Alcian blue 8G (CI 74240), discovered by Haddock and Wood in 1944. Industrially this dye provided a novel way of achieving high fastness on cotton. Unfortunately, a key intermediate proved to be carcinogenic, and production of Alcian blue by ICI Dyestuffs was abandoned. However the dye remains in use as a selective microscopic stain, for the microscopic demonstration of anionic polysaccharides and polysaccharide-protein compounds, such as chondroitin sulphate and glycoasaminoglycans. By the way, there are also many non-dye colorants used for biological staining, from labelled antibodies to fluorescent probes. But that’s another story.
So, some dyes at least don’t fade away, but instead have valuable second lives as imaging agents in biological and medicine — and, particularly, in the histopathology lab.
And finally, if you were wondering… the image at the top shows a routine paraffin section of the pancreas, with a cross-section of an artery in one corner. The section was stained with the coordination complex of hematein plus Eosin Y. This stain is traditionally, albeit inaccurately, known as Haematoxylin and Eosin, or more succinctly, as H & E. It was imaged in a light microscope.
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