Description
This specialized academic note on the Histochemistry of Amyloid and Amino Acids provides a detailed exploration of diagnostic staining techniques for tertiary students in Histopathology, Histotechnology, and Biomedical Science. Derived from authoritative texts including "Bancroft's Theory & Practice of Histological Techniques," the material first addresses amyloidosis, a disorder of protein folding, and details the gold standard diagnostic method using Congo red staining. It explains the principle of the stain, which relies on hydrogen bonding with the β-pleated sheet configuration of amyloid to produce characteristic red staining and pathognomonic green birefringence under polarized light. The note also covers alternative stains like Sirius Red and miscellaneous methods. The second section delves into histochemical methods for demonstrating specific protein-bound amino acids, including the Ninhydrin-Schiff method for α-amino groups, Millon's reaction and the Diazotization-Coupling method for tyrosine, the Performic Acid-Alcian Blue method for disulfide and sulfhydryl linkages, the DMAB-Nitrite method for tryptophan, and the Sakaguchi reaction for arginine. Each method is outlined with its aim, principle, and expected results. This comprehensive guide is essential for diploma and degree students and laboratory professionals to accurately identify amyloid deposits and localize specific amino acids within tissues for research and diagnostic purposes. Download this resource to enhance your expertise in advanced histochemical staining techniques.
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Histochemistry of Amyloid and Amino Acids Amyloid •Amyloidosis is a disorder of protein folding. •The deposited proteins (amyloid) →damage the structure and function of the tissues. •Occurs in other diseases e.g. Alzheimer’s disease and type 2 diabetes mellitus. •The diagnosis of amyloidosis requires presence of amyloid in a tissue, and the gold standard technique is Congo red histology. Demonstration of Amyloid •In routine H&E staining amyloid appears amorphous, eosinophilic, hyaline, extracellular substance. •Sometimes displays green birefringence under polarized light. •Collagen has same appearance under polarized light. •Other proteins are stained pink by eosin, hence not specific. Amyloid: homogeneous and eosinophilic (pink with H&E stain) Congo Red •Is a linear dye molecule. It binds to amyloid by hydrogen bonds. •Two factors are important to the Congo red- amyloid reaction: •Linearity of the dye molecule and •The β-pleated sheet configuration vIf the spatial configuration of either is altered, even though the chemical groupings are left intact, the reaction fails. Principle of Congo Red Staining Reaction •Congo Red dye forms hydrogen bonds with amyloid and red to green birefringence occurs when viewed by polarised light due to parallel alignment of the dye molecules on the linearly arranged amyloid fibrils. Congo Red •Disadvantage: its not specific for amyloid. •Any tissue component that binds Congo red in a linear way also exhibits green birefringence in polarized light. •E.g. collagen fibers •The specificity can be increased by: •Using an alkaline Congo method •Alcoholic method combined with high ion strength & High pH (The high pH enhances the non-polar hydrogen bonding of congo red and amyloid) •Green birefringence under polarized light. Congo Red Same field viewed by polarized light (crossed polarizer and analyzer). The characteristic ‘green birefringence’ is clearly seen against the dark background. Congo Red Results •Amyloid →red •Nuclei →blue Sirius Red •Sirius red F3B (direct red 80), is similar to Congo red •Also gives green birefringence with polarized light. •Gives a more intense staining reaction which is valuable for photographic purposes. Sirius Red Results Amyloid……rose red. Nuclei……...blue. Background…clear to pale pink. Miscellaneous Methods for Demonstrating Amyloid •Alcian blue •Toluidine blue •Periodic acid Schiff’s •Methenamine silver method of Haga. Amino acids •Are molecules containing one amino group, one carboxyl group, one H atom, and a side- chain (R group) that is specific to each amino acid. •R group varies in structure, size, electric charge and influence the solubility of amino acid in water. Amino acids •Amino acids are basic structural building blocks of protein. •Amino acid histochemical methods can demonstrate some amino acids rather than the whole protein. •E.g. protein bound amino group such as lysine (Ninhydrin Schiff method)
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