Trichrome Stain
The trichrome stain is frequently used to demonstrate the presence of extracellular substances such as collagen. It is sometimes used in research investigation of soft and skeletal tissue, but is has minimal diagnostic applicabilities. As the name implies, the technique uses three dyes that stain nuclei, cytoplasm, and extracellular matrix, primarily the collagen.
This technique is rarely used in the diagnosis of tumors, but is frequently used for histomorphometric evaluation of mineralized versus nonmineralized osteoid in metabolic bone disease.
Stains for microorganisms
Of a long list of stains used to visualize infectious organisms, only a few are routinely used in diagnostic pathology.
Grams’s stain is used to classify bacteria into two major categories: those that retain crystal violet-iodine dye after alcohol and acetone treatment (gram positive) and those that are decolorized by this technique (gram negative).
The acid-fast stain (Ziehl-Neelsen) is used to document the presence of mycobacteria. The degree of resistance of acid hydrolysis depends on the lipid content in the wall of the bacteria. Bacteria with a high content of complex lipids retain carbolfuchsin after decolorization with acid alcohol.
Other stains occasionally used to document the presence of infectious organisms are
- Grocott’s silver Fungi
- Periodic acid-Schiff-Fungi
- Dieterle’s-spirochetes
- Warthin-Starry -Spirochetes and Rickettsiae.
Argentaffin and argyrophilic stains
Before the advent of immunohistochemistry, argentaffin and argyrophilic reactions were frequently used to disclose the neuroendocrine and neural differentiation of cells. In skeletal pathology, these reactions are used mainly in the diagnosis of metastatic tumors. The Fontana-Masson modification of the argentaffin reaction is used as a stain for melanin.
In the argentaffin reaction, the presence of the phenolic groups, mainly in catecholamines and indolamines, reduces the silver salt concentration and generates an insoluble black precipitate. The argyrophilic reaction requires an external reducing agent, usually hydroquinone or formalin.
The argyrophilic reaction produces the best results when tissue is fixed in formalin-free fixative such as Bouin’s solution. The most frequently used argyrophilic reaction in Grimelius stain.
Reticulin Stains
Reticulin fibers represent fine fibrillary material deposited in the intercellular matrix throughout the body. The major component of these fibres is type III collagen. The histochemical stains used to identify reticulin fibers are silver based, such as Gomori’s, Wilder’s, or Gordon and Sweet’s stains. These stains are nonspecific, and positive reaction is related to the presence of interstitial proteoglycans.
Therefore all elements rich in proteoglycans stain positive with these stains. Before the discovery of cell markers and immunohistochemistry, reticulin stains were used to disclose the patterns of reticulin fibers, to distinguish epithelial and nonepithelial neoplasia, and to subclassify some of the mesenchymal tumors. Epithelial neoplasms are characterized by the presence of reticulin fibers surrounding nests of tumor cells.
In mesenchymal tumors, reticulin fibers are sparsely distributed among individual tumor cells. In addition, some of the mesenchymal tumors have a distinct pattern of reticulin fibers (e.g., longitudinal parallel arrangement in tumors of Schwann cells). In fibrosarcoma, reticulin fibers encircle individual tumors cells. In the era of immuno-histochemistry, reticulin stains are seldom used for diagnostic purposes.
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