| Summary: | Immunohistochemistry (IHC) is a valuable technique applied in many veterinary laboratories for both diagnosis and research of infectious and neoplastic diseases in a variety of animals, based on the demonstration of antigens (Ags) within tissue sections by means of specific antibodies (Abs). Once antigen–antibody (Ag-Ab) binding occurs, it is demonstrated with a colored histochemical reaction visible by light microscopy. As most commercial Abs used as tissue markers are directed against human Ags, veterinary pathologists face many challenges when performing IHC because of the diversity of species studied and no guarantees that Abs will cross-react among different species. When dealing with fish tissues these challenges are far more arduous, because of compelling differences between mammal and fish tissues, and cross-reactivity has not been extensively tested. The aim of this research was to test a large panel of commercial Abs commonly used in veterinary laboratories on a variety of fish tissues to verify their cross-reactivity, if any, when compared with mammal tissues. Different species were sampled, including marine (sea bream - Sparus aurata, sea bass - Dicentrarchus labrax, tub gurnard - Trigla lucerna, sole - Solea solea), cold freshwater (salmon - Salmo salar, trout - Oncorhynchus mykiss), warm freshwater (tench - Tinca tinca, catfish - Ictalurus melas, eel - Anguilla anguilla), ornamental fishes and Elasmobranchs. Various tissues (skin, gills, heart, muscle, liver, pancreas, kidney, spleen, stomach, intestine, gonads, brain, pineal gland, eye, thyroid) were formalin-fixed and paraffin-embedded. A tissue array was constructed for each fish, in order to have several tissues portions in the same paraffin block; replicate sections were cut at 4 mm and stained with a standard IHC protocol. The Abs tested were directed against epithelial (cytokeratin AE1/AE3, CK 5/6, CK 14, CK 19, E-cadherin and b-catenin), mesenchymal (vimentin, desmin, smooth-muscle actin, Von Willebrand factor VIII-RA), haemopoietic (CD3, ...
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