2Six fish species studied in this book include: grass carp or white amur (Ctenopharyngodon idella), Nile catfish (Clarias gariepinus), Nile tilapia (Oreochromis niloticus), redtail shark (Epalzeorhynchos bicolor), guppy (Poecilia reticulata), and molly fish (Poecilia sphenops). Immediately after death or euthanasia, the tissue or organ is cut into small pieces and fixed in Bouin’s fluid for light microscopical studies. The fixed materials were further processed and sections were obtained at 3 µm and stained with Harris hematoxylin and eosin (HE). Hematoxylin stains the cell nucleus and other acidic structures in blue. In contrast, eosin stains the cytoplasmic proteins and a variety of extracellular structures from pink to red. Trichrome procedures help to differentiate collagen from muscle cells. In addition, Verhoff’s and Weigert’s elastica stains are ideal for elastic fibers. For histochemical staining, osmic acid reacts with fat to give a grey-black color; the periodic acid–Schiff (PAS) reaction and alcian blue (AB) identify varieties of glycosaminoglycans that are present in some tissues and cells. Grimelius silver impregnation displays some aspects of neuroendocrine cells. Iron HX and bromophenol blue are used to detect presence of proteins and Best’s carmine for detection of glycogen. For semithin section and electron microscopic studies, tissues were fixed in a mixture of 3% paraformaldehyde–glutaraldehyde fixative. Then the samples were processed and semithin sections are cut at 1 µm thickness and stained with toluidine blue for light microscopy. Ultrathin sections were cut at 70 nm and were stained with uranyl acetate and lead citrate and examined by JEOL 100CX II transmission electron microscope. For scanning electron microscopy, the tissues are washed in 0.1 M cacodylate buffer and transferred to a 1% solution of tannic acid. The samples were processed and mounted on aluminum stubs and sputter-coated with gold/palladium. The specimens were examined with a JEOL JSM-5400 LV Scanning Electron Microscope. Pictures of gross anatomy of longitudinal and surface sections of whole fish were provided.