الفهرس 
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Abstract
In the present study, forty healthy early mature Nile tilapia (Oreochromis niloticus) fish of both sexes were utilized. The gill system excised rapidly after slaughter of the fish collected from ,,Abbasa,, fish farm at Sharkia Province.
The samples for light microscopy were immediately fixed in 10% buffered neutral formalin and processed for paraffin technique. Sections of 6 μm thickness were obtained. They underwent H & E stain, Crossmon’s trichrome stain, Masson’s trichrome stain, Orcein, Ab and PAS techniques. Another tissue pieces for scanning electron microscope were fixed in a buffered GA/FA fixative. Then processed till sputter-coated with gold and examined. Another minute (about 1 mm³) tissue pieces for transmission electron microscopy were fixed in a buffered GA/FA fixative. Then processed till ultrathin sections were cut and stained with uranyl acetate and lead citrate.
Gills present within two interconnected gill chambers. They were bounded ventrally by the mandible, dorsally by the roof of the oral and pharyngeal cavities and laterally by the operculum, while medially they were continuous with each other and laterally covered by the operculum. The gill system consisted of four pairs of gills. Each gill was semilunar in shape consisting of a gill arch that carried gill rakers on its concave aspect and gill filaments on its convex aspect.