| Summary: | Tilapia is an omnivorous fish which uses different kinds of feed. The aim of this study was to evaluate the histomorphology of esophagus and intestine during the replacement of fish meal with blood powder in the diet of Nile tilapia (Oreochromis niloticus). After preparation, fish were fed with designed diets containing 0 (T0%), 25 (T25%), 50 (T50%), 75 (T75%), and 100% (T100%) blood powder for 8 weeks and each treatment was performed with three replications. After this period, the fish were anesthetized and after opening the abdominal area, 0.5 cm tissue samples were taken from the esophagus and intestine, fixed in 10% neutral buffered formalin. Then, tissue preparation steps were performed including dehydration with alcohol, clarification with xylol, blocking with paraffin, cutting 4-6 microns thick with microtome. After the hematoxylin-eosin and periodic acid-Schiff staining, the microscopic slides were examined under a light microscope. The results of measuring indices such as height and thickness of villi, the thickness of epithelium and muscle layer, and change in the number of mucous cells in the esophagus and intestine showed lower in T100% and T75% in comparison with the control (T0%) and other groups. Therefore, blood powder in the diet is somewhat tolerable and replaceable up to fifty percent for the species.
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