Abstract
The fish's gastrointestinal system is considered the primary site of digestion and absorption of nutrients and has an important role in fish immunology. For that, the present investigation aimed to highlight the histology, histochemistry, and surface architecture of the anterior intestine of koi fish. In the current study, the morphological characteristics of the gut were observed in fifteen Koi fish specimens. The epithelium of the anterior intestine was composed of lymphocytes, goblet cells, enterocytes (simple columnar epithelium), and enteroendocrine cells. The enterocytes had a large number of large vesicles, which may be indicative of pinocytotic activity towards certain nutrients. Apart from their positive reaction to toluidine blue, the goblet cells also showed good reactions to PAS and alcian blue. Collagen and elastic fibers comprised the submucosa and lamina propria. In conclusion, the length and height of the mucosal folds of the anterior intestine serve as a substitute stomach, temporarily storing any food consumed.