Anterior Digestive Tract of Adult Nile Tilapia Oreochromis niloticus: morphological characterization

Aquaculture is currently one of the fastest growing food-producing systems due to its relevance as source of livelihood worldwide. Nile tilapia (Oreochromis niloticus), an omnivorous fish, is the third most farmed fish species, due to its rapid growth rate and ability to adapt to a wide range of environmental conditions, being appreciated by consumers for the softness of muscles, low fat content and good flavour [1]. The aim of this work is an integrated description of esophagus and stomach morphology, combining scanning electron (SEM) and optical microscopy with a histochemical characterization of complex carbohydrates produced by these tracts [2]. Ten adult female Nile tilapia, with an average weight of 200 g and 20 cm long, were used. Animal care and killing procedures were in accordance with legislative decrees concerning the protection of farmed animals. After 24 hours (h) fast, fish were suppressed at lower temperature. Samples of esophagus and stomach were removed, washed in phosphate buffer (PBS) pH 7.4 and fixed a) in Bouin’s liquid for 24 h at room temperature for morphological and basic histochemical analyses; b) in 2.5% glutaraldehyde solution in PBS pH 7.4 for 24 h at 4° C for SEM analyses. A morphometrical evaluation of the organs’ wall was performed, measuring the thickness of the superficial and glandular epithelium, submucosa, and inner and outer musculature of the stomach by digital imaging and Image J software. We highlighted the presence of five different portions in the stomach: 1) esophagus-gastric lumen passage; 2) descending glandular; 3) fundic; 4) ascending glandular; 5) gastric-pyloric transition. Morphometrical analysis of muscle layers showed that the outer layer displays a constant thickness exerting a key role in food stirring, while, the inner layer, which significantly increases along the stomach, ensures the progression of digested feed. Histochemical investigation showed the presence of carboxylated and sulphated mucins along the esophagus and stomach capable of attracting a great amount of water and enhancing the protective function of the mucosa mucous gel, especially in the highly acidic stomach environment. Our study provides new insights concerning the morphological structure of the anterior digestive tract of Nile tilapia, relevant to better understand the several aspects related to physiological growth and the health status of this species, aimed at improving production rates while also increasing animal welfare.