A Histological study on the development of the digestive tract of Caspian salmon, Salmo trutta caspius (Kessleri), from hatching to parr stage

Histological development of digestive tract of the Caspian salmon, Salmo trutta caspius, from hatching to 6 month post-hatching was studied using light microscope. The digestive tract of newly hatched larvae was a simple, undifferentiated tube which continued from mouth to anus. In 10 days post hatching larvae, the alimentary canal became more differentiated and secretion was started activity, with appearance of goblet cells in epithelium of the mouth, pharynx and esophagus. A thin layer of neutral mucopolysaccharid components covered the digestive tract's epithelium in this stage. At the onset of exogenous feeding (25 days post hatch, dph, at 70C), the digestive tract became completely differentiated into mouth, pharynx, esophagus, glandular and non-glandular stomach, primary pyloric cacea, anterior and posterior parts of the intestine and rectum, similar to those of juveniles and adults. The alimentary canal was structurally completed and lined by a stratified squamous epithelium in the mouth, pharynx and rudimentry part of the esophagus at 25 dph, while the terminal part of esophagus, the stomach and the intestine were lined by a columnar epithelium. During this period, striated muscle layer became distinct from mouth to esophagus, while smooth muscle layer was appeared at the stomach and intestine. This pattern did not change up to six month post hatch, except in development of the digestive organs. Histological analysis of the present study prepare some basic scientific knowledge on the Caspian salmon's digestive histology which may be used in further studies on digestive system, such as histochemical studies.