Formation of the diffuse pancreas and the development of digestive enzyme synthesis in larvae of the Japanese flounder Paralichthys olivaceus

The development of the diffuse pancreas and the stage at which the pancreas acquired exocrine function was investigated in larvae of the Japanese flounder Paralichthys olivaceus. The larval primordial pancreas differentiated from the anterior part of intestine at 1 day post-hatching (dph). Pancreatic zymogens were first detected at 2 dph using an anti-eel trypsinogen antibody (anti-eTrg). There was strong staining at 3 dph (first feeding day), and zymogen granules were secreted from the pancreatic glandular cells at 3 dph. These results suggest that the larval pancreas begins to synthesize digestive enzymes at 2 dph, and that enzymes are secreted from 3 dph onwards. The pancreas, which is located at the boundary between the oesophagus and intestine, was a compact organ at 3 dph. It began to elongate posteriorly along veins on the intestine at 20 dph. After metamorphosis (45 dph), the pancreas was localized along the veins running toward the porta hepatis from the stomach, pyloric appendages and intestine. Pancreatic tissue had also begun to invade the liver along the hepatic portal vein, thereby forming a diffuse pancreas. Since the gastric glands of the stomach wall are also differentiated at metamorphosis, it is concluded that the digestive system of the flounder assumes the adult form in the early juvenile stage following metamorphosis.