Tohru Suzuki

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

Abstract 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.

Quantification of exogenous protease derived from zooplankton in the intestine of Japanese sardine (Sardinops melanotictus) larvae

It has been suggested that exogenous enzymes derived from live food contribute to digestion in fish during the larval stage; however, the extent of this contribution is unclear. Therefore, we estimated the ratio of protease activity derived from zooplankton to total protease activity in sardine larva intestine. Larvae (10 days old) were starved for 24 h (0 h postfeeding; hpf), fed on rotifers for 2 h (2 hpf) and starved again for 10 h (12 hpf). Larvae were dissected into two parts; one part contained the pancreas, and the other part contained the intestine. The samples were analyzed for total protease activity using casein as a substrate and the protein content derived from rotifers by ELISA. Rotifer protease activity per unit weight of rotifer protein was also determined. Rotifer specific protease activity was estimated from these data by multiplying the rotifer protease activity per unit weight of rotifer protein by the amount of rotifer protein found in the intestine. While the total protease activity of the pancreas did not differ between 0, 2 and 12 hpf, that of the intestine was significantly higher at 2 hpf than at 0 and 12 hpf. At 2 hpf, the protease activity derived from rotifers was estimated to be 0.60% of the total protease activity in the intestine. Therefore, the contribution of exogenous protease to digestion in sardine larvae appears to be insignificant.