Posthatching developmental changes involving the liver and intrahepatic pancreas of grass carp (Ctenopharyngodon idella) with a focus on the distribution pattern of pancreatic-biliary-vascular tracts.

The grass carp (Ctenopharyngodon idella) is a cyprinid fish frequently used for aquaculture, ecotoxicology, and developmental biology-related studies. In the present study, developmental features involving the liver and intrahepatic pancreas were studied in C. idella from 1-day post-hatching (1 dph) to 365 dph. Various techniques, including morphometry, light, scanning, and transmission electron microscopy, were employed. The liver primordium was detected along the dorsal aspect of the cranial part of the yolk sac at 1 dph. The right and left hepatic lobes were formed at 2 dph. The hepatic cords and gall bladder were formed at 4 dph. Several hepatocytes were differentiated at 10 dph. Kupffer cells were observed inside the hepatic sinusoids at 20 dph and became numerous at 90 dph, concomitant with the expansion of the liver and its intimate contact with the pancreas. Pancreatic acini started to penetrate the hepatic parenchyma along the course of the portal vein and main bile duct at 20 dph, and the intrahepatic pancreas was clear from 90 dph onwards. At 365 dph, the intrahepatic pancreatic acini surrounded the portal vein branches (afferent hepatic veins), forming pancreatic venous tracts (P-VT); afferent hepatic veins and bile duct radicals, forming pancreatic venous biliary tracts (P-VBT); and afferent hepatic veins, bile duct radicals, and hepatic artery branches, forming pancreatic venous biliary arterial tracts (P-VBAT). Ultrastructurally, zymogen granules of intrahepatic pancreatocytes displayed a spherical-shaped electron-dense appearance at 90 dph with an increase in density and size at 365 dph. A large proportion (∼50%) of hepatocytes contained eccentric nuclei and abundant glycogen granules and lipid droplets at 365 dph, reflecting the gain of high metabolic activity of the liver by this age. The present study reports important developmental features in the liver of C. idella and will help to understand the hepatic biology of the studied fish and other related species.