Histology of Liver Vascular-Biliary Tracts and Associated Exocrine Pancreas in Guppies (Poecilia reticulata) at Three Post-Hatch Ages.

To describe the structural development of the liver in young guppies (Poecilia reticulata) and complement the adult characterisation, we conducted a histological study at 7, 17, and 28 days post-birth. We used serial sections to trace the organisation of blood vessels, bile ducts, and pancreatic tissue within and around the liver. At all ages, the liver was highly cellular and composed mainly of hepatocytes. Pigmented melanomacrophage centres or aggregates were not observed, whereas rodlet cells were rare and restricted to extrahepatic connective tissue. At 7 days, we identified four distinct venous entry points. This multi-afferent pattern persisted at Days 17 and 28. Arterioles remained restricted to the outer hilar region and were not observed within the liver. The gallbladder and common hepatic duct were recognisable at Day 7. In contrast, intrahepatic bile ducts were not discernible at Day 7, were scarce at Day 17, and were more frequent, although still limited, at Day 28. Exocrine pancreatic tissue was mainly extrahepatic at Day 7; intrahepatic pancreatic acini were detected in one fish at this age and were increasingly observed at Days 17 and 28 in association with afferent venous tracts. No vascular-biliary associations comparable to those described in adult guppies were observed. These findings establish a histological baseline for normal post-hatching liver development in guppies and provide a reference for future studies in comparative anatomy and of environmentally associated changes in liver structure during early life stages.