The Development of Form and Function in Fishes and the Question of Larval Adaptation

Functional Development of the Liver and Exocrine Pancreas in Teleost Fish

Katja Hoehne-Reitan and Elin Kjørsvik

doi: https://doi.org/10.47886/9781888569582.ch2

Abstract.—The liver and bile system, as well as the exocrine pancreas, with their digestive enzymes are of fundamental importance for metabolic processes and digestion of exogenous food in developing fish. The pancreatic tissue, liver hepatocytes, the gallbladder, and bile system develop when embryos/larvae are still solely depending on endogenous yolk reserves as sources for metabolic energy and structural components. Due to the central role of the liver in different metabolic pathways, its structure clearly reflects the nutritional status and condition in feeding larvae. Stored glycogen and synthesis of lipoproteins in the liver have been reported during the endotrophic phase for different species. Bile components are synthesized in the liver and excreted at first feeding and thereby contribute to improved lipid digestion and uptake in the gut epithelium cells. The hepatic energy stores respond sensitively to nutritional changes and physiological unbalances. The exocrine pancreas is functional at the time of hatching or mouth opening, which is well documented by histological localization of zymogen granules, as well as by immunochemical and molecular detection of several pancreatic enzymes or their precursors. The later development is mainly characterized by quantitative growth, while no new structural elements develop. As in adults, fish larvae seem to possess the most important, if not all, pancreatic enzymes. Amylase, trypsinogen, chymotrypsinogen, and, more recently, phospholipase A₂ and bile salt-dependent lipase are some of the enzymes that have been detected at very early larval stages and show that fish larvae are able to digest ingested food. The developmental sequence of cell differentiation, like the synthesis of enzymes, and bile appear to be genetically programmed, while a dietary influence has been detected on both the organ structure and enzyme levels after the endotrophic phase. The shift from endogenous to exogenous stimulation occurs during the endo–exotrophic phase, and a decline or a stagnation of liver and pancreatic function is reported during this period for many pelagic marine fish larvae. Whether this is a normal development or an environmentally/dietary-caused delay in digestive functionality remains to be solved. Numerous in vitro studies have shown that the activity and secretion of pancreatic enzymes are affected in different degrees by food ration size, nutritional status, and dietary composition. The ingestion rate of food seems to regulate pancreatic responses at an earlier time than the dietary composition. Fish larvae, in general, are capable of digesting their food without being dependent upon exogenous enzymes from live prey in terms of digestive capacity.