Organogenesis of the exocrine pancreas in the sea bass, Dicentrarchus labrax L., reared extensively and intensively

Abstract

Organogenesis of the exocrine pancreas in the sea bass, Dicentrarchus labrax, was found to comprise three main phases: (i) appearance of a primordium at hatching, in the form of a dorsal bud on the digestive tract which migrated to the right side, (ii) differentiation of exocrine cells and appearance of excretory ducts and blood vessels during the second half of the prelarval phase, and (iii) growth of the organ in larvae and juveniles. The pancreatic primordium appeared as a stratification of the dorsal wall of the digestive tract. Its migration was related to morphogenetic movements combined with twisting of the digestive tract. The first visible sign of exocrine cell differentiation was polarization. The first zymogen granules appeared 3 days after hatching and became abundant at the onset of the trophic phase, regardless of the rearing method. This synthesis coincided with the appearance of tubules and the common duct, but no excretion could yet be seen in their lumina. Until the time of mouth opening, the developmental sequence of cell differentiation did not depend on the rearing conditions and appeared to be genetically programmed. In contrast, the activity of the exocrine gland during the larval and juvenile periods appeared to depend more on dietary conditions. During these stages, the exocrine pancreas of intensively reared individuals fed ad libitum on Artemia was more active than that of extensively reared individuals, whose food was much less abundant. Larvae fed on artificial granules did not appear to secrete enzymes even though their digestive tract was full of food.