FUNCTION
The fish pancreas consists of an exocrine part, which secretes digestive enzymes, e.g. zymogen into the intestine and an endocrine part, the Islets of Langerhans which secrete hormones, insulin and glucagon.

ANATOMICAL CHARACTERISTICS
The exocrine pancreas is a diffuse organ in teleosts, extending over the surface of the intestine, along the portal vessels, and is interspersed with the adipose and other connective tissue and blood vessels. In salmonids it is primarily found diffusely distributed in the fatty tissue which surrounds the pyloric caecae. In some species, the exocrine pancreas surrounding the portal vessels extends into the liver and occasionally the spleen. Hence the name "hepatopancreas". The islets of Langherans are usually situated within the bounds of the exocrine pancreas, but not within the hepatopancreas. In some species, e.g. dab, Limanda limanda, cod, Gadus morhua, wolfish Anarkichas lupus, islets are very large and can be seen macroscopically in adult fish (Thomas 1975). These large islets are sometimes referred to as Brockman bodies.

HISTOLOGICAL CHARACTERISTICS
The exocrine pancreas is a compound acinar gland, with secretory units or acini scattered throughout the connective and fatty tissue which surrounds the pyloric caecae and spleen and /or within the liver .

Each acinus consists of a single layer of pyramidal broad based cells, that rest on a basal lamina. The nucleus of the acinar cell lies towards the base of the cell and the basophilic cytoplasm towards the lumen of the acinus contains zymogen granules that are acidophilic and refractile. These zymogen granules are secreted into the small central acinar lumen, which in turn lead into small pancreatic ducts. These ducts are composed of simple columnar epithelial cells surrounded by an outer connective tissue sheath which becomes thicker as the ducts get larger. A recent study by Einarsson & Davies (1997) in Atlantic salmon suggests that the pancreatic ductal system comprises a collective system, draining into one single duct, in combination with smaller ducts entering the intestine directly.

The endocrine components of the pancreas, the islets of Langerhans, are enclosed in a thin capsule and consist of poorly stained cords of fusiform cells with large distinct nuclei, interspersed with, blood sinuses. All vertebrate islets except those in cyclostome have three functionally independent cell type, alpha cells producing glucagon, beta cells producing insulin and delta cells. The islets are ductless glands.

(Diagram adapted from A..W. Ham. Histology 7th ed.)