Fish do things differently. Fishes have evolved some organs that mammals haven't, one of them being a combination of the liver and pancreas into a single organ, the hepatopancreas.
This specimen in the image at left is from a fish, and the appearance is typical of the hepatopancreas. The liver portion (Hp) lacks the distinct lobulation typical of mammals, but the hepatocytes (which comprise most of its bulk) are reasonably similar in appearance and closely similar in function. The really striking thing about this arrangement is the "islands" of pancreatic material scattered throughout the liver. In fish, as in mammals, the liver and the pancreas arise as separate organs, but the growing pancreas "invades" the liver during larval development. Pancreatic tissue (P) proliferates along branches of the hepatic artery (BV) and eventually appears as you see it here, as scattered regions surrounded by liver.
In this image the relationships of the pancreatic tissue to the
liver and to the blood vessels are clearly seen. A blood vessel runs
right through the "island." The erythrocytes inside are
nucleated, as they are in all nonmammalian vertebrates!
The pancreatic cells themselves are virtually indistinguishable from their mammalian counterparts: densely basophilic basal regions and bright eosinophilic secretory granules. There is a duct system into which they empty, though you can't see it in this image.
Not all fish have a hepatopancreas exactly like this, though most do. Fishes are a very old group in
evolutionary terms, by far the oldest group of vertebrates. They have
had so many millions of years in a fairly stable environment that
adaptive radiation has produced some oddities in their anatomy and
physiology that might well have been weeded out in a terrestrial
setting. Some fish have their pancreatic tissue formed into discrete
organs; some have it scattered as clumps of parenchyma in the
mesenteries that support the intestine. There are even some that have
carried things to the logical extreme and have their pancreatic
tissue scattered diffusely in the mucosal lining of the intestine!
But the hepatopancreatic layout seems to have been the dominant
theme.
