IMMUNOCYTOCHEMICAL DETECTION OF ISLET HORMONES IN THE DIGESTIVE-SYSTEM OF PROTOPTERUS ANNECTENS

The presence, distribution, and interrelationships of the four typical pancreatic islet hormones were investigated in the digestive system o f Protopterus annectens by single and double immunohistochemical metho ds. Insulin-, glucagon-, and somatostatin-immunoreactive (IR) elements were detected in both the pancreas and the gut. Pancreatic polypeptid e (PP)-IR endocrine cells were always present in the gut, but were onl y present in the pancreas of a few specimens. Some of the latter cells also seemed to react with glucagon antiserum. In the pancreas the imm unopositive cells were organized into islets of different sizes, and t heir organizations were studied by the double immunohistochemical tech niques. In the few large islets insulin-IR cells were present in the c entral zone, glucagon- and PP-IR cells at the periphery, and somatosta tin-IR cells intermingled with both the peripheral and the central end ocrine cells. In the smaller islets, the number and the staining inten sity of glucagon- and PP-IR endocrine cells varied markedly. In the gu t, insulin-, somatostatin-, and PP-IR cells were of the open type; glu cagon-containing cells were very few and had no luminal contact. They were differently distributed along the intestinal epithelium. Somatost atin-IR nerve fibers and somatostatin-IR neuron cell bodies were also observed in the intestinal wall. The organization of pancreatic endocr ine cells in P. annectens is similar to that observed in the majority of teleosts even if a different topographical association can be found . Furthermore, islets of different sizes seem to display a different m etabolic turnover, and the detection of pancreatic PP-immunoreactivity varied according to the specimens utilized. In the intestinal portion insulin-IR cells, in addition to PP-, glucagon- and somatostatin-IR c ells are present: this suggests that intestinal insulin-like immunorea ctivity may be more widespread than previously supposed. (C) 1996 Acad emic Press, Inc