The Agnathan Enteropancreatic Endocrine System: Phylogenetic and Ontogenetic Histories, Structure, and Function

The extant jawless fishes (Agnatha) include the hagfishes andlampreys whose ancestry can be traced through a conserved evolutionto the earliest of vertebrates. This review traces the studyof the enteropancreatic (EP), endocrine cells and their productsin hagfishes and lampreys over the past two centuries. ErikaPlisetskaya is one of several prominent comparative endocrinologistswho studied the development, distribution or function of theagnathan EP system. Her physiological studies in Russia laidthe foundation for her subsequent isolation in North Americaof the first lamprey EP peptides (insulin and somatostatin)and providing the first homologous radioimmunoassay for agnathan(lamprey) insulin. This review also emphasizes the nature andthe method of development of the agnathan endocrine pancreas(islet organ), for it reflects the earliest vertebrate endocrinepancreas originating from intestinal and/or bile-duct epithelia.The lamprey life cycle includes a protracted larval period anda metamorphosis when the adult EP system develops. Differencesin morphogenesis during metamorphosis of southern- and northern-hemispherelampreys dictate that a single cranial mass (islet organ) appearin the former and both a cranial and a caudal principal isletcomprises most of the islet organ in holarctic species. Thereare differences in distribution of cell types and in the primarystructure of the peptides in the definitive islet organ of hagfishesand lampreys. The primary structures of insulin, somatostatins,glucagons, glucagon-like peptide, and peptide tyrosine tyrosineare now available for three lamprey species representing threegenera and two of the three families. Differences in structureof peptides within, and between, families is providing supportfor earlier views on the time of divergence of the familiesand the different genera. It is concluded that due to the ancientlineage and successful habitation of lampreys and hagfishes,and the importance of the EP system to their survival, thattheir EP systems should be a research focus well into the nextcentury.