Insulin and other islet hormones (somatostatin, glucagon and PP) in the neuroendocrine system of some lower vertebrates and that of invertebrates - A minireview

Onto- and phylogenetical studies of the evolution of cells, producing regulatory peptides, belonging to the 'hormone families' of insulin, somatostatin, glucagon, and PP (the pancreatic polypeptide), have shown that the islets of Langerhans in vertebrates form a substantial part of the large neuroendocrine system (NES). The NES consists of three major parts, viz. (i) neuronal cells of the central and peripheral nervous systems, (ii) disseminated cells in the mucosa of the alimentary tract (and that of other hollow organs), (iii) the parenchymal cells of the classical endocrine glands. In the NES of coelenterates no evidence of islet hormone production has been obtained, so far. In invertebrates, belonging to the protostomian evolution line, the neuronal parts of the NES predominate markedly, and in the most highly developed phyla, such as artropods and molluscs, clear-cut evidence has been obtained for the presence of cells producing members of the islet hormone families. A 'brain-gut axis' for all the four islet hormones is well established in the NES of the pro-craniates, i.e. in the invertebrates of the deuterostomian evolution line. Here, the gut endocrine cells are cells of the disseminated type in the epithelium of the mucosa. A separate islet organ does not occur in the NES until the appearance of the first vertebrates, viz. the Agnatha, some 500 million years ago. Here, a grossly visible islet organ exists, free from exocrine, acinar, pancreatic parenchyma. It is a two-hormone organ with insulin and somatostatin cells only. The first pancreatic gland in evolution appears in the Holocephalan cartilaginous fish; it is equipped with large islets of Langerhans, containing - in addition to insulin and somatostatin - also glucagon-/glicentin-producing cells. The final evolutionary stage - the four-hormone islet parenchyma of mammalian type - is, however, reached already in the plagostomian cartilaginous fish (sharks, rays). Studies by means of analyses of the amino-acid sequences have revealed interesting features of the molecular evolution of some of the members of the four islet hormone families.