Phylogeny Of Vertebrate Pancreas

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Consideration of invertebrate and lower vertebrate organisms provides enormous insight into the evolution of the pancreas.6 Although no islets of Langerhans exist in any invertebrate, the insulin signaling pathway itself is highly conserved across a broad phylogenetic range. A range of invertebrates, from arthropods7 to worms8 to mollusks,9 have been found to express genes orthologous to mammalian insulins; in these invertebrates, insulin orthologs are typically expressed within cells of the nervous system (Figure 1.1). The Drosophila melanogaster genome, for example, contains seven insulin-like peptides and one identified insulin receptor. Loss of function mutations in the Drosophila insulin receptor produces a pheno-type similar to that observed in flies whose neuronal insulin producing cells have been ablated through a cytotoxic transgene.7,10 Both groups of flies experience growth retardation, as measured by the length of the larval stage and wing size, and carbohydrate levels in transgenic Droso-phila hemolymph are elevated during the brief fasting period of larval development, suggesting a conserved role in glucostasis for Drosophila insulin-like peptides.7 Insulin signaling has been recognized in Caenorhab-ditis elegans since the late 1990s and has been shown to play roles in

Reptiles Avians

Arthropods

Worms

Mollusks

Protochordates Cyclostomes Cartilaginous fish Bony fish (incl. teleosts) Amphibians

Reptiles Avians

Arthropods

Worms

Mollusks

Protochordates Cyclostomes Cartilaginous fish Bony fish (incl. teleosts) Amphibians

Mammals

Endocrine hormone producing cells within nervous system

Endocrine cells in intestinal tract Primitive islet organ Pancreatic organ with exocrine and endocrine compartments i

Figure 1.1 Schematic phylogenetic tree illustrating the evolutionary versions of the modern vertebrate pancreas. Animal phylogeny above coelenterate organisms is broadly divided into protostomian and deuterostomian lines. The protostomes include the arthropods, worms, and mollusks and possess endocrine hormone producing cells within their nervous systems (see text). In protochordates, primitive members of the deuterostomian line, endocrine hormone producing cells are no longer found in the nervous system, but are instead scattered throughout the intestinal tract. Cyclostomes, the most primitive vertebrates, possess a primitive islet organ but no exocrine parenchyma. A modern pancreas, with specialized exocrine and endocrine cells and a ductal system, is first seen in holocephan cartilaginous fish.

glucostasis and longevity. As in Drosophila, insulin-like ligands are produced within neuronal cell types in C. elegans.8

The evidence for conserved functions in these two organisms suggests that other invertebrate examples of endocrine hormone-producing cells may have functions analogous to their mammalian counterparts. The ancient colocalization of endocrine-like cells and neuronal tissue is echoed today in the observation that many transcription factors controlling pancreatic differentiation (e.g., ngn3, neuroD, Ptf1a) are also expressed, at least transiently, in the developing nervous system. The recent identification of short-lived Pdx1-regulated somatostatin expressing cells in the developing rat brain provides further evidence for this relationship.11

As one conceptually ascends the phylogenetic tree to protochordates, the immediate precursors to true vertebrates, these endocrine-like cells are no longer predominantly located in brain, but instead are found scattered throughout the intestinal mucosa.6 The first islet organ appears in hagfish and lampreys, primitive jawless fish of the cyclostome class. In both organisms, endocrine cells are primarily concentrated in a single organ attached to the bile duct (in hagfish) or gut (in lampreys), and tissue expressing exocrine zymogens are enmeshed within the liver.12 These organisms also bear the first recognizable thyroid and pituitary glands, suggesting a general trend toward condensation of endocrine tissue to form specialized endocrine organs. A modern pancreas is found in the next higher order of vertebrates, the holocephan cartilaginous fish. These possess a pancreas with an exocrine architecture, a main pancreatic duct, and organized islets.6 Ascending the phylogenetic tree further, one finds surprising conservation of embryonic pancreatic morphogenesis among a wide variety of higher vertebrates, from fish13 and reptiles14 to avians15 and mammals (Figure 1.1).5

In this review, we compare and contrast pancreatic development in a variety of vertebrate organisms. In so doing, we emphasize molecular regulatory factors required for normal cellular differentiation and morphogenesis. Even though details of morphogenesis may vary from species to species in association with evolutionary changes in body plan, the molecular mechanisms governing pancreatic development appear to be highly conserved.

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