Stem Cell Biology and Regenerative Medicine

The development of the pancreas and the pancreatic islets has been an area of particular scientific interest over the last several years as our attention has turned toward the possible engineering of progenitor cells and stem cells into pancreatic β cells and pancreatic islets. Pancreatic development is a highly complex process in which two morphologically distinct tissue types must derive from one simple epithelium. Although the parent endoderm from which the exocrine tissue (including acinar cells, centroacinar cells, and ducts) and the endocrine islets are derived appears to be homogeneous, it is clear that there are selected cells within the early endoderm that are destined to become either endocrine or nonendocrine lineages. The identification of these cells and the processes that determine whether or not they will become islets is of paramount importance to the engineering of stem cells into β cells. Moreover, there is a repertoire of events that allows these endocrine progenitor cells to disconnect from the epithelial lining during development. In this chapter we discuss the various key elements of basic pancreatic development. Specifically, we focus on the intercellular factors, such as growth factors, that may influence these developmental processes, as well as the important known intracellular transcription factors, which have been shown to establish a developmental hierarchy that determines lineage selection and cell fate. 1.1 Basic Pancreas Embryology and Development of Pancreatic Endocrine Cells The first morphologic evidence of the pancreas is a condensation of mesenchyme overlying the dorsal aspect of the endodermal gut tube in the foregut, just distal to the stomach, on the 26th day of gestation in humans and at approximately 9.5 days gestation in mice (approximately the 25th somite stage). Some 2–4 h later G.K. Gittes (B) Division of Pediatric Surgery, Children’s Hospital of Pittsburgh, Pittsburgh, PA 15213, USA e-mail: [email protected] 3 S. Efrat (ed.), Stem Cell Therapy for Diabetes, Stem Cell Biology and Regenerative Medicine, DOI 10.1007/978-1-60761-366-4_1, C © Humana Press, a part of Springer Science+Business Media, LLC 2010 4 G.K. Gittes et al. Fig. 1.1 Schematic representation of epithelial–mesenchymal interactions. Mesenchyme condensation at E9–E9.5 leads within the next 12–24 h to epithelium evagination. This is followed shortly by the onset of branching morphogenesis, which results in exclusion of most of the mesenchyme from within the clefts of the branched epithelium (see inset). This relative exclusion of mesenchyme may predispose to endocrine differentiation, since absence of contact with mesenchyme is thought to lead to endocrine differentiation (putative endocrine progenitor region shown in the green dotted-line box in the inset) there is evagination of the dorsal epithelium; then about 12 h later in the mouse and 6 days later in humans, the ventral bud begins to arise from the caudal aspect of the hepatic–biliary bud evagination. Ventral bud evagination occurs through a process that morphologically resembles that of the dorsal bud, but is regulated by a markedly different set of molecules. The pancreatic buds undergo a unique pattern of branching morphogenesis. Unlike the more typical right-angle outgrowth of branches seen in, for example, lung and kidney, the pancreas undergoes a more arboreal pattern with acute-angle branching. Thus, owing to the resulting proximity of adjacent branches to one another, this branching tends to exclude intervening mesenchyme (Fig. 1.1). This exclusion may in turn influence the amount of epithelial–mesenchymal contact, and hence lineage selection. Owing to gut rotation, together with elongation of the dorsal and ventral stalks, the two buds come into contact with one another within the forming C-loop of the duodenal anlage. This contact and subsequent fusion of the buds occur around E12– E13 in the mouse and on day 37–42 in humans. Coalescence of the two buds leads to formation of the future duct of Wirsung, whereas the future duct of Santorini 1 Pancreas and Islet Development 5 (smaller and accessory) originates from the proximal portion of the dorsal bud epithelium. Around E13–E14 in the mouse, dramatic changes occur in the cellular architecture of the pancreas, such as major amplification of endocrine cell numbers, particularly β cells (termed the secondary transition), and rapid branching morphogenesis with acinar cell differentiation. Glucagon-containing α cells are the first endocrine cell types seen in the mouse, at E9 (Pictet et al., 1972), whereas significant numbers of insulin-containing β cells are not typically seen until the secondary transition period. Hormone-positive cells in the epithelium typically lose connection with the epithelial lumen (possibly through a change in cell division polarity) from perpendicular to parallel to the basement membrane (Pictet, 1972). It has been suggested that this loss of epithelial connection parallels the epithelium-to-mesenchyme transformation that occurs in other tissues. Over the next several days the delaminating endocrine cells accumulate along the ducts and blood vessels in a linear pattern, often referred to as the “cord region” of the E14–E18 mouse pancreas. These “cordlike” endocrine cell collections coalesce over the next few days into aggregates that represent the first islets of Langerhans. In adults, the islets of Langerhans constitute approximately 1–2% of the mass of the pancreas and consist of β cells that produce insulin and amylin, α cells producing glucagon, δ cells producing somatostatin, PP cells producing pancreatic polypeptide, and cells producing ghrelin. 1.2 Early Tissue Interactions Pancreas development has classically been described as an epithelial–mesenchymal interaction, but earlier key tissue interactions occur before the appearance of the pancreatic mesenchyme.