The discovery of insulin by Banting and Best in the early 1920s marked the beginning of a new era of diabetes research that focused on the study of insulin and the biology of the hormone-producing cells of the pancreas. In the ensuing decades, although much was learned regarding the synthesis, structure, and action of insulin, the developmental origins of insulin-producing β cells remained ambiguous. In the 1970s, the long-held belief that β cells might arise from the neural crest was refuted by elegant interspecies cell transplantation studies, which demonstrated that radiolabeled quail neural crest cells do not populate pancreatic endocrine tissues in host chick embryos . This seminal finding, and the advent of techniques to manipulate the mouse genome, has resulted in a greater understanding of how the pancreas develops and matures. The lessons from developmental biology have also elicited speculation that maintenance of endocrine cell mass in the adult pancreas follows a similar paradigm, whereby a pool of proliferative progenitor cells differentiates as needed to repopulate the endocrine compartment. It is becoming increasingly recognized that processes that maintain or increase β-cell mass rely on signals distinct from those in the developing pancreas, and therefore the mechanisms of cell specification and maintenance differ depending upon the age of the organism. This chapter reviews the fundamentals of pancreas organogenesis, proceeds with an overview of cytodifferentiation with an emphasis on the β cell, and concludes with a review of β-cell growth and maintenance in the adult pancreas.
The pancreas is a compound digestive gland, comprised of both exocrine and endocrine components that secrete digestive enzymes and hormones, respectively. The exocrine component makes up approximately 95% of pancreas mass, and consists of acinar cells and duct cells. Acinar cells produce and secrete proteases, lipases, amylases, and nucleases that are necessary for the digestion of nutrients. Duct cells form a tubular network throughout the pancreas, and also secrete mucins and fluids that flush the acinar secretions to the intestine. The mature endocrine component of the pancreas, organized into structures known as the islets of Langerhans, comprises ∼2% of the total organ mass. In the adult, islets of Langerhans are composed primarily of five discrete hormone secreting cell types: α cells produce glucagon, β cells produce insulin and islet amyloid polypeptide, δ cells produce somatostatin, PP cells produce pancreatic polypeptide, and ϵ cells produce ghrelin. β Cells comprise the majority (60–80%) of the cells that make up the islet in vertebrate animals. The progression of pancreas development in vertebrates can be segmented into five major events (summarized in Figure 4.1): (1) induction of definitive endoderm, (2) formation of the primitive gut tube and patterning of endoderm into organ-specific progenitor zones, (3) induction of dorsal and ventral pancreatic buds, (4) outgrowth, branching, and fusion of the pancreatic buds, and (5) cytodifferentiation.
Induction of the definitive endoderm
During early development, pluripotent stem cells in the gastrula stage embryo evolve into the multipotent progenitor cells of the three primary germ layers known as ectoderm, mesoderm, and definitive endoderm. Broadly speaking, mesodermal derivatives provide support and movement, whereas ectodermal derivatives provide for sensation of, and protection from, the environment. The definitive endoderm is the innermost germ layer of metazoan embryos, and its derivatives mediate exchanges with the environment, including nutrient absorption and gas exchange. Definitive endoderm gives rise to the pancreas, liver, intestine and other digestive organs, as well as the thyroid and parathyroid glands and the respiratory tract.
Model systems must be employed to study the mechanisms of development. Considering its evolutionary proximity to humans and the wealth of sophisticated tools for genetic manipulation, the mouse (Musmusculus) is the most pervasively utilized model organism. However, much of what is known about endoderm induction and morphogenesis has been learned through the study of lower vertebrates. Model systems such as the zebrafish (Danio rerio), the African clawed frog (Xenopus laevis), and the chicken (Gallus gallus) each exhibit species-specific experimental advantages (see Table 4.1).
During gastrulation, cells delaminate from the epiblast cell layer, and ingress through a transitory structure called the primitive streak in amniotes (e.g. mammals and birds) or the marginal zone in fish and frog, and emerge as endoderm cells. Although the morphogenetic events of gastrulation vary widely in model organisms, these events are regulated by a conserved set of regulatory molecules. The most fundamental endoderm induction signal is Nodal, a secreted TGFβ-like molecule that is highly conserved in amniotes, amphibians, and fish. Nodal signaling is necessary and sufficient in all vertebrates to trigger a genetic cascade leading to endoderm formation . Nodal is expressed at the site of gastrulation, where it initially generates a transitory mesendodermal precursor cell population. This population is subsequently subdivided into mesoderm and endoderm by the morphogenetic properties of Nodal, with the highest levels of Nodal signaling producing endoderm. Expression of the gene encoding Nodal is initiated in divergent ways depending upon the species. In mice, the gene is induced by WNT signaling at the interface of embryonic and extra-embryonic tissues , whereas in zebrafish and frogs it is induced by maternally provided factors that are localized to the site of Nodal induction [4,5].However, in all vertebrates studied, Nodal expression is enhanced by high levels of WNT signaling and maintained by a positive paracrine feedback loop . The targets of Nodal include the Mix-like (Mixl) family of homeobox genes, the Foxa family of forkhead genes, and the high mobility group box gene Sox17. Together, these genes comprise an essential transcription factor network that stabilizes the endodermal fate while simultaneously segregating it from mesoderm [7,8].
Patterning of the gut tube into organ domains
All endoderm generated during gastrulation is not equivalent: cells acquire positional identity upon specification, which is dependent upon the time at which the cells are specified. The foregut endoderm is specified first, followed by midgut, then hindgut. As gastrulation concludes, the foregut expresses unique spatial markers, including Hhex, which is directly induced by Nodal, as well as Sox2 and Foxa2. These three transcription factors are essential for the development of foregut organs [2,9,10]. Moreover, the antero-posterior (A-P) patterning of the endoderm is dictated by the release of several signaling molecules from the mesoderm. In addition to Nodal, which induces anterior endoderm at the highest levels of expression and posterior fates at progressively lower levels, a core set of secreted signaling molecules are encountered by migrating cells. Fibroblast growth factors (FGFs), Wingless/Int (WNTs), bone morphogenetic proteins (BMPs), and retinoic acid (RA) are secreted from a posterior location, serve to suppress anterior fates, and to partition further the gut tube into regions that express genes encoding the crucial transcription factors Pdx1 (intermediate gut region) and Cdx1/2/4 (hindgut region) [3,8]. Moreover, as discussed later, this set of signaling molecules acts iteratively within the pancreatic endoderm throughout development to generate the mature pancreas.
Immediately after formation, the germ layers are arranged as three flat, stacked sheets of cells. The endoderm rapidly transitions to a tubular morphology and becomes enveloped by mesoderm. In amniotes, the morphological transformation begins with the folding together of the lateral edges of the anterior and posterior ends of the sheet to form foregut and hindgut pockets. The lateral edges of the endoderm sheet are then fused together by zipper-like morphogenetic behavior at the ventral midline that progressively seals from the anterior and posterior ends; this process ultimately generates a hollow gut tube. In zebrafish and frogs, the endodermal sheet first forms a solid rod through mass endodermal cell migration toward the midline.This rod later hollows by the mechanism of cavitation, which may involve lumen-directed fluid transport.
The foregut endoderm gives rise to multiple organs, including pancreas, liver, thyroid, and lung. The gut tube becomes subdivided into organ forming regions through reciprocal interactions between the endodermal cells and the adjacent mesenchyme, a process requiring FGF, BMP and RA signaling pathways. Explant studies using uncommitted mouse foregut originally revealed the role of FGF emanating from the septum transversum mesenchyme. Low levels of FGF led to expression of Pdx1 in ventral pancreatic progenitors, intermediate levels led to hepatic fates, whereas the highest levels specified lung and thyroid. Mouse genetic experiments later confirmed this model in vivo, and the requirement for FGF is conserved in both zebrafish and frogs [4,5,11]. BMP signals, which also originate from the septum transversum mesenchyme in mammals and from the lateral plate mesoderm in fish, regulate pancreatic specification [6,12]. BMP signals repress pancreatic development from a common pool of progenitors [7,8,13]. RA signaling biases endoderm to more posterior fates, and may have a role in partitioning the expression domains of the genes encoding Pdx1 (Pancreatic and duodenal homeobox 1) and Cdx1 (Caudal type homeobox 1), which are considered master regulators of pancreatic and intestinal fates, respectively. Furthermore, RA may have a direct role in dorsal bud induction, as both mice and zebrafish deficient in RA signaling lack the dorsal pancreas [14,15].
Induction of the dorsal and ventral pancreatic buds
The pancreas is derived from the merger of distinct buds that are induced dorsally and ventrally from the foregut endoderm by the adjacent mesoderm. In all vertebrates, both dorsal and ventral buds are marked by expression of Pdx1 and Ptf1a (Pancreatic transcription factor 1a), two transcription factors essential for proper development of all pancreatic cell types . The ventral bud is specified by low-level FGF signaling from the mesenchyme in the absence of BMP signaling, as described earlier. Dorsal bud fate is specified when FGF2 and Activin signals secreted from the notochord mitigate Sonic Hedgehog (Shh) expression in the dorsal midline of the gut tube . In contrast to the suppressive role of Shh in early pancreatic bud induction, Shh later plays a positive role in the expansion of the pancreatic epithelium and in β-cell function . Additionally, dorsal bud induction requires RA  as well as yet unidentified factors secreted from the dorsal aorta. In contrast to amniotes and frogs, dorsal fate specification likely occurs by a divergent mechanism in zebrafish, as Shh has a positive regulatory role in dorsal bud induction . Furthermore, signaling from the dorsal aorta does not appear to have a fundamental role in zebrafish, as cloche mutants of zebrafish that lack all vasculature still demonstrate normal bud induction .