In many organisms the allocation of primordial germ cells (PGCs) is determined by the inheritance of maternal factors deposited in the egg. However, in mammals, inductive cell interactions are required around gastrulation to establish the germ line. Here, we show that Bmp4 homozygous null embryos contain no PGCs. They also lack an allantois, an extraembryonic mesodermal tissue derived, like the PGCs, from precursors in the proximal epiblast. Heterozygotes have fewer PGCs than normal, due to a reduction in the size of the founding population and not to an effect on its subsequent expansion. Analysis of ␤-galactosidase activity in Bmp4 lacZneo embryos reveals that prior to gastrulation, Bmp4 is expressed in the extraembryonic ectoderm. Later, Bmp4 is expressed in the extraembryonic mesoderm, but not in PGCs. Chimera analysis indicates that it is the Bmp4 expression in the extraembryonic ectoderm that regulates the formation of allantois and primordial germ cell precursors, and the size of the founding population of PGCs. The initiation of the germ line in the mouse therefore depends on a secreted signal from the previously segregated, extraembryonic, trophectoderm lineage.

Pancreas oganogenesis comprises a coordinated and highly complex interplay of signaling events and transcriptional networks that guide a step-wise process of organ development from early bud specification all the way to the final mature organ state. Extensive research on pancreas development over the last few years, largely driven by a translational potential for pancreatic diseases (diabetes, pancreatic cancer, and so on), is markedly advancing our knowledge of these processes. It is a tenable goal that we will one day have a clear, complete picture of the transcriptional and signaling codes that control the entire organogenetic process, allowing us to apply this knowledge in a therapeutic context, by generating replacement cells in vitro, or perhaps one day to the whole organ in vivo. This review summarizes findings in the past 5 years that we feel are amongst the most significant in contributing to the deeper understanding of pancreas development. Rather than try to cover all aspects comprehensively, we have chosen to highlight interesting new concepts, and to discuss provocatively some of the more controversial findings or proposals. At the end of the review, we include a perspective section on how the whole pancreas differentiation process might be able to be unwound in a regulated fashion, or redirected, and suggest linkages to the possible reprogramming of other pancreatic cell-types in vivo, and to the optimization of the forward-directed-differentiation of human embryonic stem cells (hESC), or induced pluripotential cells (iPSC), towards mature b-cells. Developmental Dynamics 240:530-565,

E ndocrine and exocrine cells originate from a precursor epithelial cell during pancreatic organogenesis (1,2). Various differentiation factors are required to achieve the mature phenotype characteristic of islet ␤-cells. The use of a knockout mouse model for islet duodenal homeobox-1 (IDX-1) (also termed IPF-1/STF-1 and PDX-1) has significantly contributed to the elucidation of the specific role played by different genes in the differentiation of insulin-secreting cells. Mice lacking IDX-1 fail to develop a pancreas (3). Islet-1, a homeodomaincontaining protein, is necessary for the development of the dorsal pancreas and is required for the generation of islet cells (4). Inactivation of NeuroD/Beta2 or Pax4 genes cause a striking reduction in the number of insulin-producing cells and a failure to develop mature islets (5,6).Growth and differentiation of islet ␤-cells is not limited to the embryological state. A constant remodeling of size and function of the islets of Langerhans occurs during the entire life of individuals and is likely to play an essential role in the prevention of diabetes. In adult rats, two independent pathways are used for the proliferation of pancreatic endocrine cells. In the first pathway, new endocrine cells arise from the division and differentiation of cells within the islets, whereas in the second pathway of proliferation, the islets cells originate from precursor cells located in the pancreatic ductal epithelium (7). It is likely that a coordinated activation of multiple differentiation factorsin a fashion similar to the sequence of events occurring during fetal development-is required for the cellular growth of the endocrine pancreas of adults. The mechanism(s) for the activation of such a complex regulatory network in adulthood is not known. Recently, Xu et al. (8) demonstrated that an analog of the incretin hormone glucagonlike peptide (GLP)-1, termed exendin-4, was able to increase islet mass in adult animals previously subjected to subtotal pancreatectomy. Similarly, we recently demonstrated that the treatment of glucose-intolerant aging Wistar rats with GLP-1 restored normal glucose tolerance and induced islet cell proliferation (9). These studies suggest that exogenously administered stimuli are able, in vivo, to increase the mass of insulin-secreting cells and ameliorate glucose tolerance by inducing neogenesis of islet cells. In the present study, we investigated the ability of human recombinant GLP-1 to differentiate ductal epithelial cells into insulin-secreting cells.From the

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