Introduction Research in our laboratory is focused on the developmental biology of the pancreas. We wish to understand how the pancreas normally develops and use that information to grow and develop pancreatic cells (Islets of Langerhans) in culture. One goal of this project is to explain how vertebrates make an organ from undifferentiated embryonic cells. Another longer-term goal has practical significance: if our studies are successful, it should be possible to apply our conclusions to human cells and provide a source of insulin producing ß cells for transplantation into diabetics. Our goals challenge us to understand the precursor or stem cells that give rise to the pancreas and to characterize the key gene products that specify cell fates and functions during organogenesis. To this end, we use several vertebrate organisms, including frogs, chickens, and zebrafish, but the majority of our studies are done with mice. We use a wide variety of techniques including functional genomics and gene arrays for gene discovery, tissue explants and grafting for analyzing inductive signals, and developmental genetics for direct assays of gene function. The aim of all our experiments is to understand the genes, cells, and tissues that direct pancreatic organogenesis. Our pursuit of these goals includes collaborations with other faculty in the department, notably the laboratory of Andrew McMahon. In addition, we have joint group meetings with the McMahon, Robertson, Dulac and Hunter labs. Development of the endoderm and pancreas The pancreas develops as an evagination from the embryonic endoderm. In mice and humans, two independent buds form as shown in the diagram below. One of the first steps required for pancreatic development is an inductive interaction between the endoderm (yellow cup) and adjacent mes/ectoderm (red/blue cup). This interaction sets up a prepattern in the endoderm for various organ-forming regions, including the pancreas, but pancreatic specific genes aren’t turned on at this stage. One possibility is that soluble factors secreted by the mes/ectoderm or endoderm itself set up the pre-pattern in the endoderm (step 1).
Subsequent inductive interactions (step 2) occur between the notochord (red rod) and the endodermal epithelium. These permissive inductions allow the pancreatic buds to emerge and continue development. About this time the first pancreatic specific genes are expressed. The three blue dots within the endodermal epithelium mark the expression of one such gene, Pdx1. When the epithelial sheet folds up to make a tube, the two lateral regions fuse to form the site where the ventral bud will emerge. The middle region forms the dorsal pancreatic bud. The two pancreatic buds (dorsal and ventral) require interactions with adjacent mesenchyme (red dots) for further pancreatic growth and differentiation (step 3). As development continues, the two buds merge to form one organ while exocrine and endocrine cytodifferentiation proceeds (step 4). For each of the steps outlined in the figure above, we aim to identify the genes that regulate development. In addition to work on genes that regulate pancreatic organogenesis, many of the projects in our lab are aimed at the identification of pancreatic precursor or stem cells. One goal of our work is to turn stem cells into islets or ß cells in culture. If we understood the gene products that signal precursor or stem cells to become islets, this information could also be used to treat patients directly by stimulating the growth and differentiation of new ß cells in vivo.
This figure shows a map for this approach. The lineage tree illustrates the idea that at each step of development, from egg or stem cell to a functional pancreas, decisions are made that affect the fate of cells. For example, an embryonic stem cell can be directed to one of the three germ layers, ectoderm, mesoderm or endoderm, the latter being the germ layer that will give rise to the pancreas. Subsequently, the endoderm is subdivided into different organ regions, including the thymus, lung, liver, stomach, intestines and pancreas. Once cells are set aside to form the pancreas, additional decisions are made to parse cells into the ductal, exocrine or endocrine lineages. An Islet of Langherans, a kind of "mini" endocrine organ, is shown as a confocal image in the figure. Work from our lab and others has identified genes that are involved in these various steps. Yet we still do not know all the genes or steps that would be needed to drive cells from their immature embryonic state at the top to a fully formed ß cell at the bottom. Nonetheless, progress in recent years and technological advances have shown that this is a feasible approach to producing pancreatic tissue ex vivo. Summary Our laboratory is investigating the normal development of the pancreas in order to understand how islets and ß cells are produced. We wish to understand the genes and stem cells needed for pancreatic organogenesis. In the longer term, we wish to use that information to direct the production of new islets, in vivo or ex vivo, for the treatment of diabetes. |
