EXEL · Formation, maintenance and differentiation of the extraembryonic endoderm lineage""

Formation, maintenance and differentiation of the extraembryonic endoderm lineage

Cell-fate specification and differentiation, which determine the cellular identity, are central issues for developmental, stem-cell and tumour biology, and therefore relevant for applications in regenerative medicine and cancer therapy. Early mammalian embryo is a model of choice to study these processes and derivation of stem cell lines from the first embryonic cell lineages has generated an increasing interest for this period of development. Indeed, it is a relatively simple model consisting of a limited number of cells and cell types (~100 cells and three lineages at the time of implantation). Nevertheless, it represents an integrated paradigm where the complex processes of cell specification/differentiation, cell proliferation, programmed cell death, cell migration, etc. are all at work to establish distinct cell lineages. In addition, preimplantation mouse embryos can easily develop ex vivo for 3-4 days in defined culture conditions. It is thus possible to assay the function of signalling pathways by supplementing culture medium with ligands and/or pharmacological inhibitors. Moreover, numerous methods and tools now provide the ability to engineer genetically modified mice and to follow morphogenetic events by time-lapse imaging. There are also cell culture models derived from each of the three cell lineages present in the mouse blastocyst, thus allowing investigations to be conducted both in vivo and in vitro. Embryonic stem (ES) cells were derived more than 25 years ago and have been a pillar of developmental biology and stem cell biology studies ever since. TS cells and XEN cells, that derive from the trophoblast and from the PrE lineages, respectively, are more recent innovations, at the centre of much of the excitement the field of preimplantation development is currently generating. Access to cell culture models allows approaches that the scarcity of embryonic material previously forbade. EXEL project was designed to get a better understanding of the molecular mechanisms driving the maintenance and differentiation of two cell lineages, the epiblast (Epi) and the primitive endoderm (PrE), from a common progenitor. The epiblast is pluripotent and will give rise to the embryo proper while the primitive endoderm will give rise to the extraembryonic yolk sac. The PrE is a critical and fascinating tissue that has been long neglected because of its extraembryonic origin. However, it is now evident that it assumes complex roles, providing nutrients to sustain development and signals to control differentiation of embryonic tissues and pattern the embryo. EXEL project focused on the biology of the PrE and combined in vivo and in vitro innovative strategies at the interface between stem cell et developmental biology. The first aim of the project was to dissect the signalling pathways involved in PrE versus EPI formation. We have determined the distinct roles of several receptor tyrosine kinases including the Fibroblast Growth Factor (FGF) receptor in the specification of the PrE and the Platelet Derived Growth Factor (PDGF) receptor in the survival of PrE cells. These data are currently extended to other mammalian species. The second aim of the project was to identify new signalling pathways involved in the differentiation of the PrE. To do this, we used XEN stem cells as an in vitro model of PrE. We found that Wnt signalling is an important player in the regulation of XEN cell homeostasis. Taken together, EXEL project has contributed to a better understanding of developmental and stem cell biology of the first embryonic lineages and may also provide in the future new options in regenerative medicine.

Data: CORDIS, © European Union