Recent cell lineage and knockout experiments have raised the possibility that pre-gastrulation movements of cells from the extra-embryonic visceral endoderm are important to determine where the future anterior and posterior of the embryo will develop. These movements are indeed asymmetric, however it remains unclear how this is revealed at the cellular level and how this is regulated by molecular signalling. To gain insight into cell behaviour during the implantation period, which cannot be reproduced in vitro, the general approach proposed here consists of labelling cells before implantation, at the blastocyst stage, and monitoring their distribution after implantation, in cultures of embryos between days 5.5 and 6.5. I am interested in characterising whether the asymmetry of the apparent movements of visceral endoderma/ ce//s result from active cell migration, differential proliferation or cell rearrangements, based on time-lapse following of fluorescent labelled cells. I also aim to understand the relationships between the movement of visceral endoderm! cells in the different regions of the egg cylinder : a transgenic line expressing GFP under the control of the Cer-lpromoter will be used, as a marker of the movement of cells from the distal visceral endoderm. Lastly I shall examine which components of these morph genetic cell movements are important for the establishment of anterior-posterior patterning. To this end RNA interference with the expression of Wnt signalling, which is important in anterior-posterior patterning and which acts through different pathways, will be carried out in clones of visceral endoderma/ cells. Their behaviour will be analysed together with the induction of axial markers. The project fits well the strategic objectives of the European Community.