Signals governing extraembryonic endoderm formation in the mouse: involvement of the type 1 parathyroid hormone-related peptide (PTHrP) receptor, p21Ras and cell adhesion molecules

The formation of parietal endoderm (PE) from primitive endoderm (PrE) immed iately after implantation of the early mouse embryo can be seen as the earl iest example of an epithelio-mesenchyme transition (EMT) in murine developm ent. Since EMT and EMI (epithelium-mesenchyme interactions) are at the very heart of morphogenesis, identifying molecular mechanisms governing these p rocesses is of utmost importance. An excellent in vitro model system to stu dy RE formation, i.e. F9 embryonal carcinoma cells, is available to this en d. In the present paper we review our own recent results and those of other s using these cells, and present our current view on the molecular mechanis ms involved in RE formation.