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) immediately after implantation of the early mouse embryo can be seen as the earliest example of an epithelio-mesenchyme transition (EMT) in murine development. Since EMT and EMI (epithelium-mesenchyme interactions) are at the very heart of morphogenesis, identifying molecular mechanisms governing these processes is of utmost importance. An excellent in vitro model system to study PE formation, i.e. F9 embryonal carcinoma cells, is available to this end. In the present paper we review our own recent results and those of others using these cells, and present our current view on the molecular mechanisms involved in PE formation.