Building the mouse gastrula: signals, asymmetry and lineages.

The mouse embryo is built by assembling the progenitors of various tissue types into a body plan. Early postimplantation development involves the establishment of anatomical asymmetries and regionalized gene expression in the conceptus, the specification of tissue lineages, and the coordination of cell movement for correct positioning of the lineage progenitors before and at gastrulation. Recent findings reveal that Wnt and Tgfbeta signalling function is instrumental in delineating the anterior-posterior embryonic axis by defining the site of primitive streak formation and by directing the movement of the visceral endoderm. These signalling activities are also required for the specification of anterior and posterior fates of the epiblast cells and for the induction and navigation of the primordial germ cells.