The orderly allocation of mesodermal cells to the extraembryonic structures and the anteroposterior axis during gastrulation of the mouse embryo

The prospective fate of cells in the primitive streak was examined at early , mid and late stages of mouse gastrula development to determine the order of allocation of primitive streak cells to the mesoderm of the extraembryon ic membranes and to the fetal tissues. At the early-streak stage, primitive streak cells contribute predominantly to tissues of the extraembryonic mes oderm as previously found. However, a surprising observation is that the er ythropoietic precursors of the yolk sac emerge earlier than the bulk of the vitelline endothelium, which is formed continuously throughout gastrula de velopment. This may suggest that the erythropoietic and the endothelial cel l lineages may arise independently of one another. Furthermore, the extraem bryonic mesoderm that is localized to the anterior and chorionic side of th e yolk sac is recruited ahead of that destined for the posterior and amnion ic side. For the mesodermal derivatives in the embryo, those destined for t he rostral structures such as heart and forebrain mesoderm ingress through the primitive streak early during a narrow window of development. They are then followed by those for the rest of the cranial mesoderm and lastly the paraxial and lateral mesoderm of the trunk. Results of this study, which re present snapshots of the types of precursor cells in the primitive streak, have provided a better delineation of the timing of allocation of the vario us mesodermal lineages to specific compartments in the extraembryonic membr anes and different locations in the embryonic anteroposterior axis.