SnapShot: Mouse Primitive Streak

Gastrulation is the central event in organizing the body plan of the embryo. It transforms a single layer of epithelial cells, called the epiblast, into the three germ layers—ectoderm, mesoderm, and endoderm—that give rise to all of the tissues and organs of the embryo. In addition to changes in gene expression that define the different germ layers, gastrula-tion is also an epithelial-to-mesenchymal transition (EMT) that generates migratory cells of the mesoderm and endoderm. The gastrulation EMT in the mouse takes place at a structure called the primitive streak (light blue) at the posterior pole of the embryo. Gastrulation in the mouse initiates at embryonic day 6.25 (e6.25) and continues for several days.Cells adopt different fates depending on when they leave the streak. The first cells to exit from the streak give rise to extraembryonic, cardiac, and cranial mesoderm, as well as definitive endoderm. Later cells give rise to lateral plate and then paraxial mesoderm. Thus, the primitive streak is a stable entity that continually proliferates and generates an ordered series of cell types through an EMT. Recent evidence suggests that, if cells are forced to undergo EMT, they adopt a stem cell-like state and now contribute to a variety of cell fates. This SnapShot depicts the signals and transcription factors, which are required for the establishment, maintenance, and termination of the primitive streak, and illustrates how this process endows stem cell-like fate on the cells that have delaminated from the epiblast.