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How does the primitive streak guide the migration of mesodermal cells in vertebrate embryos?

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embryos of different animals — Animal development The embryos of many animals appear similar to one another in the earliest stages of development and progress into their specialized forms in later stages.

In vertebrate embryos, such as those of birds, reptiles, and mammals, the primitive streak serves as the essential site for the formation and migration of the mesoderm.12

During gastrulation, the embryo consists of a disk-shaped group of cells, primarily the epiblast. The primitive streak develops as a thickened, elongated seam along the midline of this epiblast. Mesodermal cells are generated when epiblast cells migrate toward this midline, turn downward, and sink into the interior of the embryo.12

Once these cells emerge beneath the epiblast, they move laterally—spreading out to the left and right—to occupy the space between the newly formed ectoderm and the endoderm. Additionally, cells proliferating from the anterior end of the streak, known as the Hensen’s node (or primitive knot), migrate forward to form the notochord, which establishes the longitudinal axis of the developing embryo. This process ensures that the germ layers are positioned correctly to begin the subsequent stage of organogenesis.321