Tracking Cell Migration during Muscle Formation in the X. laevis Embryo

During embryo development, mesoderm cells elongate and form transient structures called somites, which eventually give rise to muscle, vertebrae, and other tissues in the adult. To understand how mesoderm cells of the early gastrula embryo organize to become the muscle fibers of the somites, we tracked cell migration in Xenopus laevis embryos. We examined the movements that position these cells three dimensionally within somites and along the embryo’s axis using a cell-transplantation approach. We show that gastrula cells positioned in the upper lateral lip region (ULL) are directly adjacent to the notochord as they enter the presomitic mesoderm (PSM). These cells will eventually form muscle fibers in the central region of somites positioned along most of the anteroposterior axis. In contrast, cells positioned in the lower lip (LL) region of the gastrula migrate dorsally around the blastopore lip and appear to enter the PSM from the lateral edge. This population of cells then splits to flank ULL cells dorsally and ventrally to form muscle fibers in the dorsal and ventral quadrants of somites along the trunk and tail axis. Together, these results show that cells in the gastrula undergo different trajectories to give rise to muscle fibers positioned in distinct locations within somites and along the anteroposterior axis. These results offer new insights into how cells migrate to eventually form muscle tissue and vertebrae in the early embryo. Characterizing these embryonic migratory pathways adds to our understanding of congenital disorders such as spina bifida.