Cell behaviors associated with somite segmentation and rotation in Xenopus laevis.

During vertebrate development the formation of somites is a critical step, as these structures will give rise to the vertebrae, muscle, and dermis. In Xenopus laevis, somitogenesis consists of the partitioning of the presomitic mesoderm into somites, which undergo a 90-degree rotation to become aligned parallel to the notochord. Using a membrane-targeted green fluorescent protein to visualize cell outlines, we examined the individual cell shape changes occurring during somitogenesis. We show that this process is the result of specific, coordinated cell behaviors beginning with the mediolateral elongation of cells in the anterior presomitic mesoderm and then the subsequent bending of these elongated cells to become oriented parallel with the notochord. By labeling a clonal population of paraxial mesoderm cells, we show that cells bend around their dorsoventral axis. Moreover, this cell bending correlates with an increase in the number of filopodial protrusions, which appear to be posteriorly directed toward the newly formed segmental boundary. By examining the formation of somites at various positions along the anteroposterior axis, we show that the general sequence of cell behaviors is the same; however, somite rotation in anterior somites is slower than in posterior somites. Lastly, this coordinated set of cell behaviors occurs in a dorsal-to-ventral progression within each somite such that cells in the dorsal aspect of the somite become aligned along the anteroposterior axis before cells in other regions of the same somite. Together, our data further define how these cell behaviors are temporally and spatially coordinated during somite segmentation and rotation.