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-toventral 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. Developmental Dynamics 235:3268 -3279, 2006.

Disruption of axis specification leads to defects in dorsal tissue patterning and cell movements. Here, we examine how ␤-catenin coordinately affects gastrulation movements and dorsal mesoderm differentiation. The reduction of ␤-catenin protein levels by morpholino oligonucleotides complementary to ␤-catenin mRNA causes a disruption in gastrulation movements. Time-lapse imaging of ␤-catenin morphants during gastrulation reveals that involution occurs simultaneously around the blastopore in the absence of convergent extension cell movements. Transplantation experiments show that morphant cells grafted from the marginal zone into wild-type hosts differentiate into notochord and muscle. However, wild-type mesoderm cells grafted to the marginal zone of ␤-catenin morphants do not form dorsal tissues. These data argue that ␤-catenin is not required for the initial establishment of dorsal mesoderm cell competency, but it is required for the maintenance of that competency. We propose that tissue interactions that occur during convergent extension movements are necessary for maintaining dorsal tissue competency. Developmental Dynamics 236:3007-3019, 2007.