Cell coherence during production of the presomitic mesoderm and somitogenesis in the mouse embryo.

In this study, we investigated (in the early mouse embryo) the clonal properties of precursor cells which contribute to the segmented myotome, a structure derived from the somites. We used the laacZ method of single cell-labelling to visualise clones born before segmentation and bilateralisation. We found that clones which contribute to several segments both unilateral and bilateral were regionalised along the mediolateral axis and that their mediolateral position was maintained in successive adjacent segments. Furthermore, clones contributed to all segments, from their most anterior to their most posterior borders. Therefore, it appears that mediolateral regionalisation of myotomal precursor cells is a property established before bilateralisation of the presomitic mesoderm and that coherent clonal growth accompanies cell dispersion along both the mediolateral and anteroposterior axes. These findings in the mouse correlate well with what is known in the chick, suggesting conservation of the mode of production and distribution of the cells of the presomitic mesoderm. However, in addition, we also found that the mediolateral contribution of a clone is already determined in the pool of self-renewing cells that produces the myotomal precursor cells and thus that this pool is itself regionalised. Finally, we found that bilateral clones exhibit symmetry in right and left sides in the embryo at all levels of the mediolateral axis of the myotome. All these properties indicate synchrony and symmetry of formation of the presomitic mesoderm on both sides of the embryo leading to formation of a static embryonic structure with few cell movements. We suggest that sequential production of groups of cells with an identical clonal origin for both sides of the embryo from a single pool of self-renewing cells, coupled with acquisition of static cell behaviour, could play a role in colinearity of expression of Hox genes and in the segmentation system of higher vertebrates.