1 Early Events of Somitogenesis in Higher Vertebrates: Allocation of Precursor Cells During Gastrulation and the Organization of a Meristic Pattern in the Paraxial Mesoderm

Through the fate mapping of mouse and avian embryos, the localization of the somite precursors in the germ layers, the primitive streak, and the tail bud has been extensively elucidated. A stage-by-stage study of the localization of the precursor population and the distribution of the clonal descendants of labeled or transplanted cells has enabled the reconstruction of the morphogenetic movement of the prospective somitic cells during gastrulation and early organogenesis. Studies mouse and avians have revealed substantial homology in the allocation of cell lineages, the morphogenesis of the paraxial mesoderm, and the specification of the somites, such that some generalization may be warranted for both species. Results of fate mapping experiments have shown that the primitive streak and the tail bud are continuous sources of new cells for somitogenesis. It is currently not known how this self-renewing population is maintained and how the prospective somitic cells are incorporated into the paraxial mesoderm. Of significant interest is how the somitogenic potential of this self-renewing population is regulated, especially toward the cessation of somitogenesis.