During vertebrate embryogenesis, the primary body axis grows posteriorly and is concomitantly segmented into somites, the precursors of the vertebral column, skeletal muscle, and dermis. The somites arise sequentially, with the anterior somites that give rise to the cervical vertebrae created early. The more posterior somites that become the thoracic, lumbar, and sacral vertebrae form at progressively later times. During the axis elongation period, the embryo must parse the somite precursors appropriately so that there are enough cells remaining to make the most posterior somites at the end of somitogenesis.How the embryo allocates these cells is not well understood. However, in this issue, Szeto and Kimelman (2006) address this question by showing that cells are specified to give rise to anterior trunk, posterior trunk, and tail somites. They find that this cell fate decision occurs surprisingly early in zebrafish development, prior to gastrulation, in response to nodal, fgf, and bmp signaling (Fig. 1; Szeto and Kimelman 2006). Their data link the processes of mesoderm induction and patterning with vertebrate segmentation and elucidate a mechanism by which the embryo reserves a somite precursor population for the formation of the more posterior body segments.
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