Editors' ChoiceDevelopment

Timing Isn't Everything

Science Signaling  18 Feb 2014:
Vol. 7, Issue 313, pp. ec48
DOI: 10.1126/scisignal.2005183

Somites are hollow balls of polarized mesoderm that form along both sides of the neural tube and give rise to muscle and bone. Somites are derived from dorsal presomitic mesoderm and form sequentially in an anterior-to-posterior fashion at regular time intervals. The expression of several genes oscillates in the presomitic mesoderm, and these oscillations cease after the cells are incorporated into somites. The dominant model for somite formation is the clock-and-wavefront model, in which somites form where the gene expression oscillations (segmentation clock) intersect a posteriorly moving wave of signals that stops the clock. Dias et al. report that the clock is not required for the formation of somites in chick. The authors explanted ventral presomitic mesoderm, which would not normally give rise to somites, from quail or from green fluorescent protein (GFP)–expressing chick embryos, treated it with the bone morphogenetic protein antagonist Noggin to transform it into dorsal presomitic mesoderm, and then transplanted the tissue into an extraembryonic region of a host embryo along with Noggin-coated beads to maintain the transplants' dorsal fate. Surprisingly, the transplants formed structures comparable to somites in size, morphology, and molecular markers but did not show oscillatory gene expression in the transplanted tissue prior to formation of the structures. Unlike true somites, which appear in an anterior-to-posterior sequence and have distinct anterior and posterior compartments, these ectopic somites formed nearly simultaneously in a cluster rather than in a linear array and were not subdivided into anterior and posterior compartments. The ectopic somites gave rise to normal somite-derived vertebral and muscle tissues when they were grafted in place of endogenous somites in a secondary host, demonstrating that dorsal presomitic mesoderm can intrinsically form somites independently of the segmentation clock. The authors hypothesize that cell polarization and elongation followed by extracellular matrix deposition is sufficient to drive the formation of these spherical masses. Computational simulations confirmed that local cell-cell interactions and physical constraints could account for the relatively uniform size of these self-organizing ectopic and endogenous somites. In a Perspective, Kondo considers the extent to which these findings are compatible with the current clock-and-wavefront model. Although the clock and wavefront may not be important for determining the shape, size, and identity of somites, the system probably regulates the temporal progression of somitogenesis.

A. S. Dias, I. de Almeida, J. M. Belmonte, J. A. Glazier, C. D. Stern, Somites without a clock. Science 343, 791–795 (2014). [Abstract] [Full Text]

S. Kondo, Self-organizing somites. Science 343, 736–737 (2014). [Abstract] [Full Text]