New insight into the control of the mammalian body plan

Figure 1: Somites ‘line up’ along the neural tube of the developing embryo.

Reproduced from Developmental Cell 13, 298–304 (2007), Copyright (2007) with permission from Elsevier

Researchers from Japan and the United States have uncovered a role for another signaling pathway in the determination of the mammalian body plan.

The vertebrae, ribs, dermis and muscles of mammals arise from structures called somites that appear at regular intervals along the neural tube (Fig. 1). As the body lengthens new somites arise at the posterior of the growing embryo from cells in that area called the presomitic mesoderm. This development is a result of periodic oscillations in the expression of key transducers of signaling pathways that involve master regulators, such as Notch and Wnt. Now, Ryoichiro Kageyama and his colleagues have added fibroblast growth factor (Fgf) to the mix¹.

Using a series of knockout mice, the team has shown that Fgf signaling oscillates in the posterior region of the presomitic mesoderm, which kickstarts the oscillation of a target gene, Hes7. The oscillatory expression of Hes7 is then amplified and propagated towards the anterior region of the presomitic mesoderm by the action of Notch and its target genes, thus specifying the new somite. This process is repeated as the body lengthens and new somites arise. Without Hes7, however, Fgf and Notch signaling molecules are not properly coordinated, resulting in severe fusion of somites.