Further Development 13.6: Determining the Sonic Signal

Neural Tube Formation and Patterning

The importance of Sonic hedgehog in patterning the ventral portion of the neural tube has been confirmed by experiments that again demonstrate the principles of “find it, lose it, move it” (see Further Development 7.8 and the associated Dev Tutorial). If a piece of notochord is removed from an embryo, the neural tube adjacent to the deleted region will have no floor plate cells (Placzek et al. 1990). Moreover, if notochord fragments are taken from one embryo and transplanted to the lateral side of a host neural tube, the result will be to induce another set of floor plate cells in the adjacent host neural tube and bilaterally positioned sets of ectopic motor neurons around the induced floor plate (Figure 1). The same results can be obtained if the notochord fragments are replaced by pellets of cultured cells secreting Sonic hedgehog, demonstrating that Shh alone is sufficient for the induction of floor plate and affiliated motor neurons (Echelard et al. 1993).

**Figure 1** Notochord-derived Shh induces ventral neural tube structures. (A) Cells closest to the notochord become the floor plate neurons; motor neurons emerge on the ventrolateral sides. (B) If a second notochord, floor plate, or any other Sonic hedgehog-secreting cell is placed adjacent to the neural tube, it induces a second set of floor plate neurons as well as two other sets of motor neurons. (After M. Placzek et al. 1998. *Science* 250: 985–988.)

Literature Cited

Echelard, Y., D. J. Epstein, B. St.-Jacques, L. Shen, J. Mohler, J. A. McMahon and A. McMahon. 1993. Sonic hedgehog, a member of a family of putative signaling molecules, is implicated in the regulation of CNS polarity. Cell 75: 1417–1430.
PubMed Link

Placzek, M., M. Tessier-Lavigne, T. Yamada, T. Jessell and J. Dodd. 1990. Mesodermal control of neural cell identity: Floor plate induction by the notochord. Science 250: 985–988.
PubMed Link

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