COMBINATORIAL SIGNALS FROM THE NEURAL-TUBE, FLOOR PLATE AND NOTOCHORDINDUCE MYOGENIC BHLH GENE-EXPRESSION IN THE SOMITE

The neural tube, floor plate and notochord are axial tissues in the ve rtebrate embryo which have been demonstrated to play a role in somite morphogenesis. Using in vitro coculture of tissue explants, we have mo nitored inductive interactions of these axial tissues with the adjacen t semitic mesoderm in chick embryos. We have found that signals from t he neural tube and floor plate/notochord are necessary for expression of the myogenic bHLH regulators MyoD, Myf5 and myogenin in the somite. Eventually semitic expression of the myogenic bHLH genes is maintaine d in the absence of the axial tissues. In organ culture, at early deve lopmental stages (HH 11-), induction of myogenesis in the three most r ecently formed somites can be mediated by the neural tube together wit h the floor plate/notochord, while in more rostral somites (stages IV- IX) the neural tube without the floor plate/notochord is sufficient. B y recombining somites and neural tubes from different axial levels of the embryo, we have found that a second signal is necessary to promote competence of the somite to respond to inducing signals from the neur al tube. Thus, we propose that at least two signals from axial tissues work in combination to induce myogenic bHLH gene expression; one sign al derives from the door plate/notochord and the other signal derives from regions of the neural tube other than the floor plate.