The homeobox gene Msx1 is expressed in a subset of somites, and in muscle progenitor cells migrating into the forelimb

In myoblast cell cultures, the Msx1 protein is able to repress myogenesis a nd maintain cells in an undifferentiated and proliferative state. However, there has been no evidence that Msx1 is expressed in muscle or its precurso rs in vivo. Using mice with the nlacZ gene integrated into the Msx1 locus, we show that the reporter gene is expressed in the lateral dermomyotome of brachial and thoracic somites, Cells from this region will subsequently con tribute to forelimb and intercostal muscles, Using Pax3 gene transcripts as a marker of limb muscle progenitor cells as they migrate from the somites, we have defined precisely the semitic origin and timing of cell migration from somites to limb buds in the mouse. Differences in the timing of migrat ion between chick and mouse are discussed, Somites that label for Msx1(nlac )Z transgene expression in the forelimb region partially overlap with those that contribute Pax3-expressing cells to the forelimb, In order to see whe ther Msx1 is expressed in this migrating population, we have grafted somite s from the foreIimb level of Msx1(nlac)Z mouse embryos into a chick host em bryo. We show that most cells migrating into the wing field express the Msx 1(nlacZ) transgene, together with Pax3, In these experiments, Msx1 expressi on in the somite depends on the axial position of the graft. Wing mesenchym e is capable of inducing Msx1 transcription in somites that normally would not express the gene; chick hindlimb mesenchyme, while permissive for this expression, does not induce it. In the mouse limb bud, the Msx1(nlac) trans gene is downregulated prior to the activation of the Myf5 gene, an early ma rker of myogenic differentiation. These observations are consistent with th e proposal that Msx1 is involved in the repression of muscle differentiatio n in the lateral half of the somite and in limb muscle progenitor cells dur ing their migration.