Dorso-ventral and rostro-caudal sequential expression of M-twist in the postimplantation murine embryo.

M-twist is the murine homolog of the Drosophila twist gene which is a zygotic target for maternal genes that establish embryonic dorso-ventral polarity and is necessary for mesoderm formation. We recently showed that before gastrulation, M-twist transcripts are detected in morulae and blastocysts, then in extra-embryonic tissues of early implanted mouse embryos before the onset of gastrulation, and we suggested that M-twist might be involved in embryonic polarity (Stoetzel et al., submitted). Here, using in situ hybridization on whole mount embryos, we present the expression pattern of M-twist from primitive streak stage up to 10.5 days p.c. In implanted embryos, M-twist is first expressed in extra-embryonic tissues, then in embryo proper around egg cylinder stage within some embryonic ectodermal cells of the primitive streak. Slightly later, scattered cells within the amniotic cavity apparently detached from the primitive streak also express the gene. Then, M-twist transcripts accumulate in head mesenchyme, the first aortic arches, somites and lateral mesoderm and, as development proceeds, successively the second, third and fourth branchial arches, the anterior limb buds and, finally, the posterior limb buds. Thus M-twist expression in implanted embryos occurs first along a dorso-ventral gradient pattern until the headfold stage, then it is gradually observed along the rostro-caudal axis of the embryos as development procedes in the mesodermal cell layer and in neural crest cell derivatives. In addition, we show the existence of some previously undescribed subsets of scattered cells that express M-twist and thus might participate in murine embryo development.