We have demonstrated previously that a combination of signals from the neural tube and the floor plate/notochord complex synergistically induce the expression of myogenic bHLH genes and myogenic differentiation markers in unspecified somites. In this study we demonstrate that Sonic hedgehog (Shh), which is expressed in the floor plate/notochord, and a subset of Wnt family members (Wnt-1, Wnt-3, and Wnt-4), which are expressed in dorsal regions of the neural tube, mimic the muscle inducing activity of these tissues. In combination, Shh and either Wnt-1 or Wnt-3 are sufficient to induce myogenesis in somitic tissue in vitro. Therefore, we propose that myotome formation in vivo may be directed by the combinatorial activity of Shh secreted by ventral midline tissues (floor plate and notochord) and Wnt ligands secreted by the dorsal neural tube.

We describe here the isolation of cDNA and genomic clones corresponding to the mouse gene encoding anti-Müllerian hormone, and the use of these clones as molecular probes to study AMH gene expression. We constructed a 14.5 days post coitum (dpc) mouse fetal testes library and isolated a cDNA clone using bovine, human and rat partial cDNAs as probes. This clone contained a 1 kb insert, which was confirmed by sequencing to be the mouse homologue of AMH. Probes derived from the mouse cDNA clone were used to screen genomic libraries and a 12 kb DNA fragment containing the complete coding region of mouse AMH was isolated. In situ hybridisation was used to determine the precise timing and localisation of AMH expression in male and female embryos and postnatal testes and ovaries. AMH transcripts were first detected in fetal testes at 12.5 Ape when differences between testes and ovaries first become visible. The signal was specific for the Sertoli cells of the testes. Other fetal tissues or female embryos were negative for AMH transcripts. During male development, AMH expression is shut off postnatally. In the female, the expression of AMH was first detected at day 6 after birth and is restricted to granulosa cells. We have correlated the pattern of AMH expression in both sexes with cellular events occurring in gonadal development and discuss some implications that this may have for its function and regulation.