Differential effects of N-cadherin-mediated adhesion on the development of myotomal waves.

Myotomal fibers form by a first wave of pioneer myoblasts from the medial epithelial somite, and by a second wave from all four lips of the dermomyotome. Then, a third wave of mitotic progenitors colonizes the myotome, initially stemming from the extreme lips and, later, from the central dermomyotome sheet. In vitro studies have suggested that N-cadherin plays a role in myogenesis, but its role in vivo remains poorly understood. We find that during the growth phase of the dermomyotome sheet, when the orientation of mitotic spindles is parallel to the mediolateral extent of the epithelium, N-cadherin protein is inherited by both daughter cells. Prior to dermomyotome dissociation into dermis and muscle progenitors, when mitoses become perpendicularly oriented, N-cadherin remains associated only with the apical cell located in apposition to the myotome, generating molecular asymmetry between basal and apical progeny. Local gene missexpression confirms that N-cadherin-mediated adhesion is sufficient to promote myotome colonization, whereas its absence drives cells towards the subectodermal domain, hence coupling the asymmetric distribution of N-cadherin to a shift in mitotic orientation and to fate segregation. Site-directed electroporation to additional, discrete somite regions, further reveals that N-cadherin-mediated adhesion is necessary for maintaining the epithelial configuration of all dermomyotome domains while promoting the onset of Myod transcription and the translocation into the myotome of myofibers and/or of Pax-positive progenitors. By contrast, N-cadherin has no effect on migration or differentiation of the first wave of myotomal pioneers. Altogether, we show for the first time that the asymmetric localization of N-cadherin during mitosis indirectly influences fate segregation by differentially driving the allocation of progenitors to muscle versus dermal primordia, that the adhesive domain of N-cadherin maintains the integrity of the dermomyotome epithelium, which is necessary for myogenic specification, and that different molecular mechanisms underlie the establishment of pioneer and later myotomal waves.