Dorsoventral axis determination in the somite: a re-examination

We have repeated classic dorsoventral somite rotation experiments (Aoyama a nd Asamoto, 1988, Development 104, 15-28) and included dorsal and ventral g ene expression markers for the somitogenic tissue types, myotome and sclero tome, respectively. While the histological results are consistent with thos e previously published, gene expression analysis indicates that cells previ ously thought to be 'sclerotome' no longer express Pax1 mRNA, a sclerotome marker, These results, together with recent quail-chick transplantation exp eriments indicating that even very late sclerotome tissue fragments are mul tipotential (Dockter and Ordahl, 1998, Development 125, 2113-2124), lead to the conclusion that sclerotome tissue remains phenotypically and morphogen etically plastic during early embryonic somitogenesis. Myotome precursor ce lls, by contrast, appear to be determined within hours after somite epithel ization; a finding consistent with recent reports (Williams and Ordahl, 199 7, Development 124, 4983-4997). Therefore, while these findings support a c entral conclusion of Aoyama and Asamoto, that axis determination begins to occur within hours after somite epithelialization, the identity of the resp onding tissues, myotome versus sclerotome, differs, A simple model proposed to reconcile these observations supports the general hypothesis that deter minative aspects of early paraxial mesoderm growth and morphogenesis occur in early and late phases that are governed principally by the myotome and s clerotome, respectively.