ABSTRACT
We have repeated classic dorsoventral somite rotation experiments (Aoyama and Asamoto, 1988, Development 104, 15-28) and included dorsal and ventral gene expression markers for the somitogenic tissue types, myotome and sclerotome, respectively. While the histological results are consistent with those previously published, gene expression analysis indicates that cells previously thought to be ‘sclerotome’ no longer express Pax1 mRNA, a sclerotome marker. These results, together with recent quail-chick transplantation experiments indicating that even very late sclerotome tissue fragments are multipotential (Dockter and Ordahl, 1998, Development 125, 2113-2124), lead to the conclusion that sclerotome tissue remains phenotypically and morphogenetically plastic during early embryonic somitogenesis. Myotome precursor cells, by contrast, appear to be determined within hours after somite epithelization; a finding consistent with recent reports (Williams and Ordahl, 1997, Development 124, 4983-4997). Therefore, while these findings support a central conclusion of Aoyama and Asamoto, that axis determination begins to occur within hours after somite epithelialization, the identity of the responding tissues, myotome versus sclerotome, differs. A simple model proposed to reconcile these observations supports the general hypothesis that determinative aspects of early paraxial mesoderm growth and morphogenesis occur in early and late phases that are governed principally by the myotome and sclerotome, respectively.
It is predicted that the dorsolateral mesenchyme derived from a stage III somite in a dorsoventral axis inversion experiment would exhibit multipotentiality and integration into host anatomical structures when examined at a later time point. To test this hypothesis, chimeric embryos with inverted stage I-III somites would be harvested at a later time point to allow the examination of the ultimate differentiated cell types formed. Unfortunately, the low success rate (6/31 chimeric embryos) in obtaining correct dorsoventral inversion of the grafted stage I-III somites renders meaningful interpretation of such experiments difficult, due to the inability to determine whether the results came from correctly or incorrectly oriented somite grafts.
