STATE OF COMMITMENT OF PROSPECTIVE NEURAL PLATE AND PROSPECTIVE MESODERM IN LATE GASTRULA EARLY NEURULA STAGES OF AVIAN EMBRYOS

We examined the ability of epiblast regions of known prospective fate from the late gastrula/early neurula stage of avian embryos to self-di fferentiate when placed heterotopically, testing their state of commit ment. Three sires were examined: paranodal prospective neural plate ec toderm, containing cells fated to form a portion of the lateral wall o f the neural tube at essentially all rostrocaudal levels of the neurax is; prospective mesoderm from the caudolateral epiblast, containing ce lls fated to ingress through the primitive streak and to form lateral plate mesoderm; and prospective mesoderm from one level of the primiti ve streak, containing cells fated to continue ingressing and form para xial mesoderm, Grafts from all sites exhibited plasticity. Grafts from the prospective neural plate ectoderm could readily substitute for re gions of prospective mesoderm, when transplanted to either the epiblas t or primitive streak, undergoing an epithelial-mesenchymal transition and, where appropriate, expressing paraxis, a gene expressed in parax ial mesoderm. Similarly, grafts containing prospective mesoderm from t he epiblast could readily substitute for regions of the prospective ne ural plate ectoderm, undergoing convergent-extension movements charact eristic of neuroectodermal cells and expressing appropriate genes such as Engrailed-2 and Hoxb-1. Grafts containing prospective mesoderm fro m the primitive streak could also incorporate into the neural plate an d undergo convergence-extension movements of neurulation, although the ir principal contribution was to mesodermal and endodermal structures. Collectively, our results demonstrate that at the late gastrula/early neurula stage, germ layer-specific properties are not irrevocably fix ed for prospective ectodermal and mesodermal regions of the blastoderm . Moreover, the signals responsible for the induction of these two tis sue types must still he present and available at these late stages. (C ) 1997 Academic Press