DE-NOVO INDUCTION OF THE ORGANIZER AND FORMATION OF THE PRIMITIVE STREAK IN AN EXPERIMENTAL-MODEL OF NOTOCHORD RECONSTITUTION IN AVIAN EMBRYOS

We have developed a model system for analyzing reconstitution of the n otochord using cultured blastoderm isolates lacking Hensen's node and the primitive streak, Despite lacking normal notochordal precursor cel ls, the notochord still forms in these isolates during the 36 hours in culture, Reconstitution of the notochord involves an inducer, which a cts upon a responder, thereby inducing a reconstituted notochord, To b etter understand the mechanism of notochord reconstitution, we asked w hether formation of the notochord in the model system was preceded by reconstitution of Hensen's node, the organizer of the avian neuraxis, Our results show not only that a functional organizer is reconstituted , but that this organizer is induced from the responder, First, fate m apping reveals that the responder forms a density, morphologically sim ilar to Hensen's node, during the first 10-12 hours in culture, and th at this density expresses typical markers of Hensen's node, Second, th e density, when fate mapped or when labeled and transplanted in place of Hensen's node, forms typical derivatives of Hensen's node such as e ndoderm, notochord and the floor plate of the neural tube, Third, the density, when transplanted to an ectopic site, induces a secondary neu raxis, identical to that induced by Hensen's node, And fourth, the den sity acts as a suppressor of notochord reconstitution, as does Hensen' s node, when transplanted to other blastoderm isolates, Our results al so reveal that the medial edge of the isolate forms a reconstituted pr imitive streak, which gives rise to the normal derivatives of the defi nitive primitive streak along its rostrocaudal extent and which expres ses typical streak markers, Finally, our results demonstrate that the notochordal inducer also induces the reconstituted Hensen's node and, therefore, acts like a Nieuwkoop Center, These findings increase our u nderstanding of the mechanism of notochord reconstitution, provide new information and a novel model system for studying the induction of th e organizer and reveal the potential of the epiblast to regulate its c ell fate and patterns of gene expression during late gastrula/early ne urula stage in higher vertebrates.