Many patterning structures have been identified by microsurgical manipulation of chick embryosin ovo, such as ablation or heterotopic grafting experiments. Among the structures studied, the notochord has received much attention, since it plays a crucial role in the development of the surrounding tissues. In the overlying neural tube (Fig. 1 ), the notochord induces the ventral midline structure, the floor plate, which subsequently specifies neuronal cell types in the ventral half of the neural tube (reviewed inref.2 ). In the paraxial mesoderm that flanks the neural tube and notochord, the latter induces the sclerotome and, in synergy with signals from dorsal neural tube or surface ectoderm, the epaxial myotome ( 3 , reviewed inref.4 ). Studies on zebrafish notochord mutants suggest that the notochord also acts in the formation of the dorsal aorta ( 5 ). Finally, the proximity of notochord and the subjacent endoderm during early phases of development suggests that the notochord may play a role in the development of the roof of the gut. Fig. 1. Repatterning of the neural tube after ectopic notochord insertion ( 1 ). (A,B) sections of embryos hybridized with the dorsal neural tube markerPax3. (C,D) Sections of embryos hybridized with the intermediate markerPax6. A, C, controls, B, D, sections of operated embryos. Where the ectopic notochord (N′) has established contact to the neural tube, it induces a wedge-shaped structure normally found ventrally in the neural tube: the floor plate. Simultaneously, the graft suppresses the expression of the dorsal markerPax3and the intermediate markerPax6.
