Notochord Grafts
互联网
534
Many patterning structures have been identified by microsurgical manipulation of chick embryos in 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 in ref. 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 in ref. 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 marker Pax3 . ( C,D ) Sections of embryos hybridized with the intermediate marker Pax6 . 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 marker Pax3 and the intermediate marker Pax6 .
