Reconstitution of the organizer is both sufficient and required to re-establish a fully patterned body plan in avian embryos

Lateral blastoderm isolates (LBIs) at the late gastrula/early neurula stage (i.e., stage 3d/4) that lack Hensen's node (organizer) and primitive strea k can reconstitute a functional organizer and primitive streak within 10-12 hours in culture. We used LBIs to study the initiation and regionalization of the body plan. A complete body plan forms in each LBI by 36 hours in cu lture, and normal craniocaudal, dorsoventral, and mediolateral axes are ree stablished, Thus, reconstitution of the organizer is sufficient to re-estab lish a fully patterned body plan. LBIs can be modified so that reconstituti on of the organizer does not occur. In such modified LBIs, tissue-type spec ific differentiation (with the exception of heart differentiation) and reco nstitution of the body plan fail to occur. Thus, the reconstitution of the organizer is not only sufficient to reestablish a fully patterned body plan , it is also required. Finally, our results show that formation and pattern ing of the heart is under the control of the organizer, and that such contr ol is exerted during the early to mid-gastrula stages (i.e,, stages 2-3a), prior to formation of the fully elongated primitive streak.