CONTINUING ORGANIZER FUNCTION DURING CHICK TAIL DEVELOPMENT

Development of the posterior body (lumbosacral region and tail) in ver tebrates is delayed relative to gastrulation. In amniotes, it proceeds with the replacement of the regressed node and primitive streak by a caudal blastema-like mass of mesenchyme known as the tail bud. Despite apparent morphological dissimilarities, recent results suggest that t ail development in amniotes is in essence a continuation of gastrulati on, as is the case in Xenopus. However, this has been inferred primari ly from the outcome of fate mapping studies demonstrating discrete, re gionalized cell populations in the tail bud, like those present at gas trulation, Our analysis of the tail bud distribution of several molecu lar markers that are expressed in specific spatial domains during chic k gastrulation confirms these results. Furthermore, we present evidenc e that gastrulation-like ingression movements from the surface continu e in the early chick tail bud and that the established tail bud retain s organizer activity. This 'tail organizer' has the expected propertie s of being able to recruit uncommitted host cells into a new embryonic axis and induce host neural tissue with posteriorly regionalized gene expression when grafted to competent host cells that are otherwise de stined to form only extra-embryonic tissue. Together, these results in dicate that chick tail development is mechanistically continuous with gastrulation and that the developing tail in chick may serve as a usef ul experimental adjunct to investigate the molecular basis of inductiv e interactions operating-during gastrulation, considering that residua l tail organizing activity is still present at a surprisingly late sta ge.