TAIL FORMATION AS A CONTINUATION OF GASTRULATION - THE MULTIPLE CELL-POPULATIONS OF THE XENOPUS TAILBUD DERIVE FROM THE LATE BLASTOPORE LIP

Three fines of evidence suggest that tail formation in Xenopus is a di rect continuation of events initiated during gastrulation. First, the expression of two gene markers, Xbra and Xnot2, can be followed from t he blastopore lip into distinct cell populations of the developing tai lbud. Second, the tip of the tail retains Spemann's tail organizer act ivity until late stages of development. Third, lineage studies with th e tracer DiI indicate that the cells of the late blastopore are fated to form specific tissues of the tailbud, and that intercalation of dor sal cells continues during tail elongation. In particular, the fate ma p shows that the tip of the tail is a direct descendant of the late do rsal blastopore lip. Thus, the tailbud is not an undifferentiated blas tema as previously thought, but rather consists of distinct cell popul ations which arise during gastrulation.