Analysis of the developing Xenopus tail bud reveals separate phases of gene expression during determination and outgrowth

We have studied Xenopus tail development from the end of gastrulation to the commencement of outgrowth at the tail bud stage. We show that an early group of genes are expressed at the stage of tail bud determination, at the end of gastrulation, and a late group are expressed at around stage 27 just before tail bud outgrowth. Together, these genes define seven distinct regions of the tail bud as outgrowth commences. We have previously shown that formation of a tail bud depends on the interaction of three tissue regions, called N, M and C, at stage 13. Here we show that expression of the late group of genes is dependent on this NMC interaction. We describe molecular correlates of two of these regions, M and C, which were formerly unobservable and whose existence was inferred from embryological experiments.