A tale of tails

‘shield’, considered to be the fish equivalent of the Spemann organizer — and transplanted the cells into host embryos. The result was extra tails, consisting of donoras well as host-derived cells.Agathon et al. also showed that inactivating the Spemann organizer did not interfere with this ‘ectopic’ tail formation, implying that the two organizers are indeed independent. The authors concede, however, that the Spemann organizer is required for ‘proper’ tail formation, as the ectopic tails were incomplete: for instance, they lacked the stiff supporting rod known as the notochord, which is derived from the Spemann organizer. The authors also established that this tailorganizer activity involved growth-factor proteins of the Wnt, bone morphogenetic protein (BMP) and Nodal families. These signalling molecules were good candidates for such a role because they are known to be present in the ventral margin, and interfering with the signalling pathways to which they contribute inhibits tail formation. Agathon et al. now show that misexpressing these growth factors in combinations, but not singly, leads to the generation of extra tails — which are, again, incomplete. So all three growth factors are involved in news and views