Some thirty years ago, a model was proposed to explain patterning of the vertebrate limb along its proximal–distal axis. This model was based on the ability of cells to measure time to assess their position. Two recent publications in Nature present data that may be difficult to reconcile with the early model. One report proposes that the limb is already specified in minute form in the early limb bud, whereas the other focuses on fibroblast growth factor (FGF) signalling from the apical ridge.
- Cheryll Tickle
- Lewis Wolpert
