Characterisation of Hoxa gene expression in the chick limb bud in responseto FGF

We tested a diffusion gradient model for setting up overlapping domains of Hoxa gene expression in the chick limb bud. The model is based on morphogen production at the limb bud tip where the apical ridge is located and assum es that cells respond to a series of concentration thresholds. Consistent w ith the model, Hoxa13 gene expression rapidly switches off when the ridge i s removed from stage 21/22 buds, while Hoxa11 and Hoxa10 expression is stab le; Hoxa13 expression can be initiated and maintained in absence of the rid ge by FGF soaked beads; the Hoxa13 domain first expands quickly and then sl ows up and the size is related to the dose of FGF4. Contrary to the model, addition of FGF4 to early limb buds does not activate Hoxa13 prematurely no r extend the Hoxa13 expression domain proximally. Therefore FGF4 signalling is necessary but not sufficient for Hoxa gene expression in the limb bud. (C) 2001 Wiley-Liss, Inc.