Spatial response to fibroblast growth factor signalling in Xenopus embryos

We have examined the spatial pattern of activation of the extracellular sig nal-regulated protein kinase (ERK) during Xenopus development, and show tha t it closely resembles the expression of various fibroblast growth factors (FGFs), Until the tailbud stage of development, all ERK activation domains are sensitive to the dominant negative FGF receptor, showing that activatio n is generated by endogenous FGF signalling. ERK is not activated by applic ation of other growth factors like BMP4 or activin, nor is endogenous activ ation blocked by the respective dominant negative receptors, This shows tha t various domains of FGF expression, including the periblastoporal region a nd the midbrain-hindbrain boundary, are also sites of FGF signalling in viv o. Wounding induces a transient (<60 minutes) activation of ERK which is not s ignificantly reduced by the dominant negative FGF receptor, An artificial FGF source, created by injection of eFGF mRNA into cleavage s tage embryos, provokes ERK activation outside of its injection site over a range of several cell diameters, The range and extent of ERK activation out side the source region is unchanged by co-injection of a dominant negative form of Ras, which blocks ERK-activation within the source. This suggests p rotein can diffuse over several cell diameters.