Formation of a functional morphogen gradient by a passive process in tissue from the early Xenopus embryo

In early development much of the cellular diversity and pattern formation o f the embryo is believed to be set up by morphogens. However, for many morp hogens, including members of the TGF-beta superfamily, the mechanism(s) by which they reach distant cells is unknown. We have used immunofluorescence to detect, at single cell resolution, a morphogen gradient formed across ve rtebrate tissue. The TGF-P ligand is distributed in a gradient visible up t o 7 cell diameters (about 150-200 mum)from its source, and is detectable on ly in the extracellular space. This morphogen gradient is functional, since we demonstrate activation of a high response gene (Xeomes) and a low-respo nse gene (Xbra) at different distances from the TGF-P source. Expression of the high affinity type II TGF-P receptor is necessary for detection of the gradient, but the shape of the gradient formed only depends in part on the spatial variation in the amount of receptor. Finally, we demonstrate that the molecular processes that participate in forming this functional morphog en gradient are temperature independent, since the gradient forms to a simi lar extent whether the cells are maintained at 4 degreesC or 23 degreesC. I n contrast, TGF-beta1 internalisation by cells of the Xenopus embryo is a t emperature-dependent process. Our results thus suggest that neither vesicul ar transcytosis nor other active processes contribute to a significant exte nt to the formation of the morphogen gradient we observe. We conclude that, in the model system used here, a functional morphogen gradient can be form ed within a few hours by a mechanism of passive diffusion.