Uncommitted Xenopus blastula cells can be directed to uniform muscle gene expression by gradient interpretation and a...

The animal cap cells of Xenopus blastulae behave as multi-potent stem cells in so far as they can differentiate along many unrelated pathways according to the kind and amount of signal factor that they experience. At first, animal cap cells activate early zygotic genes across a broad range of TGFbeta concentrations; soon after this, they activate later genes more intensely and over a narrow concentration range. Here we show that uncommitted blastula cells can be directed, by the sequential influence of a particular concentration of a TGFbeta morphogen and an FGF-mediated community effect, to form a homogeneous single cell type. As a result of these two signalling processes, an entire population of animal cap cells can be converted, in the absence of other signals, to a uniform population of one tissue type. Mesoderm cells that experience a particular concentration of activin increase their XMyoD expression by 10-fold and become distinct from neighbouring cells that received lower or higher concentrations of activin. The signalling processes that we employ here may be important in normal development and useful in guiding stem cell differentiation.