Morphological differences in Xenopus embryonic mesodermal cells are specified as an early response to distinct threshold concentrations of activin.

The involution of presumptive mesoderm that occurs during amphibian gastrulation is a complex process requiring the coordinated action of a diverse range of cells. We show that cells with distinct morphologies, resembling each of those normally found in the involuting tissue of the Xenopus embryo, are induced in dispersed animal pole cells by different doses of the potent mesoderm-inducing factor activin. Each cell type is induced within a restricted dose range of activin concentrations, the boundaries of which are well demarcated shortly after activin treatment. In contrast, Brachyury and goosecoid, two genes thought to pattern the presumptive mesoderm, and the gene encoding platelet-derived growth factor receptor alpha, which is expressed in the mesoderm of gastrula stage embryos, are induced by broad, overlapping ranges of high activin concentrations at such early times. Similarly, the response of the gene encoding platelet-derived growth factor A, which is expressed normally in ectoderm of gastrula stage embryos, diminishes gradually as the activin concentration increases. Dose windows for the expression of these four genes narrow and become distinct from one another in cell aggregates after several hours in culture, suggesting that activin prompts a dynamic program of gene expression in induced mesoderm.