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 ga strulation 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 t he Xenopus embryo, are induced in dispersed animal pole cells by diffe rent doses of the potent mesoderm-inducing factor activin. Each cell t ype is induced within a restricted dose range of activin concentration s, the boundaries of which are well demarcated shortly after activin t reatment. In contrast, Brachyury and goosecoid, two genes thought to p attern the presumptive mesoderm, and the gene encoding platelet-derive d growth factor receptor alpha, which is expressed in the mesoderm of gastrula stage embryos, are induced by broad, overlapping ranges of hi gh activin concentrations at such early times. Similarly, the response of the gene encoding platelet-derived growth factor 4, which is expre ssed normally in ectoderm of gastrula stage embryos, diminishes gradua lly as the activin concentration increases. Dose windows for the expre ssion of these four genes narrow and become distinct from one another in cell aggregates after several hours in culture, suggesting that act ivin prompts a dynamic program of gene expression in induced mesoderm.