Activin redux: specification of mesodermal pattern inXenopusby graded concentrations of endogenous activin B

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Abstract
Mesoderm formation in the amphibian embryo occurs through an inductive interaction in which cells of the vegetal hemisphere of the embryo act on overlying equatorial cells. The first candidate mesoderm-inducing factor to be identified was activin, a member of the transforming growth factor type β family, and it is now clear that members of this family are indeed involved in mesoderm and endoderm formation. In particular, Derrière and five nodal-related genes are all considered to be strong candidates for endogenous mesoderm-inducing agents. Here, we show that activin, the function of which in mesoderm induction has hitherto been unclear, also plays a role in mesoderm formation. Inhibition of activin function using antisense morpholino oligonucleotides interferes with mesoderm formation in a concentration-dependent manner and also changes the expression levels of other inducing agents such as Xnr2 and Derrière . This work reinstates activin as a key player in mesodermal patterning. It also emphasises the importance of checking for polymorphisms in the 5′ untranslated region of the gene of interest when carrying out antisense morpholino experiments in Xenopus laevis .