Expression of activin mRNA during early development in Xenopus laevis

Activins are members of the transforming growth factor-β superfamily, a class of peptide growth factors that can regulate the growth and differentiation of a variety of cell types. In mesoderm induction assays, activins A and B were shown to be very potent inducers and it was only recently demonstrated that they are crucial for initial mesoderm induction in Xenopus embryos. To determine the source of activin protein for initial mesoderm induction and to investigate whether activins may play further roles in embryonic development we have examined the localization of the mRNAs encoding the activin β(A) and β(B) subunits in Xenopus embryos. Activin β(A) and β(B) mRNAs are found in the follicle cells surrounding oocytes but not in oocytes themselves or fertilized eggs. During embryogenesis activin mRNA is first detected after the midblastula transition and expression increases as development proceeds. Activin β(B) mRNA is homogeneously distributed during blastula and early gastrula stages but restricted to the dorso-anterior region in neurula stage embryos. At the early tailbud stage activin expression becomes confined to the brain, eye anlagen, visceral pouches, otic vesicles, and the anterior notochord.