DIFFERENT SPATIAL-DISTRIBUTION OF MESSENGER-RNAS FOR ACTIVIN RECEPTORS (TYPE IIA AND IIB) AND FOLLISTATIN IN DEVELOPING EMBRYOS OF XENOPUS-LAEVIS

Spatial distribution of mRNAs for activin receptors and follistatin wa s studied by Northern blot hybridization using RNAs from different par ts of dissected Xenopus embryos. mRNAs of two activin receptors (type IIA and IIB) occurred uniformly in pre-gastrular embryos, but occurred in larger amounts in ectoderm (in gastrulae), neural plate (in neurul ae) and anterior (head) regions (in tailbud embryos) than in other emb ryonic regions. By contrast, follistatin mRNA appeared almost exclusiv ely in the dorsal mesoderm including invaginating organizer region at the gastrula stage, in notochord and in dorsal ectoderm at the neurula stage, then in anterior part at the tailbud stage. The localized patt erns of the distribution of these mRNAs may be due to the regionally d ifferent zygotic expression of genes in embryos at later stages. From the relatively widespread pattern of distribution of their mRNAs, we a ssume that both type LIA and type IIB activin receptors have broad fun ctions in ectodermal and neural differentiation. On the other hand, fo llistatin mRNA showed quite a restricted pattern of expression, and th erefore, we assume that follistatin may have functions more specifical ly related to the sites of expression of its mRNA. Thus, follistatin m ay be involved in the differentiation of notochord itself and/or direc tly be responsible for organizer functions such as neural induction an d subsequent differentiation of induced neural tissues at the gastrula and later stages.