Nuclear translocation of fibroblast growth factor during Xenopus mesoderm induction

Mesoderm induction, the earliest inductive cell-cell interaction in vertebrate embryogenesis, is thought to be mediated by polypeptide growth factors including fibroblast growth factor (FGF). Here we present an immunocytochemical analysis of FGF during mesoderm induction in Xenopus laevis. Antibodies to both basic and acidic FGF were immunoreactive with oocytes and early embryos. Immunostaining was predominantly intracellular and was concentrated in the marginal zone and vegetal pole throughout cleavage and blastula stages. In addition, basic FGF (bFGF) antibodies showed intense nuclear staining in these regions, at and following the mid-blastula transition, when embryonic transcription begins. Acidic FGF (aFGF) also appeared in some nuclei at these stages. Taken together the evidence suggests that FGF is prepositioned in mesoderm-forming regions and is actively involved in mesoderm induction in vivo.