Spatial and temporal expression of basic fibroblast growth factor (FGF-2) mRNA and protein in early Xenopus development.

We have analyzed the expression pattern of bFGF (FGF-2) mRNA and protein in early Xenopus development using RNAse protections, in situ hybridization and immunocytochemical methods. We find that the maternal bFGF mRNA content is at its highest in the previtellogenic oocytes (stage I-II) but decreases during further oogenesis. This low maternal level persists into the early embryonic stages and is uniformly distributed on an RNA basis. Zygotic transcription is turned on both from anterior and posterior regions but not from the middle region in the mid-neurula stage, and the expression greatly increases during the late neurula and tailbud stages. In the tadpole stage, the expression is detected in the brain, eyes, ears and neural crest-derived mesenchyme of the head. Also, it is expressed along the mesodermal axis, the level falling as the myotomes differentiate. Immunocytochemical study shows abundant bFGF protein in early oocytes and much less in later oocytes. The localization is mainly to the nuclei of the early oocytes and to the cytoplasm of the later oocytes. Localization of maternal bFGF protein in the animal hemisphere is observed in the early embryonic stages and some reuptake into nuclei occurs by the early blastula stage. The zygotically synthesized protein starts to be expressed in the anterior region of the mid-neurula stage and soon also becomes detectable in the posterior region. By tailbud and pre-larval stages, this zygotic protein appears to be present along the entire neural and mesodermal axis. When these cases are sectioned it can be seen that bFGF protein is detected in most parts of the head. In the posterior domain, it is present in the mesoderm and clearly becomes localized in the muscle cells to the nuclei and to the cell termini adjoining the intermyotomal septa.