Fgf-4, initially isolated as a transforming gene from human tumors, is a member of the Fibroblast Growth Factor (FGF) family. It has previously been shown by northern blot hybridization analysis to be expressed in teratocarcinoma and embryonic stem cells, suggesting that it plays a role in embryonic development. We have carried out an RNA in situ hybridization analysis of Fgf-4 expression in the developing mouse embryo, from fertilization through the 14th day of gestation (E14.5). Our results show that Fgf-4 RNA is first detected at the late blastocyst stage in cells that give rise to all of the embryonic lineages (inner cell mass cells). During the early stages of gastrulation, expression becomes restricted to the primitive streak where mesoderm and definitive endoderm are formed. Expression continues in the distal (rostral) two-thirds of the streak through approx. E10, and then is detected in the tail bud, which replaces the streak as the primary source of mesoderm. Additional sites of expression are found after the three primary germ layers are established and organogenesis begins. Fgf-4 RNA is detected transiently in the branchial arch units, the somitic myotome, the apical ectodermal ridge of the developing limb bud and the tooth bud, suggesting that the gene has multiple roles during embryogenesis. These results are compared with the expression patterns of other FGF genes. Taken together, the data suggest that individual members of the gene family are expressed sequentially in developmental pathways such as mesoderm formation and myogenesis, and play a role in specific epithelial-mesenchymal interactions.

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