TARGETED OVER-EXPRESSION OF FGF IN CHICK-EMBRYOS INDUCES FORMATION OFECTOPIC NEURAL CELLS

Fibroblast growth factors (FGFs) are known to be involved mainly in me soderm formation in Xenopus embryos but their participation in other i nductive mechanisms such as neural induction has not been clearly esta blished and is now under study. Here, we provide evidence that targete d over-expression of members of this family of growth factors in the p eriphery of full-length primitive streak chick embryos produces the fo rmation of ectopic neural cells that are able to differentiate into ne urons. The supernumerary neural plate obtained derives from the epibla st layer of the blastoderm and show signs of neural differentiation 24 h after the application of FGF. We have used cell labeling and have e xamined the expression of mesodermal markers to ascertain how this exp ansion of the neural forming region of the epiblast takes place. We co nclude that the new neural cells formed are originated in the region o f the epiblast fated to be epithelia and that the induction of the ect opic neural tissue is not mediated by an increase, migration or new fo rmation of axial mesoderm. This strongly suggests that FGF is acting d irectly on epiblast cells, changing their fate from epidermal ectoderm to neural ectoderm. Therefore, our results show that FGF can induce n eural ectoderm when acting on still uncommitted cells and, therefore, it is a putative candidate for acting in normal neural induction durin g development.