Cells are believed to differentiate in neural tissue by default unless bone morphogenic proteins (BMP) inhibit such differentiation. A tissue called the "organizer" is responsible for blocking the BMP signal and allowing neural differentiation to proceed. Streit et al. identified the gene ERN1, which is induced as early as 1 hour after implantation of a heterologous organizer in chicken embryos. Embryonic tissues that expressed FGF induced ERN1, and FGF8-coated beads were able to induce ERN1 expression with the same time course as the grafted organizer. Further support for FGF as an early neural inducer came from experiments in which FGF signaling was disrupted. Under these conditions, grafted organizers did not cause increased expression of neural genes including ERN1 and Sox3. Additional data suggest that FGF is expressed in posterior cells before the formation of the organizer and may be one of the first positive signals directing neural fates.
Streit, A., Berliner, A.J., Papanayotou, C., Siruinik, A. and Stern, C.D. (2000) Initiation of neural induction by FGF signalling before gastrulation. Nature 406: 74-78. [Online Journal]