BACKGROUND: The neural tube and its overlying tissues (skin and mesenchyme) interact along the dorsal midline during early development. This has been previously demonstrated experimentally in chicken embryos by the fact that the dorsal neural tube transplanted ectopically induced expression of Msx 2 in the adjacent tissues. It is important to identify the molecules responsible for these interactions. RESULTS: We observed that BMP-4, a member of the TGFbeta-family, is expressed in the dorsal neural tube in a pattern closely correlated with that of Msx 2. In order to investigate whether BMP-4 mediates the signal between the neural tube and the skin/mesenchyme, BMP-4 was ectopically administered in ovo either by implantation of the recombinant protein or transplantation of COS cells producing BMP-4. Both manipulations resulted in ectopic induction of Msx 2 expression in the adjacent skin/mesenchyme. In addition, the activity of BMP-4 in inducing Msx 2 was counteracted by the floor plate. CONCLUSION: These data suggest that BMP-4 positively mediates the signals from the neural tube to the adjacent tissues and that this signal may be an essential step for the establishment of the dorsal midline structures.

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