Expression patterns of the bone morphogenetic protein genes Bmp-4 and Bmp-2 in the developing chick face suggest a role in outgrowth of the primordia.

Bone morphogenetic proteins BMP-4 and BMP-2 are closely-related members of the transforming growth factor-beta superfamily that have been implicated in signalling in a number of developmental systems. To determine whether they could be involved in the epithelial-mesenchymal interactions that control face development, we mapped the distribution of Bmp-4 and Bmp-2 gene transcripts in the developing chick facial primordia. At stages when primordia were becoming established, Bmp-4 transcripts were present in specific regions of epithelium in all facial primordia, but were undetectable in the mesenchyme. Bmp-4 transcripts appeared subsequently in specific regions of mesenchyme at the distal tips of the primordia. This mesenchymal expression first appeared in the frontonasal mass and then, in turn, in the lateral nasal processes, the maxillary primordia and the mandibular primordia. There was a complex relationship between domains of epithelial and mesenchymal Bmp-4 expression, and at many sites there was an inverse correlation between epithelial and mesenchymal Bmp-4 expression. Bmp-2 transcripts were found in the epithelium and mesenchyme of the maxillary and mandibular primordia at early stages in facial development. Bmp-2 transcripts appeared in the frontonasal mass and lateral nasal processes at later stages, with epithelial expression preceding mesenchymal expression. In general, mesenchymal Bmp-2 expression was associated with overlying epithelial Bmp-2 expression. The domains of Bmp-4 expression overlapped with those of Bmp-2, but detailed examination showed that there was no precise correlation between the expression patterns of the two genes. Indeed, in some places the Bmp-4 and Bmp-2 expression domains were complementary. The expression of the Bmp-4 and Bmp-2 genes in the epithelium and distal mesenchyme of the facial primordia suggests that BMP-4 and BMP-2 may be involved in the epithelial-mesenchymal interactions that control outgrowth of these primordia.