Molecular and cellular analysis of embryonic avian tongue development.

Signalling cascades first described in Drosophila have been found to regulate patterning and outgrowth in a number of structures in higher vertebrates. We sought to determine whether the evolutionarily conserved genes were important during the development of the tongue. In situ hybridisation was used to determine the temporo-spatial expression of a panel of conserved genes. Histological examination and incorporation of BrdU were used to determine the mechanism by which the tongue develops. We show that evolutionarily conserved genes were expressed in distinct dynamic patterns during tongue development. Sonic Hedgehog (Shh) and Patched (Ptc) were found only in the dorsal tongue epithelium. Shh expression was only observed in the suprabasal layers, whereas Ptc was observed in both basal and suprabasal layers. Cell division in the epithelium was concentrated in regions devoid of Shh. Expression of bone morphogenetic protein-7 (BMP) was identical to that of Shh. Shh and Ptc expression were never detected in the mesenchyme. Ectopic expression of Noggin (a potent antagonist of the BMPs) caused severe abnormalities in tongue morphology, including swelling of the mesenchymal component and a thickening of the epithelial layer. Data from this study suggests that the epithelium and mesenchyme express quite different genes during development. However BMP activity acts to inhibit growth in both tissues.