Epithelial-mesenchymal interactions in the outgrowth of limb buds and facial primordia in chick embryos.

The facial primordia in the chick embryo begin as rounded swellings that surround the primitive mouth and these grow out to form the beak. The control of proximodistal outgrowth is not well understood but may involve similar mechanisms to the limb bud. In order to test this hypothesis, combinations were made between epithelium and mesenchyme from facial primordia and limb buds. Signals from all three types of facial mesenchyme (frontonasal mass, mandibular, and maxillary) maintained the thickened apical ectodermal ridge of limb epithelium for up to 48 h. Combinations of tissues from the frontonasal mass mesenchyme and limb epithelium underwent substantial and correct morphogenesis. In contrast, poor development was observed in combinations with mandibular mesenchyme. Signals from frontonasal mass epithelium promoted outgrowth and morphogenesis of limb mesenchyme whereas mandibular and maxillary epithelium did not support joint morphogenesis. The results suggest that signals employed in the epithelial-mesenchymal interactions in facial primordia are similar but not identical to those signals used in the limb bud.