Epithelial-mesenchymal interactions in the development of chick facial primordia and the target of retinoid action.

The development of the chick face involves outgrowth of buds of tissue, accompanied by the differentiation of cartilage and bone in spatially defined patterns. To investigate the role of epithelial-mesenchymal interactions in facial morphogenesis, small fragments of facial tissue have been grafted to host chick wing buds to continue their development in isolation. Fragments of the frontonasal mass give rise to typical upper-beak-like structures: a long central rod of cartilage, the prenasal cartilage and an egg tooth. Meckel's cartilage, characteristic of the lower beak, develops from fragments of the mandible. Removal of the ectoderm prior to grafting leads to truncated development. In fragments of frontonasal mass mesenchyme only a small spur of cartilage differentiates and there is no outgrowth. The mandible is less affected; a rod of cartilage still forms but the amount of outgrowth is reduced. Retinoid treatment of chick embryos specifically affects the development of the upper beak and outgrowth and cartilage differentiation in the frontonasal mass are inhibited. The mandibles, however, are unaffected and develop normally. In order to investigate whether the epithelium or the mesenchyme of the frontonasal mass is the target of retinoid action, recombinations of retinoid-treated and untreated facial tissue have been grafted to host wing buds. Recombinations of retinoid-treated frontonasal mass ectoderm with untreated mesenchyme develop normally whereas recombinations of untreated ectoderm with retinoid-treated mesenchyme lead to truncations. The amount of outgrowth in fragments of mandibular tissue is slightly reduced when either the ectoderm or the mesenchyme has been treated with retinoids. These recombination experiments demonstrate that the mesenchyme of the frontonasal mass is the target of retinoid action. This suggests that retinoids interfere with the reciprocal epithelial-mesenchymal interactions necessary for outgrowth and normal upper beak development.