The effects of retinoids on cartilage differentiation in micromass cultures of chick facial primordia and the relationship to a specific facial defect.
Retinoids produce facial defects in chicken embryos. Outgrowth of the frontonasal mass with accompanying cartilage differentiation and pattern formation is inhibited. In contrast, the development of the mandibular primordia that give rise to the lower beak proceeds normally. To investigate whether the upper beak defect is based on the inhibition of cartilage differentiation specifically in the frontonasal mass, the effects of retinoids on chondrogenesis in micromass (high density) cultures of cells from facial primordia have been studied. When either 10(-6) M retinoic acid or 10(-8) M (E)-4-[2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-napthalenyl-1- propenyl]benzoic acid (TTNPB; a stable retinoid) is added to the culture medium, cartilage differentiation is inhibited. Both frontonasal mass and mandible cultures are equally affected. The concentration of TTNPB found in both facial primordia in vivo, after a treatment that produces the defect, is also about 10(-8) M. This rules out preferential accumulation of the retinoid by the frontonasal mass as an explanation for the defect. In fact, the concentration of retinoid found in vivo, should, from the culture studies, be sufficient to markedly inhibit chondrogenesis in both the frontonasal mass and mandibles. The effects of exposure to retinoids in the intact face appear to be different to those in culture. Furthermore, when cells from retinoid-treated facial primordia are cultured in micromass, the extent and pattern of chondrogenesis in frontonasal mass cultures is identical to that of cells from untreated primordia. Cartilage differentiation in mandible cultures is slightly affected. These findings suggest that retinoids do not produce the specific facial defect by directly interfering with cartilage differentiation.