ABSTRACT
FACE described in the retarded retinoic acid-treated :rru1mmalian ~-,"'-·~"~~
brachygnathia, and hypoplastic maxilla. have been performed in the chick embryo, but in this organism the facial defects show an interesting differential distribution which present us with an ideal opportunity for
the moiecular basis of the action of RA effects on the chick embryo face are at the same
limb. Thus AA beads are implanted into the limb bud to 'JHJmlce duplications and at the same time RA diffuses out of the bead, diffuses through the embryonic medium, and affects the face. Here it results in the absence of the upper beak and but the lower beak is unaffected This differential effect occurs in cultured fragments of frontonasal mass and mandible taken from treated
and by treated and untreated epithelium mesenchyme it shown that retinoids act on the frontonasa! mass via the (125),
case for (Section II.A) and limb regenerates HI.A). However, when the treatment was performed in micromass culture both frontonasal mass and mandibular tissue were equally sensitive to retinoid inhibition of cartilage differentiation (126) and muscle differentiation (127). Thus something happens in vivo which cannot be duplicated in vitro and HPLC measurements have shown that treatment vivo results an equivalent uptake of retinoid into the frontonasal mass mandible, so differential uptake cannot explain these results {126).
