The retinobenzoic acids Am80, Am580 and Ch55 are synthetic stable analogs of retinoic acid (RA), and show very strong differentiation-inducing activity in human myelogeneous leukemia cell line HL-60. To examine the effects of these synthetic retinoids on limb pattern formation, AG1-X2 beads containing these retinoids were applied to the anterior margin of stage 19-20 chick wing buds. By implanting the beads with 1 μg/ml retinoids, normal wings were formed and extra digits 2 or 32 were rarely formed. As the retinoid concentrations increased from 10 μg/ml to 100 μg/ml, duplicated limbs 3234, 43234, 432234, 4334 were progressively produced. At higher concentrations, 1 mg/ml, the wings often truncated, although duplication occurred in some embryos. These synthetic analogs seem to have the same degree of morphogenetic potential as RA, since the activity index of these retinoids was similar to that of RA. Since these synthetic retinoids hardly bind to CRABP (cellular retinoic acid-binding protein), it may be possible that the retinoids and RA may affect limb-pattern formation without the interaction with CRABP. It is known that limb buds cannot develop distal structures when the posterior region including all ZPA (zone of polarizing activity) is removed. When beads containing the above mentioned retinoids were implanted to the anterior margin of wing buds from which the posterior one third region including all ZPA had been removed, distal growth of the wing buds and the formation of digit elements were observed. Some of the wing buds produced a completely reverse digit pattern 432. From these results, we discussed the roles of RA in limb development and pattern formation.
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