Tickle C, Lee J, Eichele G
Dev Biol. 1985 May;109(1):82-95. doi: 10.1016/0012-1606(85)90348-3.
Small, positively charged beads that slowly release known amounts of all-trans-retinoic acid have been implanted below the apical ectodermal ridge at the anterior margin (opposite somite 16) of wing buds of 3 1/2 day-old chick embryos. The continuous release of retinoic acid is shown to create an anteroposterior concentration gradient of retinoic acid in the limb field that is stable with time, despite the fact that this compound is metabolized by the limb tissue. With beads that release increasing amounts of retinoic acid, the normal 234 digit pattern is progressively altered to a 2234, to a 32234, and then to a 432234 pattern. The tissue concentrations of all-trans-retinoic acid required to change the digit pattern in this way range between 1 and 25 nM. When the same amounts of retinoic acid are released from posteriorly implanted beads (placed below the apical ectodermal ridge opposite somite border 19/20 or somite 20), the normal digit pattern is unaffected. Implantations of beads that release all-trans-retinoic acid are thus identical in their effect to grafts of cells from the limb polarizing region, which cause similar dose-dependent changes in the digit pattern when grafted to the anterior margin of the bud (but not when grafted opposite somites 19 or 20). Because of the low concentrations of retinoic acid required for its biological effect, the graded response observed, and the fact that a concentration gradient is established across the limb field, all-trans-retinoic acid closely mimics the putative morphogen that has been postulated to be emitted by polarizing region cells during normal development.
已将缓慢释放已知量全反式视黄酸的带正电荷小珠子植入3.5日龄鸡胚翼芽前缘（与第16体节相对）的顶端外胚层嵴下方。结果显示，视黄酸的持续释放会在肢体区域形成一个视黄酸的前后浓度梯度，该梯度随时间稳定，尽管这种化合物会被肢体组织代谢。随着释放视黄酸量增加的珠子，正常的234指型逐渐改变为2234型，再变为32234型，然后变为432234型。以这种方式改变指型所需的全反式视黄酸组织浓度在1至25纳摩尔之间。当从后植入的珠子（置于与第19/20体节边界或第20体节相对的顶端外胚层嵴下方）释放相同量的视黄酸时，正常指型不受影响。因此，释放全反式视黄酸的珠子植入效果与来自肢体极化区域的细胞移植相同，当移植到芽的前缘时，后者会导致指型出现类似的剂量依赖性变化（但移植到第19或20体节相对位置时则不会）。由于其生物学效应所需的视黄酸浓度较低、观察到的分级反应以及在肢体区域建立了浓度梯度这一事实，全反式视黄酸紧密模拟了在正常发育过程中被假定为由极化区域细胞发出的假定形态发生素。
