Sharpe C R, Gurdon J B
Department of Zoology, University of Cambridge, UK.
Development. 1990 Aug;109(4):765-74. doi: 10.1242/dev.109.4.765.
We have investigated the interactions between mesoderm and ectoderm that result in the formation of a regionally differentiated nervous system in Xenopus embryos. We have used genes expressed at different positions along the neural tube as regional markers of neural induction in both whole, and in experimentally manipulated embryos. By comparing transcription from the anterior marker, XIF3, with that from the posterior marker, X1Hbox6, and the general neural marker XIF6, we have shown that the normal induction process requires interactions between ectoderm and mesoderm that persist through gastrulation into the late neurula stages. We have found that competence of the ectoderm to respond to induction is lost at the same early neurula stage for all three marker genes. Using rhodamine dextran-labelled mesoderm, we have established that the duration of contact between ectoderm and mesoderm required for gene activation in conjugates is the same for each of the markers. We have, however, identified regions of the mesoderm that can induce different combinations of neural marker gene expression. The anterior mesoderm induces expression of the anterior marker, XIF3, and the later migrating posterior mesoderm induces the ectoderm overlying it to express the posterior marker X1Hbox6. It has been proposed that neural inducing signals reach the ectoderm by two different routes: from mesoderm lying directly beneath the ectoderm or along the plane of the ectoderm. We have assessed the contribution of each route in respect of our three neural markers and find that a signal passing directly from mesoderm to ectoderm fully accounts for neural gene expression. We were unable to detect an inducing signal that passes along the plane of the ectoderm.
我们研究了中胚层和外胚层之间的相互作用，这种相互作用导致非洲爪蟾胚胎中形成区域分化的神经系统。我们使用了沿神经管不同位置表达的基因，作为完整胚胎和实验操作胚胎中神经诱导的区域标记。通过比较前脑标记物XIF3、后脑标记物X1Hbox6和一般神经标记物XIF6的转录情况，我们发现正常的诱导过程需要外胚层和中胚层之间持续到原肠胚形成后期直至神经胚晚期的相互作用。我们发现，对于所有这三个标记基因，外胚层对外源诱导作出反应的能力在神经胚早期的同一阶段丧失。使用罗丹明葡聚糖标记的中胚层，我们确定了结合物中基因激活所需的外胚层与中胚层之间的接触持续时间对于每个标记物都是相同的。然而，我们已经确定了中胚层中能够诱导神经标记基因表达不同组合的区域。前中胚层诱导前脑标记物XIF3的表达，而后来迁移的后中胚层诱导其上方的外胚层表达后脑标记物X1Hbox6。有人提出，神经诱导信号通过两种不同途径到达外胚层：从直接位于外胚层下方的中胚层或沿着外胚层平面。我们评估了每种途径对我们的三个神经标记物的作用，发现从中胚层直接传递到外胚层的信号完全解释了神经基因的表达。我们无法检测到沿着外胚层平面传递的诱导信号。
