Lamb T M, Harland R M
Department of Molecular and Cell Biology, University of California, Berkeley 94720, USA.
Development. 1995 Nov;121(11):3627-36. doi: 10.1242/dev.121.11.3627.
Neural tissue in developing Xenopus embryos is induced by signals from the dorsal mesoderm. Induction of anterior neural tissue could be mediated by noggin, a secreted polypeptide found in dorsal mesoderm. We show that bFGF, a known mesoderm inducer of blastula staged ectoderm, induces neural tissue from gastrula stage ectoderm. The type of neural tissue induced by bFGF from stage 10.25 ectoderm is posterior, as marked by Hox B9 expression. When bFGF and noggin are combined on early gastrula stage ectoderm, a more complete neural pattern is generated and no mesodermal tissue is detected. Explants treated with noggin and bFGF elongate and display distinct anterior and posterior ends marked by otx2 and Hox B9 expression, respectively. Furthermore, treatment of early gastrula ectoderm with noggin and bFGF results in the induction of En-2, a marker of the midbrain-hindbrain junction and Krox 20, a marker of the third and fifth rhombomeres of the hindbrain. Neither of these genes is induced by noggin alone or bFGF alone at this stage, suggesting a synergy in anterior-posterior neural patterning. The response of later gastrula (stage 11-12) ectoderm to bFGF changes so that Krox 20 and En-2 are induced by bFGF alone, while induction of more posterior tissue marked by Hox B9 is eliminated. The dose of bFGF affects the amount of neural tissue induced, but has little effect on the anterior-posterior character, rather the age of the ectoderm treated is the determinant of the response. Thus, an FGF signal may account for posterior neural induction, and anterior-posterior neural patterning could be partly explained by the actions of noggin and FGF, together with the changing response of the ectoderm to these factors.
非洲爪蟾胚胎发育过程中的神经组织是由来自背侧中胚层的信号诱导产生的。前侧神经组织的诱导可能由头蛋白介导，头蛋白是一种在背侧中胚层发现的分泌型多肽。我们发现，碱性成纤维细胞生长因子（bFGF），一种已知的囊胚期外胚层中胚层诱导因子，可诱导原肠胚期外胚层形成神经组织。从10.25期外胚层由bFGF诱导产生的神经组织类型为后侧神经组织，以Hox B9表达为标志。当bFGF和头蛋白共同作用于早期原肠胚期外胚层时，会产生更完整的神经模式，且未检测到中胚层组织。用头蛋白和bFGF处理的外植体伸长，并分别以otx2和Hox B9表达为标志显示出明显的前端和后端。此外，用头蛋白和bFGF处理早期原肠胚外胚层会诱导En-2（中脑-后脑交界处的标志物）和Krox 20（后脑第三和第五菱脑节的标志物）的表达。在这个阶段，单独使用头蛋白或bFGF都不会诱导这两个基因的表达，这表明在前后神经模式形成中存在协同作用。晚期原肠胚（11 - 12期）外胚层对bFGF的反应发生了变化，使得单独的bFGF就能诱导Krox 20和En-2的表达，而以Hox B9为标志的更后侧组织的诱导则被消除。bFGF的剂量会影响诱导产生的神经组织数量，但对前后特征影响较小，相反，所处理的外胚层的年龄才是反应的决定因素。因此，FGF信号可能参与后侧神经诱导，前后神经模式形成可能部分由头蛋白和FGF的作用以及外胚层对这些因子不断变化的反应来解释。
