Christiaen Lionel, Jaszczyszyn Yan, Kerfant Marina, Kano Shungo, Thermes Violette, Joly Jean-Stéphane
Center for Integrative Genomics, Molecular & Cell Biology Department, University of California, Berkeley, CA 94720, USA.
Semin Cell Dev Biol. 2007 Aug;18(4):502-11. doi: 10.1016/j.semcdb.2007.06.002. Epub 2007 Jun 15.
In chordates, the oral ectoderm is positioned at the anterior neural boundary and is characterized by pituitary homeobox (Pitx) and overlapping Dlx and Six3 expressions. Recent studies have shown that the ectoderm molecular map is also conserved in hemichordates and echinoderms. However, the mouth develops in a more posterior position in these animals, in a domain characterized by Nkx2.1 and Goosecoid expression, in a manner similar to that observed in protostomes. Furthermore, BMP signaling antagonizes mouth development in echinoderms and hemichordates, but seems to promote oral ectoderm specification in chordates. Conversely, Nodal signaling appears to be required for oral ectoderm specification in sea urchins but not in chordates. The Nodal/BMP antagonism at work during ectoderm patterning thus seems to constitute a conserved feature in deuterostomes, and mouth relocation may have been accompanied by a change in the influence of BMP/Nodal signals on oral ectoderm specification. We suggest that the mouth primordium was located at the anterior neural boundary, in early chordate evolution. In extant chordate embryos, subsequent mouth positioning differ between urochordates and vertebrates, presumably as a consequence of surrounding tissues remodelling. We illustrate these morphogenetic movements by means of morphological data obtained by the confocal imaging of ascidian tailbud embryos, and provide a table for determining the tailbud stages of this model organism.
在脊索动物中，口外胚层位于前神经边界，其特征是有垂体同源框（Pitx）以及重叠的Dlx和Six3表达。最近的研究表明，外胚层分子图谱在半索动物和棘皮动物中也具有保守性。然而，在这些动物中，口在更靠后的位置发育，处于以Nkx2.1和鹅膏蕈氨酸表达为特征的区域，其方式类似于在原口动物中观察到的情况。此外，骨形态发生蛋白（BMP）信号传导拮抗棘皮动物和半索动物的口发育，但似乎促进脊索动物的口外胚层特化。相反，Nodal信号传导似乎是海胆口外胚层特化所必需的，但在脊索动物中并非如此。因此，在外胚层模式形成过程中起作用的Nodal/BMP拮抗作用似乎是后口动物的一个保守特征，口的重新定位可能伴随着BMP/Nodal信号对口外胚层特化影响的变化。我们认为，在早期脊索动物进化过程中，口原基位于前神经边界。在现存的脊索动物胚胎中，尾索动物和脊椎动物随后的口定位有所不同，大概是周围组织重塑的结果。我们通过对海鞘尾芽胚胎进行共聚焦成像获得的形态学数据来说明这些形态发生运动，并提供一个用于确定这种模式生物尾芽阶段的表格。
