Department of Zoology, Charles University, Prague, Czech Republic.
Department of Zoology, National Museum, Prague, Czech Republic.
Nature. 2017 Jul 13;547(7662):209-212. doi: 10.1038/nature23008. Epub 2017 Jul 5.
Despite the wide variety of adaptive modifications in the oral and facial regions of vertebrates, their early oropharyngeal development is considered strictly uniform. It involves sequential formation of the mouth and pharyngeal pouches, with ectoderm outlining the outer surface and endoderm the inner surface, as a rule. At the extreme anterior domain of vertebrate embryos, the ectoderm and endoderm directly juxtapose and initial development of this earliest ecto-endoderm interface, the primary mouth, typically involves ectodermal stomodeal invagination that limits the anterior expansion of the foregut endoderm. Here we present evidence that in embryos of extant non-teleost fishes, oral (stomodeal) formation is preceded by the development of prominent pre-oral gut diverticula (POGD) between the forebrain and roof of the forming mouth. Micro-computed tomography (micro-CT) imaging of bichir, sturgeon and gar embryos revealed that foregut outpocketing at the pre-oral domain begins even before the sequential formation of pharyngeal pouches. The presence of foregut-derived cells in the front of the mouth was further confirmed by in vivo experiments that allowed specific tracing of the early endodermal lining. We show that POGD in sturgeons contribute to the orofacial surface of their larvae, comprising oral teeth, lips, and sensory barbels. To our knowledge, this is the first thorough evidence for endodermal origin of external craniofacial structures in any vertebrate. In bichir and gar embryos, POGD form prominent cranial adhesive organs that are characteristic of the ancient bauplan of free-living chordate larvae. POGD hence seem arguably to be ancestral for all ray-finned fishes, and their topology, pharyngeal-like morphogenesis and gene expression suggest that they are evolutionarily related to the foregut-derived diverticula of early chordate and hemichordate embryos. The formation of POGD might thus represent an ancestral developmental module with deep deuterostome origins.
尽管脊椎动物的口腔和面部区域存在广泛的适应性变化，但它们早期的咽腔发育被认为是严格统一的。通常情况下，外胚层勾勒出外表面，内胚层勾勒出内表面，依次形成口腔和咽囊。在脊椎动物胚胎的最前端区域，外胚层和内胚层直接相邻，最早的外胚层-内胚层界面，即原始口的初步发育，通常涉及外胚层口盖凹陷，限制前肠内胚层的前向扩张。在这里，我们提供的证据表明，在现存非硬骨鱼类的胚胎中，口腔（口盖）的形成先于前脑和正在形成的口盖之间的明显前咽肠憩室（POGD）的发育。对双鳍鱼、鲟鱼和雀鳝胚胎的微计算机断层扫描（micro-CT）成像显示，前咽区的前肠外突甚至在咽囊的顺序形成之前就开始了。通过允许早期内胚层衬里的特定追踪的体内实验，进一步证实了前口中的前肠衍生细胞的存在。我们表明，鲟鱼的 POGD 有助于其幼虫的口面表面，包括口腔牙齿、嘴唇和感觉触须。据我们所知，这是任何脊椎动物中外胚层起源的外部颅面结构的第一个全面证据。在双鳍鱼和雀鳝胚胎中，POGD 形成了突出的颅粘连器官，这是自由生活脊索动物幼虫古老的 Bauplan 的特征。因此，POGD 似乎可以说是所有有颌鱼类的祖先进化支，它们的拓扑结构、咽样形态发生和基因表达表明它们与早期脊索动物和半索动物胚胎中前肠衍生的憩室在进化上有关。因此，POGD 的形成可能代表了一个具有深远后口动物起源的祖先发育模块。
