Department of Zoology, Charles University in Prague, Prague, Czech Republic.
J Anat. 2013 Jan;222(1):79-99. doi: 10.1111/j.1469-7580.2012.01540.x. Epub 2012 Jul 16.
The vertebrate oral region represents a key interface between outer and inner environments, and its structural and functional design is among the limiting factors for survival of its owners. Both formation of the respective oral opening (primary mouth) and establishment of the food-processing apparatus (secondary mouth) require interplay between several embryonic tissues and complex embryonic rearrangements. Although many aspects of the secondary mouth formation, including development of the jaws, teeth or taste buds, are known in considerable detail, general knowledge about primary mouth formation is regrettably low. In this paper, primary mouth formation is reviewed from a comparative point of view in order to reveal its underestimated morphogenetic diversity among, and also within, particular vertebrate clades. In general, three main developmental modes were identified. The most common is characterized by primary mouth formation via a deeply invaginated ectodermal stomodeum and subsequent rupture of the bilaminar oral membrane. However, in salamander, lungfish and also in some frog species, the mouth develops alternatively via stomodeal collar formation contributed both by the ecto- and endoderm. In ray-finned fishes, on the other hand, the mouth forms via an ectoderm wedge and later horizontal detachment of the initially compressed oral epithelia with probably a mixed germ-layer derivation. A very intriguing situation can be seen in agnathan fishes: whereas lampreys develop their primary mouth in a manner similar to the most common gnathostome pattern, hagfishes seem to undergo a unique oropharyngeal morphogenesis when compared with other vertebrates. In discussing the early formative embryonic correlates of primary mouth formation likely to be responsible for evolutionary-developmental modifications of this area, we stress an essential role of four factors: first, positioning and amount of yolk tissue; closely related to, second, endoderm formation during gastrulation, which initiates the process and constrains possible evolutionary changes within this area; third, incipient structure of the stomodeal primordium at the anterior neural plate border, where the ectoderm component of the prospective primary mouth is formed; and fourth, the prime role of Pitx genes for establishment and later morphogenesis of oral region both in vertebrates and non-vertebrate chordates.
脊椎动物的口腔区域代表了内外环境之间的关键界面，其结构和功能设计是其所有者生存的限制因素之一。各自口腔开口（初级口）的形成和食物处理器官（次级口）的建立都需要几个胚胎组织之间的相互作用和复杂的胚胎重排。尽管次级口形成的许多方面，包括颌骨、牙齿或味蕾的发育，都有相当详细的了解，但对初级口形成的一般认识却令人遗憾地很低。本文从比较的角度回顾了初级口的形成，以揭示其在特定脊椎动物类群中被低估的形态发生多样性。总的来说，确定了三种主要的发育模式。最常见的模式是通过一个深深的内陷的外胚层口道和随后的双层口腔膜破裂来形成初级口。然而，在蝾螈、肺鱼，以及一些青蛙物种中，嘴巴通过外胚层和内胚层共同贡献的口道领形成而替代性地发育。另一方面，在硬骨鱼类中，嘴巴通过一个外胚层楔形物形成，然后最初被压缩的口腔上皮层水平分离，可能具有混合的胚层来源。在无颌鱼类中可以看到一个非常有趣的情况：尽管七鳃鳗的初级口以类似于最常见的颌骨模式的方式发育，但与其他脊椎动物相比，盲鳗似乎经历了一种独特的口咽形态发生。在讨论可能对该区域的进化发育变化负责的初级口形成的早期胚胎形态发生相关因素时，我们强调了四个因素的重要作用：第一，卵黄组织的位置和数量；与第二，原肠胚形成期间的内胚层形成密切相关，这启动了这个过程，并限制了这个区域内可能发生的进化变化；第三，前神经板边界处口道原基的初始结构，外胚层成分的未来初级口在这里形成；第四，Pitx 基因在脊椎动物和非脊椎动物脊索动物中对口腔区域的建立和后来的形态发生都起着主要作用。
