Huysseune A, Horackova A, Suchanek T, Larionova D, Cerny R
Department of Zoology, Faculty of Science, Charles University, Vinicna 7, 128 44, Prague, Czech Republic.
Research Group Evolutionary Developmental Biology, Biology Department, Ghent University, K.L. Ledeganckstraat 35, 9000, Ghent, Belgium.
Evodevo. 2024 Oct 3;15(1):13. doi: 10.1186/s13227-024-00232-4.
Previous studies have reported that periderm (the outer ectodermal layer) in zebrafish partially expands into the mouth and pharyngeal pouches, but does not reach the medial endoderm, where the pharyngeal teeth develop. Instead, periderm-like cells, arising independently from the outer periderm, cover prospective tooth-forming epithelia and are crucial for tooth germ initiation. Here we test the hypothesis that restricted expansion of periderm is a teleost-specific character possibly related to the derived way of early embryonic development. To this end, we performed lineage tracing of the periderm in a non-teleost actinopterygian species possessing pharyngeal teeth, the sterlet sturgeon (Acipenser ruthenus), and a sarcopterygian species lacking pharyngeal teeth, the axolotl (Ambystoma mexicanum).
In sturgeon, a stratified ectoderm is firmly established at the end of gastrulation, with minimally a basal ectodermal layer and a surface layer that can be homologized to a periderm. Periderm expands to a limited extent into the mouth and remains restricted to the distal parts of the pouches. It does not reach the medial pharyngeal endoderm, where pharyngeal teeth are located. Thus, periderm in sturgeon covers prospective odontogenic epithelium in the jaw region (oral teeth) but not in the pharyngeal region. In axolotl, like in sturgeon, periderm expansion in the oropharynx is restricted to the distal parts of the opening pouches. Oral teeth in axolotl develop long before mouth opening and possible expansion of the periderm into the mouth cavity.
Restricted periderm expansion into the oropharynx appears to be an ancestral feature for osteichthyans, as it is found in sturgeon, zebrafish and axolotl. Periderm behavior does not correlate with presence or absence of oral or pharyngeal teeth, whose induction may depend on 'ectodermalized' endoderm. It is proposed that periderm assists in lumenization of the pouches to create an open gill slit. Comparison of basal and advanced actinopterygians with sarcopterygians (axolotl) shows that different trajectories of embryonic development converge on similar dynamics of the periderm: a restricted expansion into the mouth and prospective gill slits.
先前的研究报道，斑马鱼的周皮（外层外胚层）部分扩展至口腔和咽囊，但未到达咽齿发育所在的内侧内胚层。相反，独立于外周皮产生的周皮样细胞覆盖了未来的牙齿形成上皮，并且对牙胚起始至关重要。在这里，我们检验了一个假说，即周皮的有限扩展是硬骨鱼特有的特征，可能与早期胚胎发育的衍生方式有关。为此，我们对具有咽齿的非硬骨辐鳍鱼类小体鲟（Acipenser ruthenus）和缺乏咽齿的肉鳍鱼类美西钝口螈（Ambystoma mexicanum）的周皮进行了谱系追踪。
在鲟鱼中，分层的外胚层在原肠胚形成末期牢固建立，至少有一个基底外胚层和一个可与周皮同源的表层。周皮向口腔的扩展程度有限，并局限于咽囊的远端部分。它没有到达咽齿所在的内侧咽内胚层。因此，鲟鱼的周皮覆盖了颌部区域（口腔牙齿）的未来牙源性上皮，但没有覆盖咽区域的。在美西钝口螈中，与鲟鱼一样，口咽中的周皮扩展局限于开口咽囊的远端部分。美西钝口螈的口腔牙齿在开口和周皮可能扩展到口腔之前很久就发育了。
周皮向口咽的有限扩展似乎是硬骨鱼类的一个祖先特征，因为在鲟鱼、斑马鱼和美西钝口螈中都发现了这一特征。周皮行为与口腔或咽齿的有无无关，口腔或咽齿的诱导可能取决于“外胚层化”的内胚层。有人提出，周皮有助于咽囊的腔化以形成开放的鳃裂。比较基底辐鳍鱼类和高等辐鳍鱼类与肉鳍鱼类（美西钝口螈）表明，不同的胚胎发育轨迹汇聚于周皮的相似动态：向口腔和未来鳃裂的有限扩展。
