Vandenplas Sam, De Clercq Adelbert, Huysseune Ann
Evolutionary Developmental Biology, Ghent University, Ghent, Belgium.
J Exp Zool B Mol Dev Evol. 2014 Jul;322(5):281-93. doi: 10.1002/jez.b.22577. Epub 2014 May 29.
Most actinopterygians replace their teeth continuously throughout life. To address the question of where and how replacement teeth form in actinopterygians, it is advisable to investigate well-chosen representatives within the lineage. The African bichir, Polypterus senegalus, belongs to the earliest diverged group of the actinopterygian lineage with currently living representatives. Its well characterized dentition, together with its phylogenetic position, make this species an attractive model to answer following questions: (1) when and where does the replacement tooth form and how is it connected with the dental organ of the predecessor, and (2) is there any evidence for the presence of epithelial stem cells, hypothesized to play a role in replacement? Serial sections show that one tooth family can contain up to three members, which are all interconnected by dental epithelium. Replacement teeth develop without the presence of a successional dental lamina. We propose that this is the plesiomorphic condition for tooth replacement in actinopterygians. BrdU pulse-chase experiments reveal cells in the outer and middle dental epithelium, proliferating at the time of initiation of a new replacement tooth. It is tempting to assume that these cell layers provide a stem cell niche. The observed absence of label-retaining cells after long chase times (up to 8 weeks) is held against the light of divergent views on cell cycling properties of stem cells. At present, our data do not support, neither reject, the hypothesis on involvement of epithelial stem cells within the process of continuous tooth replacement.
大多数硬骨鱼在其一生中不断更换牙齿。为了解决硬骨鱼中替换牙在何处以及如何形成的问题，明智的做法是在该谱系中研究精心挑选的代表物种。非洲多鳍鱼（Polypterus senegalus）属于硬骨鱼谱系中最早分化出来的、目前仍有现存代表物种的类群。其特征明确的齿列，连同其系统发育位置，使该物种成为回答以下问题的有吸引力的模型：（1）替换牙何时何地形成，以及它如何与前一颗牙的牙器官相连，（2）是否有证据表明存在上皮干细胞，据推测其在牙齿替换中发挥作用？连续切片显示一个牙族可包含多达三个成员，它们均通过牙上皮相互连接。替换牙在没有继承性牙板的情况下发育。我们认为这是硬骨鱼牙齿替换的原始状态。BrdU脉冲追踪实验揭示了外层和中层牙上皮中的细胞，在新替换牙开始形成时增殖。很容易推测这些细胞层提供了一个干细胞龛。在长时间追踪（长达8周）后观察到缺乏标记保留细胞，这与关于干细胞细胞周期特性的不同观点相悖。目前，我们的数据既不支持也不拒绝上皮干细胞参与连续牙齿替换过程的假说。
