Teeth are highly mineralized structures that develop in specific morphogenetic zones along the jaws within a distinct superficial epithelial unit, the dental lamina.^[1^] Both represent shared derived characters of all living vertebrates with jaws,^[2^] but form, number, and arrangement of teeth vary greatly so that they produce either homodont (all teeth are similar) or heterodont (teeth differ morphologically) dentitions. Additionally, teeth might be distributed all over the inner surface of the mouth and branchial cavities to varying degrees or restricted to definite locations. The early evolution of teeth and mechanisms resulting in the various patterns, however, are a subject of continuous debate. The recent paper by Sadier, Jackman, Laudet, and Gibert is important because it proposes a generalized model in which tooth rows develop sequentially from a single first‐formed tooth starting the inhibitor cascade for subsequent tooth development.^[3^]

This hypothesis is not novel, but the new study integrates recent evidence obtained from zebrafish and mouse into a unifying hypothesis. Both are model organisms, but they differ significantly in arrangement and location of teeth. The Zebrafish has teeth in the branchial chamber, which are continuously replaced, but lack teeth in the mouth cavity, a phenomenon that represents a secondary loss, evolutionarily speaking. The mouse, conversely, displays the very specific mammal dental heterodonty and arrangement of teeth into a single tooth row along the jaws. The mouse differs from other mammals in that it shows highly derived and adapted functional dentitions with reduced tooth types consisting only of continuously growing incisors and non‐replaced molars. Both tooth types are separated by a diastema, which is the result of a discontinuous dental lamina characterizing the specialized dentition of rodents.

The mouse, as all mammals also lacks pharyngeal teeth. Molar tooth row development in the mouse is initiated by an inhibitor tooth, as is pharyngeal tooth development in zebrafish. Anterior and posterior teeth in mammals, however, develop from different germ layers during embryogenesis with anteriors arising from the ectoderm while the molars are formed by the endoderm, as are the pharyngeal teeth in the zebrafish. The developmental boundary between anterior and posterior dentitions could indicate different evolutionary pathways for anterior and posterior dental row partitions representing distinct tooth families and might be the reason why not all mammals follow the inhibitory cascade. Changes in activation/inhibition signals during jaw development are assumed to result in variations of tooth development between species and clades.^[3^]

So far, the inhibitor cascade only has been ascertained for molar tooth rows in mammals and pharyngeal teeth in fishes, which are both of endodermal origin. And yet, the dental lamina only extends into the molar, but not into the branchial region.

Whether distinct parts of a discontinuous dental lamina initiate unconnected inhibitor cascades in the mouth cavity or a single inhibitor cascade creates a complete oral tooth row even across developmental boundaries remains ambiguous. The lack of a dental lamina in the pharyngeal zebrafish dentition, however, might imply that tooth row development is independent of such a tissue: It possibly represents an ancient conserved feature such as the development of oral tooth replacement in the bichir, which plesiomorphically is without a dental lamina.^[4^] Fishes, such as medaka, with both oral and pharyngeal teeth, or basal mammals having the complete set of heterodonty, could represent expedient model organisms for future research directions focusing on embryonic tooth family development and correlated inhibitor cascades and genetic patterns.

牙齿是高度矿化的结构，沿着独特的浅表上皮单位（牙板）内的颌骨在特定的形态发生区发育。^[ 1 ^]都代表所有有颚的活脊椎动物的共享衍生特征，^[ 2 ^]但是牙齿的形状，数量和排列变化很大，因此它们会产生同齿（所有牙齿相似）或异齿（牙齿在形态上不同）的牙齿。另外，牙齿可能以不同的程度或限制在确定的位置分布在整个口腔和分支腔的内表面。牙齿和机制的早期演变导致各种模式，这是不断争论的主题。Sadier，Jackman，Laudet和Gibert的最新论文非常重要，因为它提出了一种广义模型，其中，从单个先成形的牙齿开始依次排布齿排，并开始抑制剂级联，然后进行后续的牙齿发育。^[ 3 ^]

这个假设并不新颖，但是这项新研究将最近从斑马鱼和小鼠获得的证据整合到一个统一的假设中。两者都是模型生物，但是它们在牙齿的排列和位置上有很大的不同。斑马鱼的chamber腔内有牙齿，这些牙齿不断被替换，但在口腔中却没有牙齿，从进化上讲，这是继发性损失的一种现象。相反，鼠标显示出非常特殊的哺乳动物牙齿异齿症，并且牙齿沿颌骨排列成单个牙齿排。小鼠与其他哺乳动物的不同之处在于，它显示出高度衍生和适应的功能性牙列，牙齿种类减少，仅由不断增长的门牙和未更换的臼齿组成。两种类型的牙齿之间都被扩张，

与所有哺乳动物一样，小鼠也没有咽齿。老鼠的臼齿排发育是由抑制齿引起的，斑马鱼的咽齿发育也是如此。然而，哺乳动物的前牙和后牙在胚发生过程中从不同的胚层发育，前牙由外胚层产生，而磨牙由内胚层形成，斑马鱼的咽齿也是如此。前牙和后牙之间的发育边界可能表明代表不同牙齿家族的前牙和后牙排分区的不同进化途径，并且可能是并非所有哺乳动物都遵循抑制级联的原因。假设下颌发育过程中激活/抑制信号的变化会导致物种与进化枝之间牙齿发育的变化。^[ 3 ^]

到目前为止，仅对于哺乳动物的磨牙齿排和鱼类的咽齿，都确定了抑制剂级联反应，这两种都是内胚层来源的。然而，牙板仅延伸至磨牙，而不延伸至分支区域。

不连续的牙齿薄层的不同部分在口腔中引发未连接的抑制剂级联反应还是单个抑制剂级联反应产生了完整的口腔齿排，甚至跨越了发育边界仍然是模棱两可的。但是，咽斑马鱼牙列中缺乏牙板，可能意味着牙齿排的发育与这样的组织无关：它可能代表了一种古老的保守特征，例如在比比希尔开发了口腔替代牙齿，而整形没有牙板。^[ 4 ^] 鱼类（例如具有高齿和咽齿的牙aka）或具有完整异齿性的基础哺乳动物，可以代表权宜的模式生物，可用于未来研究方向，重点是胚胎牙齿家族的发展以及相关的抑制剂级联和遗传模式。