Ohazama Atsushi, Haworth Kim E, Ota Masato S, Khonsari Roman H, Sharpe Paul T
Department of Craniofacial Development, Dental Institute, King's College London, Guy's Hospital, London, United Kingdom.
Genesis. 2010 Jun;48(6):382-9. doi: 10.1002/dvg.20634.
Mammalian dentitions consist of different shapes/types of teeth that are positioned in different regions of the jaw (heterodont) whereas in many fish and reptiles all teeth are of similar type (homodont). The process by which heterodont dentitions have evolved in mammals is not understood. In many teleosts teeth develop in the pharynx from endoderm (endodermal teeth), whereas mammalian teeth develop from the oral ectoderm indicating that teeth can develop (and thus possibly evolve) via different mechanisms. In this article, we compare the molecular characteristics of pharyngeal/foregut endoderm with the molecular characteristics of oral ectoderm during mouse development. The expression domains of Claudin6, Hnf3beta, alpha-fetoprotein, Rbm35a, and Sox2 in the embryonic endoderm have boundaries overlapping the molar tooth-forming region, but not the incisor region in the oral ectoderm. These results suggest that molar teeth (but not incisors) develop from epithelium that shares molecular characteristics with pharyngeal endoderm. This opens the possibility that the two different theories proposed for the evolution of teeth may both be correct. Multicuspid (eg. molars) having evolved from the externalization of endodermal teeth into the oral cavity and monocuspid (eg. incisors) having evolved from internalization of ectodermal armour odontodes of ancient fishes. The two different mechanisms of tooth development may have provided the developmental and genetic diversity on which evolution has acted to produce heterodont dentitions in mammals.
哺乳动物的牙列由位于颌骨不同区域的不同形状/类型的牙齿组成（异型齿），而在许多鱼类和爬行动物中，所有牙齿的类型都相似（同型齿）。哺乳动物异型齿列的进化过程尚不清楚。在许多硬骨鱼中，牙齿由内胚层在咽部发育形成（内胚层牙齿），而哺乳动物的牙齿则由口腔外胚层发育而来，这表明牙齿可以通过不同的机制发育（进而可能进化）。在本文中，我们比较了小鼠发育过程中咽/前肠内胚层与口腔外胚层的分子特征。Claudin6、Hnf3β、甲胎蛋白、Rbm35a和Sox2在胚胎内胚层中的表达区域与磨牙形成区域重叠，但与口腔外胚层中的切牙区域不重叠。这些结果表明，磨牙（而非切牙）由与咽内胚层具有共同分子特征的上皮发育而来。这使得关于牙齿进化提出的两种不同理论可能都正确成为一种可能性。多尖牙（如磨牙）是由内胚层牙齿向外延伸至口腔而进化而来，单尖牙（如切牙）则是由古代鱼类外胚层甲齿向内演化而来。牙齿发育的两种不同机制可能提供了发育和遗传多样性，进化在此基础上作用，从而在哺乳动物中产生了异型齿列。
