關(guān)于脊椎動(dòng)物牙齒演化的經(jīng)典觀點(diǎn)是所謂的“由外向內(nèi)”（outside-in）模型,，即口腔的空腔和口腔中的牙齒都是通過內(nèi)陷作用從外胚層形成的。現(xiàn)在,，對(duì)轉(zhuǎn)基因蠑螈所做的一項(xiàng)研究表明，這個(gè)過程要比上述模型所描述的更為復(fù)雜。
對(duì)胚胎中細(xì)胞的命運(yùn)所做分析顯示，口腔中的牙齒既來自外胚層，又來自內(nèi)胚層：它們中有些甚至同時(shí)起源于外胚層和內(nèi)胚層,。這說明，神經(jīng)冠間充質(zhì)相對(duì)于上皮細(xì)胞在牙齒形成中起主導(dǎo)作用,。這一發(fā)現(xiàn)在演化上的意義是,，神經(jīng)冠細(xì)胞的牙齒形成能力是牙齒演化的必要因素，而不管上皮細(xì)胞是經(jīng)過了一個(gè)由外向內(nèi)還是由內(nèi)向外的過程,。（生物谷Bioon.com）
生物谷推薦原始出處：
Nature 455, 795-798 (9 October 2008) | doi:10.1038/nature07304
Dual epithelial origin of vertebrate oral teeth
Vladimír Soukup1, Hans-Henning Epperlein2, Ivan Horácek1 & Robert Cerny1
Department of Zoology, Charles University in Prague, Vinicna 7, 128 44 Prague, Czech Republic
Department of Anatomy, TU Dresden, Fetscherstrasse 74, 01307 Dresden, Germany
The oral cavity of vertebrates is generally thought to arise as an ectodermal invagination1, 2. Consistent with this, oral teeth are proposed to arise exclusively from ectoderm, contributing to tooth enamel epithelium, and from neural crest derived mesenchyme, contributing to dentin and pulp3, 4, 5. Yet in many vertebrate groups, teeth are not restricted only to the oral cavity6, 7, 8, 9, but extend posteriorly as pharyngeal teeth that could be derived either directly from the endodermal epithelium, or from the ectodermal epithelium that reached this location through the mouth or through the pharyngeal slits6. However, when the oropharyngeal membrane, which forms a sharp ecto/endodermal border10, is broken, the fate of these cells is poorly known. Here, using transgenic axolotls with a combination of fate-mapping approaches, we present reliable evidence of oral teeth derived from both the ectoderm and endoderm and, moreover, demonstrate teeth with a mixed ecto/endodermal origin. Despite the enamel epithelia having a different embryonic source, oral teeth in the axolotl display striking developmental uniformities and are otherwise identical. This suggests a dominant role for the neural crest mesenchyme over epithelia in tooth initiation and, from an evolutionary point of view, that an essential factor in teeth evolution was the odontogenic capacity of neural crest cells, regardless of possible 'outside-in'11 or 'inside-out'12 influx of the epithelium.
