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Mouth Development Advanced Review Mouth development Justin Chen,1,2† Laura A. Jacox,1,3,4†‡ Francesca Saldanha,1§ and Hazel Sive1,2* A mouth is present in all animals, and comprises an opening from the outside into the oral cavity and the beginnings of the digestive tract to allow eating. This review focuses on the earliest steps in mouth formation. In the first half, we con- clude that the mouth arose once during evolution. In all animals, the mouth forms from ectoderm and endoderm. A direct association of oral ectoderm and digestive endoderm is present even in triploblastic animals, and in chordates, this region is known as the extreme anterior domain (EAD). Further support for a single origin of the mouth is a conserved set of genes that form a ‘mouth gene program’ including foxA and otx2. In the second half of this review, we discuss steps involved in vertebrate mouth formation, using the frog Xenopus as a model. The vertebrate mouth derives from oral ectoderm from the anterior neural ridge, pharyngeal endoderm and cranial neural crest (NC). Vertebrates form a mouth by breaking through the body covering in a precise sequence including specifica- tion of EAD ectoderm and endoderm as well as NC, formation of a ‘pre-mouth array,’ basement membrane dissolution, stomodeum formation, and buccophar- yngeal membrane perforation. In Xenopus, the EAD is also a craniofacial organ- izer that guides NC, while reciprocally, the NC signals to the EAD to elicit its morphogenesis into a pre-mouth array. Human mouth anomalies are prevalent and are affected by genetic and environmental factors, with understanding guided in part by use of animal models. © 2017 The Authors. WIREs Developmental Biology published by Wiley Periodicals, Inc. How to cite this article: WIREs Dev Biol 2017, 6:e275. doi: 10.1002/wdev.275 INTRODUCTION ulticellular animals need to eat and a mouth is Mthe organ that allows food into the digestive sys- † These authors contributed equally. tem. It comprises the opening from the outside of the ‡ Present address: Orthodontics Department, University of North animal, the oral cavity that is connected to the opening Carolina School of Dentistry, Chapel Hill, NC, USA and the beginning of the digestive system, the pharynx. § Present address: Department of Plastic and Oral Surgery, Boston Even some single-celled organisms like Paramecia have Children's Hospital, Harvard Medical School, Boston, MA, USA a mouth leading into a subcellular intestine.1 Many ani- *Correspondence to: [email protected] mals have accessory structures that assist eating and 1Whitehead Institute for Biomedical Research, Cambridge, mouth function, and increase complexity of this organ. MA, USA We hypothesize that the mouth arose once in evolution, 2 Department of Biology, Massachusetts Institute of Technology, and consider two lines of evidence that support this. Cambridge, MA, USA These include the understanding that the mouth is 3Harvard-MIT Health Sciences and Technology Program, Cam- bridge, MA, USA always built from ectodermal and endodermal lineages. fi 4Harvard School of Dental Medicine, Boston, MA, USA The rst multicellular animals with a clear mouth were 1–3 Conflict of interest: The authors have declared no conflicts of inter- diploblasts (with ectoderm and endoderm). Interest- est for this article. ingly, even in triploblastic animals that include Volume 6, September/October 2017 1of16 © 2017 The Authors. WIREs Developmental Biology published by Wiley Periodicals, Inc. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. Advanced Review wires.wiley.com/devbio mesoderm, the mouth still forms from a region where Mouth formation reflects this precision and involves ectoderm and endoderm directly juxtapose.2,3 In chor- many steps over a long period of development dates, we named this region the extreme anterior (~2.5 days in Xenopus, 2 weeks in humans). These domain (EAD).4,5 Another aspect of the ancient origin steps position the mouth-forming oral ectoderm and of the mouth is conservation of gene expression, lead- digestive endoderm during gastrula and neurula stages ing to the proposal of a ‘mouth gene program.’ and open the mouth as the tadpole is ready to feed. In initial studies of the Xenopus mouth, we coined The complexity of mouth formation is one reason for the term ‘primary mouth’ to indicate the initial or the many human anomalies that include this region. immature larval mouth, and ‘secondary mouth’ to indi- cate later elaboration and differentiation of structures to form the mature mouth. Although this nomenclature ORAL EVOLUTION: IS MOUTH has been useful to the community, on consideration, we DEVELOPMENT CONSERVED? think the general term ‘mouth’ is most useful through- out development. We view mouth development as a A mouth is present from the simplest multicellular continuum, where even at the earliest time after mouth organisms to humans. The commonality of mouth opening, an oral cavity and accessory structures are function is food ingestion, but auxiliary structures already forming. This review focuses on the earliest may hold, tear or grind food, such as teeth in verte- stages of mouth formation, whereby the initial mouth brates or adult sea urchins and mandibles in opening forms and the first steps of differentiation are insects. In many animals, the mouth has evolved taking place, but before the mouth is mature. extra functions, including communication and Vertebrates have a complex mouth, derived not defense, but these are secondary to its role in eating only from EAD ectoderm and endoderm that form the (Figure 1(a) and (b)). The common function of eat- oral cavity and pharynx of the digestive tract but also ing could imply that all mouths are homologous from neural crest (NC) cells that form teeth and jaws. structures, or a mouth opening may have arisen Vertebrates are ‘deuterostomes’ where a mouth breaks multiple times in evolution. To address the question through the ectodermal covering and connects to the of whether the mouths of all animals derive from endodermal digestive tract. In the Xenopus model, an ancient, conserved origin, we discuss embryonic development of the vertebrate mouth is associated with tissue contributions, associated gene expression, reciprocal signaling between the EAD and NC.4,5 and axial position. FIGURE 1 | Mouths in adult or larval animals. Frontal views of sea anemone Anthopleura elegantissima, earth worm Lumbricus terrestris,sea urchin Strongylocentrotus purpuratus,grasshopperAnacridium aegyptium,lampreyPetromyzon marinus, tadpole of frog Xenopus laevis,falconFalco cherrug, and human Homo sapiens. Red dotted line denotes the border of the oral cavity. Md, mandible; Mx, maxilla; P, pharynx; T, teeth; Tg, tongue. 2of16 © 2017 The Authors. WIREs Developmental Biology published by Wiley Periodicals, Inc. Volume 6, September/October 2017 WIREs Developmental Biology Mouth development All Mouths are Made from contributes to the pharynx. In this case, oral ecto- Ectoderm + Endoderm derm and digestive endoderm are adjacent to each One of the most persuasive pieces of evidence that other throughout gastrulation and move in the same the mouth evolved only once is that in all animals the direction towards the blastopore. mouth arises from ectodermal and endodermal germ Triploblast mouth formation is more compli- layers.1 In general, the ectoderm is characterized by cated as the digestive endoderm must move a consid- strong junctions and forms a protective outer cover- erable distance to meet the oral ectoderm. In ing. The endoderm enters the embryo during gastru- deuterostomes, blastula stage embryos have a germ lation to form the innermost layer of cells and layer position opposite that of diploblasts where ectoderm is located in the animal hemisphere and contributes to the digestive system. The mouth is 7 therefore a joint ectodermal/endodermal structure as endoderm in the vegetal hemisphere (Figure 2). The it connects an opening in the ectodermal covering to blastopore remains associated with endoderm, as in the digestive endoderm.3 diploblasts, but forms in the vegetal hemisphere. This tissue arrangement is obvious in diploblasts During gastrulation the oral ectoderm remains rela- where there are only ectodermal and endodermal tively stationary and the digestive endoderm migrates germ layers.1 Ectodermal, mesodermal, and endoder- the entire length of the embryo towards animal pole mal germ layers are present in the triploblasts, which derived ectoderm, and together these layers form the include the bilateria—divided into deuterostome and mouth. A simple example of this morphogenesis is protostome groups. Deuterostomes break a mouth found in sea urchins where the archenteron (endo- opening through the ectodermal covering. Strikingly, derm) migrates towards and meets the oral ectoderm. in deuterostomes, the mouth develops as in diplo- An additional layer of detail is present in verte- brate embryos where both mouth ectoderm and blasts from ectoderm and endoderm, and the meso- endoderm are comprised of tissues arising from dif- dermal layer is not involved. Indeed, a unique ferent locations. In sea urchins and basal deuteros- anterior region of the deuterostome embryo is devoid tomes that lack a central nervous system or brain, of mesoderm, so that future oral ectoderm and future mouth ectoderm arises from the epidermal layers.9 In foregut (anterior) endoderm are directly juxtaposed2,3 vertebrates, oral ectoderm derives also from the ante- and go on to form the mouth (Figure 2). In chordates, 4 rior neural ridge (ANR), the front of the neural we named this region the EAD. It is unclear whether plate.5 This dual source of mouth ectoderm is also mesoderm is actively prevented from entering the found in Ciona, an invertebrate chordate, whose EAD, perhaps by preferential oral ectoderm/endo- mouth is derived from the anterior neuropore.10 derm adhesion, or whether mesoderm is intrinsically Thus in chordates, development of the mouth pri- unable to migrate to the anterior.
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