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WIREs Dev Biol
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The molecular basis of craniofacial placode development

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The sensory organs of the vertebrate head originate from simple ectodermal structures known as cranial placodes. All cranial placodes derive from a common domain adjacent to the neural plate, the preplacodal region, which is induced at the border of neural and non‐neural ectoderm during gastrulation. Induction and specification of the preplacodal region is regulated by the fibroblast growth factor, bone morphogenetic protein, WNT, and retinoic acid signaling pathways, and characterized by expression of the EYA and SIX family of transcriptional regulators. Once the preplacodal region is specified, different combinations of local signaling molecules and placode‐specific transcription factors, including competence factors, promote the induction of individual cranial placodes along the neural axis of the head region. In this review, we summarize the steps of cranial placode development and discuss the roles of the main signaling molecules and transcription factors that regulate these steps during placode induction, specification, and development. WIREs Dev Biol 2016, 5:363–376. doi: 10.1002/wdev.226 This article is categorized under: Gene Expression and Transcriptional Hierarchies > Cellular Differentiation Early Embryonic Development > Gastrulation and Neurulation Early Embryonic Development > Development to the Basic Body Plan
Location of cranial placodes in the embryonic vertebrate head. Schematic representation of various types of cranial placodes in a 10‐somite stage chick embryo. (Based on Ref ). Individual placodes develop in morphologically distinct domains along the neural tube in the head region. The adenohypophyseal placode develops ventral to the forebrain and is indicated here with a dotted line.
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Regionalization and specification of individual cranial placodes from preplacodal region. Schematic depicting the major signaling pathways and some of the important genes involved in regionalization and specification of various cranial placodes from the preplacodal region. Activation (green) and repression (magenta) of signaling pathways are color coded and the arrows indicate whether a signaling pathway is activated (↑) or inhibited (↓). BMP, bone morphogenetic protein; FGF, fibroblast growth factor; PDGF, platelet‐derived growth factor; RA, retinoic acid; SHH, sonic hedgehog.
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Induction of neural crest and the preplacodal region. (a) Prior to and during gastrulation, FGF, BMP, and WNT signaling from the underlying hypoblast and mesoderm initiates differentiation in naïve ectoderm. (b) High BMP and WNT signaling along with FGF inhibition promotes non‐neural ectoderm on the lateral side of the epiblast, while high FGF signaling and reduced BMP and WNT signaling induce neural ectoderm formation in the medial side of epiblast in developing embryo. (c) The neural border is induced between non‐neural and neural ectoderm. (d) Attenuation of BMP and WNT signaling in the presence of FGF induces the preplacodal region, while high BMP and WNT promote epidermis formation. On the other hand, BMP and WNT along with FGF signaling promote neural crest induction closer to the neural plate. Activation (green) and repression (magenta) of signaling pathways are color coded and the arrows indicate whether a signaling pathway is activated (↑) or inhibited (↓). BMP, bone morphogenetic protein; FGF, fibroblast growth factor; NC, neural crest; PPR, preplacodal region.
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Overview of cranial placode development. Placode development is a multistep process that first starts with the induction of non‐neural (blue) and neural (green) ectoderm from naïve ectoderm. The neural border (not shown) is formed in between neural and non‐neural ectoderm, which gives rise to the preplacodal region (orange) and neural crest (red). The preplacodal region develops exclusively in the head region as a U‐shaped domain surrounding the anterior neural plate and is absent from the trunk region. Neural crest is absent from the anterior‐most part of the head and develops in the posterior part of head and throughout the trunk region. Some of the specific genes of each domain are also shown and color coded in accordance to their expression zone. Different cranial placodes are induced from the preplacodal domain in response to local molecular signaling and specific transcription factors. The diagram represents a simplified and consensus view of a vertebrate embryo, but is closest to amniote embryos such as birds or humans that develop and gastrulate as a disc. Although the precise stages vary considerably in different vertebrate classes, the diagram shows an embryo prior to the onset of gastrulation (blastula), after initiation of gastrulation (early gastrula), after induction of the neural plate but before the formation of clear neural folds (early neurula), and after closure of the neural tube (neurula).
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Early Embryonic Development > Development to the Basic Body Plan
Early Embryonic Development > Gastrulation and Neurulation
Gene Expression and Transcriptional Hierarchies > Cellular Differentiation