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Free Content The Role of the Placodes in the Development of the Glossopharyngeal, Vagal, and Trigeminal Ganglia

The epibranchial placodes combine with the neural crest to form the inferior and superior ganglia of the glossopharyngeal and vagal cranial nerves, respectively. By comparison, the single trigeminal ganglion is composed of both neural crest and placodal cells. The steps that lead up to these events include gastrulation and the embryology of the notochord, neural crest, and the placodes. Each of these steps is reviewed in some detail. In previous reviews in this series, the embryology related to the olfactory, otic, and lens placodes, and to the geniculate ganglia has been discussed.1-3 However, the somewhat unusual embryology of the 2 ganglia of cranial nerves IX and X was only briefly mentioned as was the development of the trigeminal ganglion.4 This present review revisits these events and specifically focuses on how these ganglia develop.

Learning Objective: The reader will learn the unusual development of the superior and inferior glossopharyngeal and the vagal ganglia as well as review the varied steps in the embryology that proceeds these events. By comparison, the development of the single trigeminal ganglion is presented and the differences in its development from that of the ganglia of cranial nerves IX and X are emphasized.

Keywords: AP-2α = a transcription factor that regulates cell migration and apoptosis; Actin polymerizing module = regulates polarized cell growth; BMP = bone morphogenetic protein, represents a group of growth factors also known as cytokines and as metabologens (includes αBMP); C-MYC = a proto-oncogene that encodes a nuclear phosphoprotein that plays a role in cell cycle progression, apoptosis, and cellular transformation; CIL = contact inhibition of locomotion is a process whereby a cell ceases motility or changes its trajectory on collision with another cell; CTF2 = breakdown segment of N-cadherin; CXCR4 = receptor for SDF1α; Cadherin = calcium-dependent adhesion is a type of cell adhesion molecule (CAM) that is important in the formation of adheren junctions to bind cells with each other; Capping protein = binds in a calcium-dependent manner to fast-growing barbed ends of actin filaments; Catenin = a family of proteins found in complexes with cadherin cell adhesion molecules; E-cadherin = epithelial cadherin; ECM = extracellular matrix; EMT = epithelial-to-mesenchymal transition; EYA = eyes absent homolog, a transcription factor; FASCIN = a protein that induces membrane protrusion and increases cell motility; FGF = fibroblast growth factor; FOXD3 = regulates pluripotent stem cell potential by simultaneously initiating and repressing enhancer activity; GBX = GBX-2 is a homeobox protein that in humans is encoded by the GBX2 gene; IRX = Iroquois homeobox proteins 1, 2, 3; LRP6 = LDL receptor-related protein 6 is a protein coding gene; MET = mesenchymal-to-epithelial transition; MID1 = midline 1 is a protein coding gene; MMP = matrix metalloproteinase; N-cadherin = a protein involved with cell-cell adhesion, differentiation, embryogenesis, invasion, and signaling; these proteins are transmembrane receptors that facilitate cell-extracellular matrix adhesions; NOTCH = this signaling pathway is a highly conserved cell signaling system present in most multicellular organisms; OTX2 = orthodenticle homeobox 2 is a protein coding gene; P120 catenin = an essential regulator of cadherin stability, adhesion-induced signaling; PAX = paired box transcription factors 2, 3, 6, 7, 8, 9, 10; PCP = planar cell polarity; PCP pathway = the WNT/PCP signaling pathway controls tissue polarity and cell movement; PDGF = platelet-derived growth factor; PP2A = protein phosphatase 2, also known as PP2A, is an enzyme that is ubiquitously expressed, accounting for a large fraction of phosphatase activity in eukaryotic cells; PPR = pre-placodal region; PROFILIN = an actin-binding protein involved in restructuring of the actin cytoskeleton; RAC = a protein found in humans that is involved in a wide variety of cellular functions such as cell polarity, vesicular trafficking, the cell cycle, and transcriptional dynamics; ROBO = Roundabout is a family of proteins and Robo-Slit is the name of a cell signaling pathway with many diverse functions, including axon guidance and angiogenesis; SDF1α = stromal cell-derived factor-1α is a chemokine that plays a major role in cell trafficking and homings; SIX = family of nuclear factors; SLIT = a family of secreted extracellular matrix proteins that play an important signaling role in the neural development; SMAD pathway = a family of structurally similar proteins that are the main signal transducers for receptors of the transforming growth factor beta superfamily and which are critically important for regulating cell development and growth; SNAIL = a family of transcription factors that promote the repression of the adhesion molecule E-cadherin to regulate epithelial-to-mesenchymal transition during embryonic development; SOX 9, 10 = gene family that encodes a family of transcription factors that belong to a superfamily of genes characterized by a homologous sequence called the HMG-box (for high mobility group); this HMG box is a DNA binding domain that is highly conserved throughout eukaryotic species; TGF-β pathway = a signaling pathway is involved in the regulation of proliferation, differentiation, and survival or apoptosis of many cells; Tubulin = part of the protein superfamily of globular proteins; it is a major component of the eukaryotic cytoskeleton; WISE = the secreted protein; has been shown to modulate WNT signaling and also to inhibit BMP signals; modulation of WNT signaling activity is brought about by an interaction with the WNT co-receptor LRP6, whereas BMP inhibition is by binding to BMP ligands; WNT = wingless/int1 family of secreted signaling molecules; WNT pathway = are a group of signal transduction pathways made of proteins that pass signals into a cell through cell surface receptors; ZIC1 Znc1 = acts as a transcriptional activator; involved in neurogenesis; r = rhombomere

Document Type: Research Article

Publication date: June 1, 2020

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