Fig. 3. Six1, Six4, and Eya1 define a panplacodal primordium as illustrated for Xenopus. (A) Six1 is first expressed in a horseshoe-shaped domain around the neural plate. (B) At tailbud stages, Six1 expression continues in all placodes. The position of the fused profundal and trigeminal ganglia is indicated by asterisks (the respective placodes are already disappearing at this stage). (C) Position of the panplacodal primordium (red) relative to the neural plate (gray) and neural crest (blue). The gray broken line indicates the border of the neural plate proper (i.e. the entire ectoderm, which will become incorporated into the neural tube). Solid colored lines enclose areas of expression of various transcription factors, while the broken lines (Dlx3 and Msx1) enclose areas, from which expression is excluded. Note that the region of the outer neural folds immediately peripheral to the hatched gray line is occupied by neural crest cells laterally but by the panplacodal primordium anteriorly. Although no fate map is available, gene expression patterns suggest that placodes arise approximately from the following regions as indicated by white letters: the preoral ectoderm gives rise to the stomodeum with the adenohypophyseal placode and to the olfactory placodes; the lateral domain of Pax6 expression gives rise to lens and trigeminal placodes; the lateral Pax3 domain gives rise to the profundal placode; finally, the domain of Pax8 and Pax2 expression gives rise to the posterior placodal area, from which otic, lateral line, and epibranchial placodes develop. Expression data are based on the following sources: Bang et al. (1997), Bellefroid et al. (1998), Zygar et al. (1998), Heller and Brändli (1999), Feledy et al. (1999), Ermakova et al. (1999), Hollemann and Pieler (1999), Zuber et al. (1999), Zhou et al. (2000), Sasai et al. (2001), Hartley et al. (2001), Luo et al. (2001a), Schweickert et al. (2001), Schlosser and Ahrens (2004). Abbreviations: Ad: adenohypophyseal placode; EB: epibranchial placodes; L: lens placode; LL: lateral line placodes; Ol: olfactory placode; Ot: otic placode; Pr: profundal placode; V: trigeminal placode.
Image published in: Schlosser G (2006)
Copyright © 2006. Image reproduced with permission of the Publisher, Elsevier B. V.
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