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Xenbase Image ID: 84071

Fig. 4. Summary of placodal development in Xenopus laevis in lateral views (modified from Schlosser and Ahrens, 2004; stages after Nieuwkoop and Faber, 1967). (A, B) Gene expression domains (colored outlines) during neural plate (A) and neural fold stages (B). The panplacodal primordium (red) is apposed to the neural plate (gray) anteriorly and to neural crest (blue) laterally (see also Fig. 3C). Arrows in panel B indicate shifts of placodal expression domains due to neural fold elevation (dorsal shift) and wedge-like expansion of anterior ectoderm accompanying the formation of the optic vesicles. Green stars identify three areas of Pax6 expression that will contribute (from anterior to posterior) to adenohypophyseal and olfactory placode (light green), lens placode (blue green), and trigeminal placode (dark green). Six1 and Eya1 expressions are downregulated at neural fold stages in the regions of prospective lens placode and cement gland (black asterisk). (C–F) Development of placodes after neural tube closure until late tailbud stages. Drawings are based on reconstructions of ectodermal thickenings from serial sections (after Schlosser and Ahrens, 2004; Schlosser and Northcutt, 2000). The adenohypophyseal placode, which is located medial to the ventral part of the olfactory placode, is not shown in these lateral views. Various green colors identify placodes or prospective placodes expressing Pax6, yellow identifies the profundal placode expressing Pax3, and brown, pink, and orange jointly identify the posterior placodal area expressing Pax2 and Pax8, with pink being reserved for the subregion forming the otic placode/ vesicle and orange for the subregions forming the epibranchial placodes. At stage 21, the posterior placodal area is divided into an anterior and a posterior subregion of thickened ectoderm, separated ventrally by an indentation and dorsally by a region of thinner ectoderm (between broken black lines), while the prospective otic placode (pink) is identifiable as a particularly prominent thickening. The broken blue lines in panel C indicate neural crest streams as reconstructed from serial sections (Schlosser and Northcutt, 2000). The arrow in panel C indicates that the posterior placodal area expands posteriorly at early tailbud stages. Brown arrowheads in panels D–F indicate developing lateral line primordia. Abbreviations: Ad/Ol: anterior placodal area, from which adenohypophyseal (Ad) and olfactory placodes (Ol) develop; AV: anteroventral lateral line placode; cg: cement gland; Hp1: first hypobranchial placode; L: prospective lens placode (hatched outline), lens placode or lens (invagination of placode between stage 27 and 33/34); LL/Ot/EB: posterior placodal area, from which lateral line (LL), otic (Ot), and epibranchial (EB) placodes develop; M: middle lateral line placode; Ol: olfactory placode; Ot: otic placode or vesicle (invagination of placode between stage 24 and 33/34); P: posterior lateral line placode; Pr: profundal placode; V: trigeminal placode; VII: facial epibranchial placode; IX: glossopharyngeal epibranchial placode; X1: first vagal epibranchial placode; X2/3: second and third vagal epibranchial placodes (fused).

Image published in: Schlosser G (2006)

Copyright © 2006. Image reproduced on Xenbase with permission of the Publisher, Elsevier B. V.

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