Molecular anatomy of placode development in Xenopus laevis

Abstract

We analyzed the spatiotemporal pattern of expression of 15 transcription factors (Six1, Six4, Eya1, Sox3, Sox2, Pax6, Pax3, Pax2, Pax8, Dlx3, Msx1, FoxI1c, Tbx2, Tbx3, Xiro1) during placode development in Xenopus laevis from neural plate to late tail bud stages. Out of all genes investigated, only the expression of Eya1, Six1, and Six4 is maintained in all types of placode (except the lens) throughout embryonic development, suggesting that they may promote generic placodal properties and that their crescent-shaped expression domain surrounding the neural plate defines a panplacodal primordium from which all types of placode originate. Double-labeling procedures were employed to reveal the precise position of this panplacodal primordium relative to neural plate, neural crest, and other placodal markers. Already at neural plate stages, the panplacodal primordium is subdivided into several subregions defined by particular combinations of transcription factors allowing us to identify the approximate regions of origin of various types of placode. Whereas some types of placode were already prefigured by molecularly distinct areas at neural plate stages, the epibranchial, otic, and lateral line placodes arise from a common posterior placodal area (characterized by Pax8 and Pax2 expression) and acquire differential molecular signatures only after neural tube closure. Our findings argue for a multistep mechanism of placode induction, support a combinatorial model of placode specification, and suggest that different placodes evolved from a common placodal primordium by successive recruitment of new inducers and target genes.

Kruppel‐like factor family genes are expressed during Xenopus embryogenesis and involved in germ layer formation and body axis patterning

“…In early tail bud embryos at stage 25, the expression pattern of klf8 is similar to that of six1, a gene that is transcribed specifically in the preplacodal region (prp) (Fig. 2E) (Schlosser and Ahrens, 2004;Ahrens and Schlosser, 2005). Subsequently, klf8 transcript was detected in a variety of specific tissues or organ precursors that are derived from ectoderm and mesoderm, including forebrain, midbrain, hindbrain, otic vesicle, eyes (ey), branchial arches, ventral blood island (vbi), and somites (sm) (Fig.…”

“…2C) (Schlosser and Ahrens, 2004). At stage 30, signals were observed in the precursor cells of prospective trigeminal placode (tg) and in cells in the deep layer of cement gland (cg) (Fig.…”

Notch signaling represses p63 expression in the developing surface ectoderm

“…Differentiation was also accompanied by an increase in the mRNA levels of the ectodermal markers MSX2, GATA2 and GBX2 ( Fig. 1D) (Davidson, 1995;Li et al, 2009;Orford et al, 1998;Schlosser and Ahrens, 2004), further indicating that ectodermal fate was induced in hESCs. At the end of the differentiation protocol, a small percentage of cells differentiated to the epidermal keratinocyte lineage, as indicated by the presence of K14 + cells (Fig.…”

“…S4B). The GFP + cell population displayed elevated expression of K14 and p63 mRNAs, whereas the GFP -population expressed higher levels of the ectodermal genes Gata2, Gbx2 and Six1 (Li et al, 2009;Orford et al, 1998;Schlosser and Ahrens, 2004) (Fig. 4A,B).…”

The doublesex-related gene, XDmrt4 , is required for neurogenesis in the olfactory system

“…Several genes are expressed in the developing olfactory placodes (41). Among these are the winged helix transcription factor XBF1 (28), the distal-less related homeobox gene XDlx5 (30), a gene related to Drosophila empty spiracles, XEmx2 (31), and the helixloop-helix factor Xebf2 (29).…”