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???displayArticle.abstract??? Cranial placodes are thickenings of the embryonic headectoderm that contribute to the paired sense organs and to the cephalic peripheral nervous system. Here we report the spatiotemporal expression pattern of transcription factor Pitx2c during Xenopus laevis cranial placode formation, focusing more specifically on key stages of trigeminal and profundal placode development. We also compare its expression to five genes that have been associated with development of these sensory placodes, namely Foxi1c, Islet1, NeuroD, Pax3, and Six1. We show that while initially expressed in both the trigeminal and profundal placodes, Pitx2c is later restricted to the prospective profundal ganglion, where it is co-expressed with Islet1, NeuroD and Pax3. This combination of factors defines a molecular signature for the characterization of the profundal versus trigeminal ganglia in Xenopus.
Fig. 1 (left). Whole-mount in situ hybridization of six placodal genes encoding transcription factors expressed at stage 14 (early neurula) and stage 17 (mid-neurula). The position of the prospective trigeminal placode is indicated (magenta arrows). For each stage, left panels are frontal views, dorsal to top, and right panels are lateral views, anterior to left, dorsal to top. Scale bar, 500 mm.
Fig. 2 (right). In situ hybridization of six placodal genes expressed at stage 21 (early tailbud). Prospective trigeminal (magenta arrows) and profundal (green arrows) placodes are indicated. Left panels are frontal views, dorsal to top, and middle panels are lateral views, anterior to left, dorsal to top. Transverse sections (right panels) were performed at the level of the optic vesicles. A white line on each side of the embryo indicates the plane of section (middle panels). br, brain; cg, cement gland; ov, optic vesicle. Scale bar for whole embryos is 500 mm, and for histological sections is 200 mm.
Fig. 3. in situ hybridization of six placodal genes expressed at the tailbud stages. Trigeminal (magenta arrows) and profundal (green arrows) ganglia are indicated. For Whole-mount in situ hybridization, lateral views, anterior to left, dorsal to top. Transverse sections (stage 25) were performed at the level of the optic vesicles. A white line on each side of the embryo indicates the plane of section (left panels). br, brain; cg, cement gland; ov, optic vesicle. Scale bar for whole embryos is 500 mm, and for histological sections is 200 mm. TABLE 1SUMMARY OF THE SPATIOTEMPORAL EXPRESSION OF SIX GENES IN THE TRIGEMINAL AND PROFUNDAL PLACODES AND GANGLIA “+†indicates gene expression, “–†indicates that the gene was not detected.
Fig. 4. Diagram summarizing the developmental expression of six placodal genes at the tailbud stages. The position of the profundal placode/ganglion (yellow) and trigeminal placode/ganglion (blue) are indicated at stage 21 (placodes) and stage 25 and 29/30 (ganglia). Based on their expression in the profundal placode/ganglion, the trigeminal placode/ganglion or both, each gene name is highlighted in yellow, blue or green, respectively. White indicates no expression. cg, cement gland; le, lens; ol, olfactory placode; ov, optic vesicle; op, otic placode. The diagram of the embryos is modified from Schlosser and Northcutt (2000).
