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The paraxis gene encodes a basic helix-loop-helix transcription factor that is expressed in paraxial mesoderm and whose mutant displays an inability to form epithelial somites. Here, the molecular characterization of Xenopus paraxis is reported. paraxis is expressed in the paraxial mesoderm and somites but is down-regulated during muscle differentiation. In addition to its paraxial mesodermal expression, described in other organisms, two novel expression domains of paraxis were found: the neural tube and the headmesoderm. paraxis expression in the neural tube was compared with the expression of the neural markers Xash and Xiro1, and we concluded that paraxis is expressed in a broad band in the prospective sulcus limitans of the neural tube.
Figure 2. paraxis is expressed in somitic mesoderm. A-D: Dorsal view of embryos at different stages as indicated in the figure. Anterior (a) is at the top; p, posterior. E-H: Sections of embryos shown in A-D, in the region indicated by the line; n, notochord. I: Stage 22 embryo showing an anterior-posterior wave of paraxis down-regulation. J: Anterior section of embryo shown in I. Arrowhead, somitic expression of paraxis is restricted to the dermatome; arrow, initial expression of paraxis in the neural tube; n, notochord. K: Posterior section of embryo shown in I. Note that the complete somite is stained; n, notochord. L: Stage 26 embryo. Note that, as muscle differentiation is more advanced in this embryo, a down-regulation of paraxis is observed, except in the most posteriorsomites.
Figure 3. paraxis is expressed in the paraxial mesoderm and neural tube. a, anterior; p, posterior. A: Stage 23 embryo stained for paraxis (purple) and the 12/101 antigen (brown) that is expressed in differentiated muscles. Note that paraxis expression is not found concurrent with markers of muscle differentiation. B: Stage 31 embryo stained for paraxis and 12/101. Note that, at this stage, most of the somitic muscles have already differentiated and no paraxis expression is observed, except in the tip of the tail bud. No expression in the neural tube is observed. C: Section of embryo shown in A. Note that, in the somite, paraxis is restricted to the dermatome (arrowhead). In addition, a clear expression in the neural tube is observed (arrows); n, notochord. D: Equivalent section of a stage 23 embryo showing expression of Xash in the sulcus limitans of the neural tube (arrows); n, notochord. E: Equivalent section of a stage 23 embryo showing expression of Xiro1 in the sulcus limitans of the neural tube (arrows); n, notochord. F: Summary of paraxis expression in the neural tube (nt) and somites (s), compared with sulcus limitans markers. n, notochord. Scale bars = 125 mu m in C-E.
Figure 4. Paraxis is expressed in the headmesoderm underlying the neural crest territory. A: Dorsal view of a stage 18 embryo stained for paraxis. Arrow, expression in a region anterior to the paraxial mesoderm. Continuous line, anterior limit of paraxial mesoderm; dashed line, section shown in D. B: Dorsal view of a stage 18 embryo stained for Slug (purple) and 12/101 (brown). Continuous line, anterior limit of paraxial mesoderm; dashed line, section shown in E. C: Higher magnification of anterior region of A. D: Section of embryo shown in A. The section was made just anterior to the somite expression. Note a broad band of expression in the headmesoderm (HM) and no expression in the prechordal mesoderm (PM). NC, neural crest; nt, neural tube. E: Section of embryo shown in B. The section was made at the border of somite expression, to include somite labeling as a landmark. s, somite. F: Summary of paraxis expression in the headmesoderm (HM); PM, prechordal mesoderm; NC, neural crest; nt, neural tube.
Figure 1
Sequence and structure of Xenopus paraxis. A: Deduced amino acid sequences from human, mouse, chick, Xenopus, and zebrafish paraxis were aligned using the ClustalW algorithm. Identical and similar amino acids are in black and gray boxes, respectively. The helixâ€loopâ€helix domain is indicated. B: Phylogenetic unrooted tree including different paraxis members as well as the XTwist protein.
