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Fox (forkhead/winged helix) genes encode a family of transcription factors that are involved in embryonic pattern formation, regulation of tissue specific gene expression and tumorigenesis. Several of them are transcribed during Xenopus embryogenesis and are important for the patterning of ectoderm, mesoderm and endoderm. We have isolated three forkhead genes that are activated during gastrulation and play an important role in the dorso-ventral patterning of the mesoderm. XFKH1 (FoxA4b), the first vertebrate forkhead gene to be implicated in embryonic pattern formation, is expressed in the Spemann-Mangold organizer region and later in the embryonic notochord. XFKH7, the Xenopus orthologue of the murine Mfh1(Foxc2), is expressed in the presomitic mesoderm, but not in the notochord or lateral plate mesoderm. Finally, XFD-13'(FoxF1b)1 is expressed in the lateral plate mesoderm, but not in the notochord or presomitic mesoderm. Expression pattern and functional experiments indicate that these three forkhead genes are involved in the dorso-ventral patterning of the mesoderm.
Fig. 1. Expression of three different forkhead genes during early Xenopus development. (A) In situ hybridization of XFKH1(FoxA4b) to a stage 12 Xenopus embryo (dorsal view). Expression is in the notochord (arrow). (B) In situ hybridization of XFKH7 to a stage 12 Xenopus embryo (dorsal view). Expression is in the presomitic mesoderm, and is absent in the notochord (arrow). (C) Transverse section of a stage 15 embryo hybridized with XFKH1(FoxA4b). Expression is in the notochord and neural floor plate. (D) Expression of XFKH7 in the trunk section of the stage 14 embryo. Expression is in the presomitic mesoderm, but it is absent in the notochord and the neural floor plate. (E) Expression of the XFD-13FoxF1b) in the trunk section of stage 15 embryo. Expression is in the lateral plate mesoderm. No expression is visible in the notochord and presomitic mesoderm. Expression is also absent in the most ventralmesoderm.
Fig. 3. Expression of XFKH7 during Xenopus development. (A) Transverse section of a stage 15 embryo showing a robust and uniform expression throughout the entire somite forming mesoderm. (B) A dorsal view of this stage 14 embryo demonstrates the expression of XFKH7 in the presomitic mesoderm and the absence of its expression in the notochord (arrow). (C) Whole mount in situ hybridization of a stage 18 embryo with the XFKH7 probe, demonstrates an intricate expression pattern of this gene at this stage. Although morphologically visible somites are not yet present, the expression of this gene clearly demonstrates a repetitive pattern of gene expression present in the presomitic mesoderm (arrow). (D) Whole mount in situ hybridization of a stage 25 embryo with XFKH7 (lateral view) demonstrating the complex expression pattern of this gene. Strong expression is present in the headmesoderm as well as in presomitic trunkmesoderm. Numbers with arrows indicate the position of transverse sections in Figs. 3E, 3F and 3G. (E) Transverse section through the anteriortrunk region (1) of the embryo in Fig. 3D. This section demonstrates the strong reduction of the expression in this part of the embryo in the somites. The remaining expression appears to be in the sclerotome (arrow). (F) Cross section through the mid-trunk region (2) of the embryo in Fig. 3D displaying a strong reduction of the expression in the somites. Only the most ventral part of the dorsal mesoderm is expressing this gene (arrow). (G) Transverse section through the posteriortrunk region (3) of the embryo in Fig. 3D. Presomitic mesoderm in the tail region is not yet differentiated and we can observe expression of XFKH7 in the entire presomitic mesoderm (arrow).
Fig. 4. Expression of XFD-13FoxF1b) during Xenopus development. (A) Transverse section through the head of a stage 35 embryo hybridized with an XFD-13FoxF1b) probe. Expression is in the neural crest cells. The plane of section is indicated in the Fig. 4C (number 1). (B) Transverse section through the trunk of a stage 35 embryo hybridized with an XFD- 13FoxF1b) probe. Expression is in the lateral and ventral regions of the embryo. The plain of section is indicated in the Fig. 4C (number 2). (C) Whole mount in situ hybridization of a stage 35 embryo with the XFD- 13FoxF1b) probe. Numbers indicate the plane of section in Figs. 4 A,B. (D) Whole mount in situ hybridization of a stage 40 embryo with XFD- 13FoxF1b) and troponin probes. Light pink staining indicates XFD- 13FoxF1b) expression, whereas the blue staining indicates troponin expression in the heart. An arrow indicates this area of expression. There is no XFD-13FoxF1b) expression in the heart.
