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???displayArticle.abstract??? Lens development provides a good model system for studying cellular and molecular mechanisms underlying embryonic induction and morphogenesis. Members of the large Maf family of transcription factors, L-Maf and c-Maf, have been shown to play key roles in chick and mouse lens development. Here we report identification of two Xenopus maf genes, XmafB and XL-maf, which exhibit unique temporal and spatial expression patterns during lens formation. XmafB can first be detected in the presumptive lens-forming ectoderm, when the primary eye vesicle makes contact with the headectoderm. XL-maf expression appears a little later, just before thickening of the lens placode, and both XmafB and XL-maf can be detected in the lens placode. During lens vesicle formation, the expression domains of XmafB and XL-maf segregated from each other, resulting in restricted expression in lens epithelial and fiber cells, respectively. When the optic cup anlagen was removed, only XmafB expression is detected. Both Mafs can induce the lens fiber cell-specific markers, betaA4- and gamma-crystallins. In animal cap assays, XmafB can induce Pax6, Xlens1 and Sox3 expression, but XL-maf fails to induce Pax6 and Xlens1 expression. These results suggest that these maf genes are involved in the regulation of cell-type specific gene expression and play roles in inductive events during Xenopus lens development.
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Fig. 2. Whole-mount in situ hybridization showing expression domains of XmafB. (A–H) Lateral view of whole embryos. (A′–G′) High magnification of the eye showing XmafB expression (A–G). (A,A′) At stage 22 XmafB is expressed in rhombomeres (r) 5 and 6 (black arrow), the presumptive blood island (white arrow) and lens-forming ectoderm (arrowhead). (B,B′) At stage 24 signal appears in somites (arrowhead) and neural crest cells (arrow). (C,C′) Stage 28. Staining is detected in the developing pronephros (arrow) and lens placode. (D,D′) The expression in r4 becomes detectable at stage 29/30 (arrow). (E,E′) Stage 32. (F,F′) Stage 33/34. Strong staining is detected in somites (arrowhead) and pronephros (arrow). (G,G′) Stage 37/38. The lens is stained. (H) XmafB is initially expressed in the anteriorneuroectoderm at stage 14 (arrow). (I) Dorsal view of stage 19 embryo. Signal is found in r5 and 6 (arrow).
Fig. 3. Sections of embryos analyzed by whole-mount in situ hybridization for XmafB expression. (A–G) Sectioning planes are indicated in schematic diagrams. (A) At stage 14 XmafB is expressed in the anteriorneuroectoderm. (B) At stage 22, signal also appears in neural crest cells (arrow). (C) Stage 24. Staining is observed in the dorsal part of the neural tube. (D) Section of stage-23 embryo at the level of the trunk showing XmafB expression in somite (arrowhead) and blood island (white arrow). (E–H) Stage 32. (E) Section at the level of rhombomere (r) 4. Bilateral staining is seen in the dorsal part of the neural tube (arrow). (F) Section through r5 and pronephros. Expression of XmafB is found in the dorsal part of the neural tube (arrowhead) and developing pronephros (arrow). (G) Section at the level of the trunk showing XmafB expression in somite (arrowhead). (H) Parasagittal section shows myotomal cells are stained. Scale bars: 200 μm.
Fig. 4. Whole-mount in situ hybridization showing the expression of XL-maf. (A–F) Lateral view of whole embryos. (A′–F′) High magnification of the eye showing XL-maf expression (A–F). (A,A′) At stage 24 XL-maf is initially expressed in the PLE (arrowhead). (B,B′) At stage 28 staining is detected in lens placode. (C,C′) Stage 31. (D,D′) Stage 32. Strong signal in developing lens placode is found. (E,E′) Stage 35/36. XL-maf is expressed in lens detached from ectoderm. (F,F′) At stage 37/38 signal also appears in retina (arrowhead).
Fig. 5. Transverse sections through the level of the eye showing expression of XmafB (A–G) and XL-maf (A′–G′) during lens formation, after whole-mount in situ hybridization. (A,A′) At stage 23 expression of XmafB is found in the sensorial layer of the ectoderm, but not XL-maf. (B,B′) Both genes are expressed in the sensorial layer which is beginning to thicken at stage 27. (C,C′) Stage 28. (D,D′) At stage 31 signals are detected in the developing lens placode. (E,E′) At stage 32 XmafB is detectable in the border between the ectoderm and lens placode, while XL-maf can be detected in the prospective lens fiber cells of the placode. (F,F′) Stage 33/34. XL-maf is also expressed in the prospective neural retina (arrows). (G,G′) Subsequently expression of XmafB is found in lensepithelium, and that of XL-maf in lens fiber cells at stage 37/38. he, headectoderm; l, lens; le, lensepithelium; lf, lens fiber cells; lp, lens placode; oc, optic cup; ov, optic vesicle. Scale bars: 100 μm.
