The morphogenesis and cell differentiation in developing teeth is governed by interactions between the oral epithelium and neural crest‐derived ectomesenchyme. The fibroblast growth factors FGF‐4, ‐8, and ‐9 have been implicated as epithelial signals regulating mesenchymal gene expression and cell proliferation during tooth initiation and later during epithelial folding morphogenesis and the establishment of tooth shape. To further evaluate the roles of FGFs in tooth development, we analyzed the roles of FGF‐3, FGF‐7, and FGF‐10 in developing mouse teeth. In situ hybridization analysis showed developmentally regulated expression during tooth formation for Fgf‐3 and Fgf‐10 that was mainly restricted to the dental papilla mesenchymal cells. Fgf‐7 transcripts were restricted to the developing bone surrounding the developing tooth germ. Fgf‐10 expression was observed in the presumptive dental epithelium and mesenchyme during tooth initiation, whereas Fgf‐3 expression appeared in the dental mesenchyme at the late bud stage. During the cap and bell stage, both Fgf‐3 and Fgf‐10 were intensely expressed in the dental papilla mesenchymal cells both in incisors and molars. It is of interest that Fgf‐3 expression was also observed in the primary enamel knot, a putative signaling center of the tooth, whereas no transcripts were seen in the secondary enamel knots that appear in the tips of future cusps of the bell stage tooth germs. Down‐regulation of Fgf‐3 and Fgf‐10 expression in postmitotic odontoblasts correlated with the terminal differentiation of the odontoblasts and the neighboring ameloblasts. In the incisors, mesenchymal cells of the cervical loop area showed partially overlapping expression patterns for all studied Fgfs. In vitro analyses showed that expression of Fgf‐3 and Fgf‐10 in the dental mesenchyme was dependent on dental epithelium and that epithelially expressed FGFs, FGF‐4 and ‐8 induced Fgf‐3 but not Fgf‐10 expression in the isolated dental mesenchyme. Beads soaked in Shh, BMP‐2, and TGF‐β1 protein did not induce either Fgf‐3 or Fgf‐10 expression. Cells expressing Wnt‐6 did not induce Fgf‐10 expression. Furthermore, FGF‐10 protein stimulated cell proliferation in the dental epithelium but not in the mesenchyme. These results suggest that FGF‐3 and FGF‐10 have redundant functions as mesenchymal signals regulating epithelial morphogenesis of the tooth and that their expressions appear to be differentially regulated. In addition, FGF‐3 may participate in signaling functions of the primary enamel knot. The dynamic expression patterns of different Fgfs in dental epithelium and mesenchyme and their interactions suggest existence of regulatory signaling cascades between epithelial and mesenchymal FGFs during tooth development. © 2000 Wiley‐Liss, Inc.

We demonstrate that Sprouty genes 1, 2 and 4 are expressed in several developing organs of the craniofacial area and trunk, including the brain, cochlea, nasal organs, teeth, salivary gland, lungs, digestive tract, kidneys and limb buds. In organs such as the semicircular canal, Rathke's pouch, nasal organs, the follicle of vibrissae and teeth, Sprouty1 and Sprouty2 are expressed in the epithelium and Sprouty4 in the mesenchyme or neuronal tissue, while in the lung Sprouties1, 2 and 4 are all expressed mainly in the epithelial tissue. In the kidney, Sprouty1 is prominent in the ureteric bud whereas Sprouty2 and 4 are expressed in both the ureteric bud and the kidney mesenchyme and glomeruli deriving from it. The expression profiles suggest roles for these Sprouties in the epithelial-mesenchymal interactions that govern organogenesis.

the early stages of organogenesis indicated a crucial role for sprouty function in coordination of epithelialmesenchymal and stromal signalling, the sites of expression of these genes. Moreover, Fgf7 induced Spry2 gene expression in vitro and led with Gdnf to a partial rescue of the SPRY2-mediated defect in ureteric branching. Remarkably, it also led to supernumerary epithelial bud formation from the Wolffian duct. Together, these data suggest that Spry genes contribute to reciprocal epithelialmesenchymal and stromal signalling controlling ureteric branching, which involves the coordination of Ffg/Wnt11/Gdnf pathways.

The embryonic kidney is a classic developmental model system for studying inductive tissue interactions that govern organogenesis. We report here that Wnt-6 is expressed in the ureter bud, and that cell lines expressing Wnt-6 induce nephrogenesis in vitro. Wnt-6 cells induce tubules with similar kinetics to spinal cord (SPC) and lead to induced expression of Pax2, Pax8, Sfrp2, and E-cadherin genes, early markers of tubulogenesis. Moreover, Wnt-6 signaling rescues tubulogenesis in mesenchyme separated from Wnt-4 mutant embryos and leads to activation of Wnt-4 transcription. Wnt-6 also induces a secondary axis in early Xenopus embryos. We conclude that Wnt-6 is a candidate for the ureter epithelium-derived signal that leads to activation of kidney tubulogenesis via Wnt-4.

Wnt-4 signaling has been implicated in female development, because its absence leads to partial female to male sex reversal in the mouse. Instead of Mullerian ducts, Wnt-4-deficient females have Wolffian ducts, suggesting a role for androgens in maintaining this single-sex duct type in females. We demonstrate here that testosterone is produced by the ovary of Wnt-4-deficient female embryos and is also detected in the embryonic plasma. Consistent with this, the expression of several genes encoding enzymes in the pathway leading to the synthesis of testosterone in the mouse is induced in the Wnt-4-deficient ovary, including Cyp11a, Cyp17, Hsd3b1, Hsd17b1, and Hsd17b3. Inhibition of androgen action with an antiandrogen, flutamide, during gestation leads to complete degeneration of the Wolffian ducts in 80% of the mutant females and degeneration of the cortical layer that resembles the tunica albuginea in the masculinized ovary. However, androgen action is not involved in the sexually dimorphic organization of endothelial cells in the Wnt-4 deficient ovary, because flutamide did not change the organization of the coelomic vessel. These data imply that Wnt-4 signaling normally acts to suppress testosterone biosynthesis in the female, and that testosterone is the putative mediator of the masculinization phenotype in Wnt-4-deficient females.