Ectopic application of FGF8 beads to the murine GT induced mesenchymal gene expression, and also promoted the outgrowth of the GT. Experiments utilizing anti-FGF neutralizing antibody suggested a growth-promoting role for FGF protein(s) in GT

Recent molecular cloning of many of the genes controlling development, including growth factors and homeobox genes, has led to the formulation of basic plans of organogenesis (Kessel and Gruss, 1990; Chisaka et al., 1992; Gehring et al., 1994; Johnson and Tabin, 1997; Hogan, 1996, 1999; Gehring, 1999). During fetal and neonatal development of mammals, the last structures to develop along the embryonic anteroposterior (AP) axis are the external genitalia, organs displaying characteristic proximodistal-oriented outgrowth adjacent to the cloacal membrane (Fig. 1). The mammalian external genitalia are highly developed structures to permit efficient internal fertilization. The external genital anlage, the genital tubercle (GT), differentiates into a penis in males and a clitoris in females (Murakami and Mizuno, 1986; Hildebrand, 1995). Because of the lack of a suitable experimental system, the mechanisms of GT formation have been very little studied by molecular developmental analysis. Pioneering study has suggested that GT development may have some similarities to limb development, with both structures exhibiting organ outgrowth (Dolle et al., 1991; Yamaguchi et al., 1999). Vertebrate limb development depends on the establishment and maintenance of the apical ectodermal ridge (AER), a specialized ectoderm region at the distal tip of the limb bud (Duboule, 1993; Tickle and Eichele, 1994; Johnson and Tabin, 1997). It has been speculated that the GT epithelium and mesenchyme develop through presumptive epithelial-mesenchymal interactions, although no discrete structures resembling an AER have been described, nor have any candidate molecules for those “AER functions“ been suggested in studies of external genitalia formation (Murakami and Mizuno, 1986; Dolle et al., 1991). Epithelial-mesenchymal interactions play an essential role in regulating a wide variety of developmental processes (Le Douarin and Jotereau, 1975; Hall 1981; Slavkin et al., 1984; Thesleff et al., 1991, 1995; Tickle and Eichele 1994; Johnson and Tabin, 1997; Hogan, 1999). Signaling between the 2471 Development 127, 2471-2479 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 DEV2522