FIGURE 17. Growth factors regulating the outgrowth and ambisexual differentiation of the external genitalia. Role of sonic hedgehog (Shh) in the outgrowth and ambisexual differentiation of the genital tubercle (see table 5 for references). Most factors, with the exception of Hoxa13, are regulated by sonic hedgehog (Shh), expressed in the urethral epithelium (light green), and are identical to those regulating limb morphogenesis. Apoptosis is also affected by Shh. Data obtained from ref. (325): Klonisch T, Fowler PA, Hombach-Klonisch S. Molecular and genetic regulation of testis descent and external genitalia development. Developmental Biology, 270:1-18 (2004). http://www.sciencedirect.com/science/article/pii/S001216060400137X.