Figure 16. Growth factors regulating the outgrowth and ambisexual differentiation of the external genitalia (see table 4 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. Subsequently, testicular Leydig cells secrete testosterone that is metabolized into dihydrotestosterone (DHT) in the genital tubercle. DHT acting via the androgen receptor provokes virilization of the genital tubercle. Data from ref. (261) .