**3. Development of the male genitalia**

The development of the male internal and external genitalia in an XY fetus requires a complex interplay of many critical genes, enzymes and cofactors (Hannema § Hughes, 2007). Wolffian ducts (mesonephric ducts) and mullerian ducts (paramesonephric ducts) are both present in early fetal life in the bipotential embryo. The wolffian ducts are the embryological structures that form the epididymis, vas deferens and seminal vesicles. T is produced by Leydig cells as early as 8 weeks of gestation and acts on the androgen receptor to stabilize the wolffian ducts (Tong et al., 1996). T and its 5-reduced end product, dihydrotestosterone (DHT), induce the formation of male external genitalia, including the urethra, prostate, penis and scrotum (Wilson, 1978). The mullerian ducts should regress in a male with the presence of the mullerian inhibiting substance produced by Sertoli cells in the testes. In addition, multiple other factors are necessary for the male phenotype to be congruent with a 46,XY genotype. The enzyme 17HSD3 is present almost exclusively in the testes and converts 4-A to T. The 5 -reductase type 2 enzyme is needed to convert T to DHT. In order for T and DHT to exert their androgenic role, there must be an intact androgen receptor. The lack of any one of these critical factors, including 17HSD3, can lead to a child with a DSD.
