The external genitals develop according to a similar pattern. Until the gonads begin releasing hormones during the 6th week, the external genital tissues of male and female fetuses are undifferentiated (I Figure 5.3). These tissues will develop into either male or female external genitals, depending on the presence or absence of a testosterone product known as dihydrotestosterone (DHT; Hotchkiss et al., 2008). DHT stimulates the labioscrotal swelling to become the scrotum and the genital tubercle and genital folds to differentiate into the glans and shaft of the penis, respectively. The genital folds fuse around the urethra to form the shaft of the penis, and the two sides of the labioscrotal swelling fuse to form the scrotum; these fusions do not occur in females. In the absence of testosterone (and possibly under the influence of a substance or substances triggered by the DSS, or femaleness gene), the genital tubercle becomes the clitoris, the genital folds become the inner vaginal lips (labia minora), and the two sides of the labioscrotal swelling differentiate into the outer vaginal lips (labia majora). By the 12th week the differentiation process is complete: The penis and scrotum are recognizable in males; the clitoris and labia can be identified in females.
Because the external genitals, gonads, and some internal structures of males and females originate from the same embryonic tissues, it is not surprising that they have corresponding, or homologous, parts. ■ Table 5.2 summarizes these female and male counterparts.
Undifferentiated before sixth week