Hormones produced by the fetal gonads are important for shaping the external genitals in a sex-specific way.
Just as the testes and ovaries originate from the same embryonic structures, so too do the external genitals of males and females originate from embryonic tissues that start out being identical in both sexes. (Note that genitals are sometimes called genitalia; these are two different words that mean the same thing.) Around the 7th week of embryonic growth, a small projection appears in the genital area. This structure, referred to as the genital tubercle, has the same appearance in males and females.
In males, the Leydig cells of the developing testis produce testosterone, the most important hormone in the class of steroids known as androgens. Testosterone triggers a cascade of events that cause the genital tubercle to grow into a penis. As part of this process, a flat piece of tissue "zips up" to form the urethra (tube through which urine is passed) starting at the base of the penis and finishing at the tip. The zipping-up of the urethra needs to be very finely co-ordinated with the extension and shaping of the penis so that the opening of the urethra comes to be at the tip of the penis. When this co-ordination is disturbed, either through incorrect gene function or sometimes by chemicals the mother is exposed to, the urethral opening ends up somewhere along the length of the penis, a condition known as hypospadias.
In females, the genital tubercle is typically not exposed to testosterone, and hence does not grow as large. Instead, it develops into the clitoris. The urethra develops beside the clitoris, not within it.
A second structure common to both sexes is the pair of labioscrotal folds (sometimes referred to as the urogenital folds), which form around the base of the genital tubercle. In males, these folds of tissue typically expand and fuse in the midline, underneath the developing penis, to form the sac called the scrotum. This process, like the formation of the penis, results from exposure to testosterone produced by the developing testes. In females, the labioscrotal folds are not exposed to testosterone and therefore remain unfused as the labia, the inner and outer folds each side of the vaginal opening.
Short schematic movies illustrating the development of the male and female external genitals from a common set of fetal tissues can be viewed below. (The movies need QuickTime to play.)
Last updated: 15 July 2015 PK
Edit history: Author P. Koopman 9/12; revised PK 5/13, 10/13