Hormones produced by the fetal gonads are important for shaping the external genitals in a sex-specific way.

 


 

Even though they end uo looking very different, the external genitals of males and females originate from embryonic tissues that start out being identical in both sexes.

(Genitals are sometimes called genitalia; these are two different words that mean the same thing: the reproductive organs on the outside of the body.)

Around the 7th week of gestation, a small nub appears between the legs of the embryo. This structure is called the genital tubercle, and it looks the same in males and females.

 

The penis in males and the clitoris in females

In males, the developing testis starts to produce testosterone, the most important hormone in the class of steroid hormones known as androgens. Testosterone triggers a cascade of events that cause the genital tubercle to become longer and grow into a penis. As part of this process, the genital tubercle starts to curl lengthwise to form an open tube, which then "zips up" along its length from the base to the tip, to form a closed tube, called the urethra (the tube through which urine is passed).

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 may end 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 so it does not grow as large. Instead, it develops into the clitoris. The urethra develops beside the clitoris, not within it.

So, the penis and the clitoris are analogous structures in terms of their origins, development and shape.

 

The scrotum in males and the labia in females

A second structure common to both sexes is a pair of skin folds (flaps) which form around the base of the genital tubercle. These are called labioscrotal folds, or urogenital folds. In males, these folds of tissue typically expand and fuse together in the midline to form a sac, underneath the developing penis, called the scrotum. This process, like the formation of the penis, results from exposure to testosterone from the developing testes.

In females, the labioscrotal folds are not exposed to testosterone, and therefore remain unfused and form the labia, the inner and outer folds each side of the vaginal opening.

Thus, the scrotum and the labia are analogous structures in terms of their origins, development and shape. As the fetus develops further, the testes in males usually move from inside the body where they first form, down into the scrotum where they remain (see Development of other sex-specific features). Ovaries in females do not usually descend in this way, but stay within the abdomen.

Short schematic movies illustrating the development of the male and female external genitals from a common set of fetal tissues can be viewed below.

Female

Male

 

-----------------------------------------------

Last updated: 5 August 2021 PK

Edit history: Author P. Koopman 9/2012; revised PK 5/2013, 10/2013, 7/2015