Figure 1.20. X-ray of urethral valves in the male.

occur, although such absence is rare. Finally, if the infratubercular segment of the urogenital sinus does not develop normally, a urogenital sinus will persist with the urethra and vagina having a common external opening.

External Genital Development

A closer look at fetal external genital development reveals that while the fetus is still in the undifferentiated stage, the perineal body becomes surrounded by a mound of tissue known as the genital tubercle. This tissue assumes the form of folds which are known as the urogenital folds which extend along each side of a groove known as the urogenital groove or sulcus, the floor of which is formed by the unruptured urogenital membrane. The remaining major portions of the genital tubercle are the glans proper and the lateral buttresses. More lateral and cephalad to the genital tubercle, two paired elevations, known as the labioscrotal swellings, arise which initially are located on the very lower aspect of the abdominal wall (see Fig. 1.21).

As fetal development continues the genital tubercle gradually elongates into a more or less cylindrical phallus which is the embryological

homologue of the penis in the male and the clitoris in the female. At about the seventh week of fetal life, the urogenital membrane in the floor of the urogenital sulcus ruptures providing the urogenital sulcus with an opening to the fetal exterior. This external orifice is now known as the urethral sulcus. It is at this point in fetal development that the presence or absence of testicular hormones impact on further development.

Male External Genital Development

In the presence of normal fetal testicular development and function, the distal urogenital sinus gives rise to the prostatic urethra below the verumontanum and also forms the membranous urethra (see Fig. 1.22). Furthermore, the genital tubercle gradually elongates into a cylindrical phallus to form the penis so that by the third month the bulbar and male penile urethra are formed by fusion of the urogenital folds. These urogenital folds fuse in the midline from behind forward until the entire sulcus to the glans penis is completely closed. The glandular urethra is then formed by canalization of the urethra plate in the glans penis.

The fused edges of these urogenital folds constitute the median raphe and failure of this closure to occur at varying stages of development leads to the varying degrees of hypospadias. The lateral buttresses form the body of the penis and labio scrotal swellings form the scrotum.

Remember that this entire sequence of changes in the male phenotype is directly related to the influence of testosterone on these androgen dependent target areas and must occur and must be completed by the 12th week of male fetal development.

Female External Genital Development

In the female that is in the absence of testicular hormones, the genital tubercle remains rather small and becomes the clitoris (see Fig. 1.23). The urethral sulcus remains short,never extends onto the glans penis as in the male and opens as the vestibule with the urogenital folds becoming the labia minora. The lateral buttresses form the body of the clitoris and the scrotal swellings form the labia majora.

Anomalies of the External Genitalia

Anomalies of the external genitalia are most commonly seen with pseudohermaphrodism. These anomalies are generally related to hormonal aberrations during pregnancy. Although absence or duplication of the penis or clitoris can

occur, this anomaly is extremely rare. More commonly, the penis may remain rudimentary or the clitoris may be hypertrophic. Failure or incomplete fusion of the urogenital folds in the male fetus will result in the various degrees of hypospadias. Congenital urethral diverticula are similarly related to abnormal fusion of these folds.

Figure 1.23. genital tubercle development in the female.

Summary of Genital

Development

In summation, sexual differentiation is bipotential with the gonads and the external genitalia developing from common primordia and the ductal structures arising from separate primordia with only one system developing based on the presence or absence of a functioning testis (presence of testosterone and MIF).

Femaleness does not require gonadal influence and the bipotential fetus will develop into a female unless normal testicular influence is present.

Testes must thus be present for:

•Male genital duct development

•Müllerian ductal structure suppression

•Male external genital differentiation

External genital differentiation of the fetus commences during the eighth week of fetal life and is testosteroneand target areadependent.

The glans of the genital tubercle gives rise to the clitoris in the female and the glans penis in the male.

The lateral buttresses form the body of the penis or clitoris.

The urogenital folds give rise to the labia minora in the female and the median raphe in the male.

The labioscrotal swellings give rise to the labia majora in the female and the scrotum in the male.

Acknowledgment We would like to acknowledge the wonderful illustrations provided by Tom Mittemeyer. The illustrations provided give a visual, graphic picture and they fully illustrate the textual content.