Sex determination takes place at the time of fertilization through the coupling of two gametes, either each with one X chromosome (XX in females) or such with an X and a Y chromosome (XY in males). Primarily, the male (female) phenotype is determined by the presence (or absence) of the Y chromosome with its genes, even though genes on other chromosomes are also involved. In addition to the genetic factors, hormonal regulation also plays an important role during the various developmental steps. During the first 6 weeks the genital system is sex-indifferent and it is only then that the gonads as well as the internal and external genitalia form under hormonal influence.
Two types of testicular cells are decisively important for the development of the male genitalia: firstly, the supporting cells (Sertoli) that surround the germ cells and form the antimüllerian hormone (AMH), which causes the paramesoneophric duct (Müller) to atrophy; secondly, the interstitial cells (Leydig) that produce testosterone, which is responsible for the differentiation of the male genitalia. On both sides of the epididymis the mesonephric duct (Wolff) forms the deferent duct, the seminal vesicle, and the ejaculatory duct and it opens into the urethra below the urinary bladder. The urethra as well as various accessory glands (prostate, bulbourethral and urethral glands) stem from the urogenital sinus. The penis arises from the genital tubercle and the urethral folds and the scrotum from the genital swellings.
In the ovarian cortex the primordial germ cells are surrounded by the follicle cells that come from the coelomic epithelium and form the primordial follicles. The development of the female genitalia is characterized by the atrophy of the mesonephric duct (Wolff) and the retention of the paramesonephric duct (Müller), out of which the fallopian tube, the uterus and a part of the vagina arise. The urogenital sinus forms the genital swellings, the urethral folds, the genital tubercle, the external genitalia (lowest part of the vagina, vaginal vestibule, labia majora and minora as well as the clitoris).
Early disruptions in the differentiation of the genitalia lead to hermaphroditism (e.g., by creating a mosaic of XX and XY cell populations). Other chromosomal aberrations (Turner or Klinefelter) also lead to abnormalities of the genital organs.
To the most frequent male genital abnormalities number the incomplete closure of the urethra, an incomplete descent of the testes, as well as inguinal hernias and hydroceles.
In women, fusion disorders in the region of the paramesonephric duct (Müller) lead to abnormalities in the utero-vaginal area.
Finally, proliferative tumors appear that stem from the primordial germ cells.