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Determination of Gender

Although genetic sex (XX or XY) is determined at fertilization, the embryo’s gender is not distinguishable for the first six weeks of development; this is known as the indifferent period of development.  Characteristics of either male or female genitalia can often be recognized by week twelve of development.

Development of External Genitalia

In both sexes about the fourth week of development an indifferent genital tubercle develops near the cloaca and elongates to form a phallus.  In a male embryo, androgens secreted by the testes cause the phallus to elongate into the penis and the urogenital folds to fuse and form the spongy urethra.  Without influence of androgens, the phallus becomes the clitoris, the urogenital folds become the labia minora, and the labioscrotal swellings become the labia majora.  The external genital organs are not fully differentiated until about the twelfth week of development.

Development of Genital Ducts

During indifferent development both pairs of genital ducts are present.  In female embryos the paramesonephric ducts (müllerian ducts) develop into most of the female genital tract, including the uterine tubes, uterus, and part of the vaginal canal. In male embryos the testes secrete müllerian inhibiting substance, which suppresses development of the paramesonephric ducts.  Instead the mesonephric ducts develops into the epididymis, ductus deferens, and ejaculatory duct.

Figure 1 - Adult persistence of embryonic ducts in the genitourinary system

Descent of the Ovaries and Testes

The ovaries and testes develop in the abdomen and descend to their adult anatomical positions before birth.  In the male the testes descend from the abdomen into the scrotum about the twenty-eighth week of development.

Clinical Correlations

Hypospadias

Incomplete fusion of the urogenital folds creates abnormal openings of the urethra on the ventral aspect of the penis.  This malformation occurs in about 1/300 infants.

Malformations of the Uterus and Vagina

If the two paramesonephric ducts fail to fuse correctly it can result in duplication of the uterus and vagina (double uterus and double vagina). If one paramesonephric duct fails to develop it results in formation of a single uterine tube and single horn of the uterus (unicornuate uterus).

Cyrptorchidism

Failure of the testes to descend into the scrotum (cryptorchidism) is the most common malformation of the male genital system, resulting in infertility and an increased risk of testicular cancer.  The testes may remain anywhere between the abdomen and the scrotum.

Intersexuality

Rare true hermaphrodites have both ovarian and testicular tissues, usually possessing a 46,XX karyotype.  The internal and external and external genitalia are variable.  Female pseudohermaphrodites are more common, possessing a 46, XX karyotype, and typically result from exposure to excess androgens during embryologic development (as in congenital virilizing adrenal hyperplasia).  Male pseudohermaphrodites have testes and a 46, XY karyotype.  This condition results from an inadequate production of androgens by the testes, or when embryonic genital tissues lack a specific receptor needed to respond to normal levels of the hormone.

Congenital Inguinal Hernia

A large patency of the tunica vaginalis can allow a loop of intestine to herniate into the scrotum.  This must typically be corrected surgically.

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