Development of the Sexual Organs in the Embryo and Fetus

Bipotential refers to the early embryo's capacity to develop either male or female reproductive organs. During the fifth week of development, gonadal ridges form from intermediate mesoderm alongside two duct systems: the mesonephric ducts and paramesonephric ducts. The embryo's genetic makeup determines which pathway develops, allowing flexibility in sexual differentiation before genetic signals activate male or female-specific development.

The SRY gene on the Y chromosome acts as the critical switch for male development by encoding the testis-determining factor (TDF), a transcription factor that initiates male differentiation. Its expression triggers the gonadal ridge to differentiate into testes by the seventh week. The developing testes then produce Müllerian-inhibiting substance, which causes the paramesonephric ducts to degenerate, preventing female structure formation.

By the eighth week, interstitial cells (Leydig cells) in the developing testes secrete testosterone under the influence of human chorionic gonadotropin. This hormone drives development of male reproductive structures including the epididymis, ductus deferens, ejaculatory duct, and seminal vesicles. Some testosterone converts to dihydrotestosterone, an androgen that promotes formation of the urethra, prostate, and external genitalia.

In female embryos lacking the SRY gene, the paramesonephric ducts persist and develop into the uterus, fallopian tubes, and the upper two-thirds of the vagina. Without Müllerian-inhibiting substance, these ducts are not degraded. The mesonephric ducts regress in females, and in the absence of dihydrotestosterone, female external genitalia including the clitoris, labia minora, and labia majora develop.

External female genitalia develop by the eighth week of embryonic development in the absence of dihydrotestosterone. The clitoris, labia minora, and labia majora form as part of female sexual differentiation. This development occurs alongside the internal reproductive structures derived from the paramesonephric ducts, completing the formation of the female reproductive system.

In female embryos, the mesonephric ducts regress and do not contribute to the reproductive system. Without the SRY gene and its associated hormonal signals, these ducts lack the stimulation needed for development. Instead, the paramesonephric ducts persist and develop into the female reproductive tract, while the mesonephric ducts degenerate as part of normal female sexual differentiation.

Müllerian-inhibiting substance (MIS) is produced by Sertoli cells in the developing testes and induces apoptosis in the paramesonephric ducts, preventing female structure formation in male embryos. This hormone is essential for male sexual differentiation, ensuring that only male reproductive structures develop. In female embryos, the absence of MIS allows the paramesonephric ducts to persist and form the uterus, fallopian tubes, and upper vagina.