31.30 : Development of the Sexual Organs in the Embryo and Fetus

Development of the reproductive organs in an embryo starts from a bipotential state. This means the early embryo can develop either male or female reproductive organs. The formation of these organs begins with the growth of gonadal ridges that arise from the intermediate mesoderm during the fifth week of development.

Near the gonadal ridges, two duct systems are present: the mesonephric ducts (Wolffian ducts) and paramesonephric ducts (Müllerian ducts). These ducts form the basis for the male and female reproductive systems, respectively, and both are connected distally to urogenital sinus.

In male embryos with one X and one Y chromosome, the SRY gene on the Y chromosome acts as the critical switch for male development. The SRY gene encodes the testis-determining factor (TDF), a transcription factor essential for initiating male differentiation. Its expression triggers the differentiation of the gonadal ridge into testes by the seventh week. The Sertoli cells of the developing testes produce Müllerian-inhibiting substance (MIS), which induces apoptosis in the paramesonephric ducts, preventing the development of female structures.

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

The gonadal ridge differentiates into ovaries in female embryos, which lack a Y chromosome and the SRY gene. Without MIS, the paramesonephric ducts persist, giving rise to the uterus, fallopian tubes, and the upper part of the vagina, while the mesonephric ducts regress. In the absence of DHT, female external genitalia, including the clitoris, labia minora, and labia majora, develop. This intricate process, regulated by genetic and hormonal signals, ensures the formation of distinct male and female reproductive organs, critical for sexual differentiation and reproduction.