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Paired ducts of the embryo that run down the lateral sides of the urogenital ridge and terminate at the Müllerian eminence in the primitive urogenital sinus. In the female, they develop to form the Fallopian tubes, uterus, cervix, and the upper two-thirds of the vagina, and in the male, they are lost.
The development of the reproductive system is the part of prenatal development that concerns the sex organs. Because it largely overlaps the urinary system, the development of these organs is often described together.
The reproductive organs develop from the intermediate mesoderm. The permanent organs of the adult are preceded by a set of structures that are purely embryonic; with the exception of the ducts, they disappear almost entirely before birth. These embryonic structures are the Wolffian and Müllerian ducts, also known as mesonephric and paramesonephric ducts, respectively. The Wolffian duct remains in males and the Müllerian duct persists in females.
The Role of the Wolffian Duct
In males, the Wolffian duct persists and forms the tube of the epididymis, the ductus deferens, and the ejaculatory duct, while the seminal vesicle arises during the third month of fetal development as a lateral diverticulum from its hinder end. A large part of the head of the mesonephros atrophies and disappears; of the remainder, the anterior tubules form the efferent ducts of the testis while the posterior tubules are represented by the ductuli aberrantes and paradidymis, which is sometimes found in front of the spermatic cord above the head of the epididymis. In females, the Wolffian bodies and ducts atrophy.
The Role of the Mullerian Duct
Shortly after the formation of the Wolffian ducts, the Müllerian ducts develop. In the female, the Müllerian ducts persist and undergo further development. The portions that lie in the genital cord fuse to form the uterus and vagina. This fusion begins in the third month, and the septum formed by their fused medial walls disappears from below upward. The parts outside this cord remain separate, each forming the corresponding Fallopian tube.
About the fifth month, a ring-like constriction marks the position of the cervix, and after the sixth month the walls of the uterus begin to thicken. For a time, the vagina is represented by a solid rod of epithelial cells. A ring-like outgrowth of this epithelium occurs at the lower end of the uterus and marks the future vaginal fornix. At about the fifth or sixth month, the lumen of the vagina is produced by the breaking down of the central cells of the epithelium. The hymen represents the remains of the Müllerian eminence.
In males, the Mullerian ducts atrophy.
The gonads are the precursors of the testes in males and ovaries in females. They initially develop from the mesothelial layer of the peritoneum. The ovary is differentiated into a central part, the medulla, covered by a surface layer, the germinal epithelium. The immature ova originate from cells from the dorsal endoderm of the yolk sac.
The periphery of the testes is converted into the tunica albuginea. Cords of the central mass run together and form a network that becomes the rete testis, and another network that develops the seminiferous tubules. The descent of the testes consists of a connection from the testis to its final location at the anterior abdominal wall, followed by the development of the gubernaculum, which subsequently translocates the testis down into the developing scrotum. Ultimately, the passageway closes behind the testis. A failure in this process causes indirect inguinal hernia.
Until about the ninth week of gestational age, the external genitalia of males and females look the same and follow common development. This includes the development of a genital tubercle and dorsal membrane covering the developing urogenital opening, as well as the development of labioscrotal folds. Even after differentiation can be seen between the sexes, some stages are common, like the disappearing of the membrane. Sex-dependent development includes further protrusion of the genital tubercle in the male to form the penis. The labioscrotal folds evolve into the scrotum in males and into labia in females.