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Question 6#

Embryology of the gonads. Which one of the following is TRUE?

A. The genital ridges develop during the 12th week
B. In female embryos, Müllerian Inhibiting Substance causes regression of the paramesonephric duct
C. Sertoli cells secrete testosterone that plays a role in embryogenesis
D. Hydatid of Morgagni is a remnant of the paramesonephric duct
E. The testes reach the inguinal region by the 24th week

Correct Answer is D

Comment:

The sex of the embryo is determined genetically at the time of fertilisation. However, it is not until the 7th week that the gonads develop features of male or female morphology. Primordial germ cells migrate during the 5th week of gestation from the yolk sac along the hindgut and its dorsal mesentery towards the mesenchyme of the posterior body wall. The mesonephros and coelomic epithelium proliferate to form a pair of genital ridges. During the 6th week, cells of the genital ridge invade the mesenchyme to form primitive sex cords, consisting of cortical and medullary regions that will eventually invest the germ cells and support their development. At the same time, paramesonephric ducts develop lateral to the mesonephric ducts in a cranio-caudal direction. Primordial germ cells carry the XY sex chromosome if the embryo is genetically male. The SRY gene located on the Y chromosome encodes for testis determining factor. This acts to stimulate proliferation of cells in the medulla and degeneration of cells in the cortical regions of the primitive sex cords. During the 7th week, the cells differentiate into Sertoli cells that become organised into testis cords and eventually to seminiferous tubules around the time of pubescence. These Sertoli cells secrete Müllerian Inhibiting Substance (MIS) that cause regression of the paramesonephric ducts between the 8th and 10th weeks, some of which remain as the Hydatid of Morgagni. Near the hilum, the testis cords break up into a network of thin strands that eventually become the rete testis. Some of these thin walled ducts connect to the mesonephric ducts from the medial aspect of the gonads. With further development, the testis cords become separated from the surface epithelium by the fibrous tunica albuginea. By the 10th week, Leydig cells develop from the mesenchymal cells of the genital ridge and secrete testosterone, initially under the influence of placental chorionic gonadotropin and later by the pituitary gonadotrophins. By the 12th week, the mesonephric (Wolffian) duct is transformed into the vas deferens under the influence of testosterone. The cranial nephric duct also degenerates whilst the part adjacent to the presumptive testis becomes the epididymis. During the 7th week, the testes are located near the kidneys, held in position by the dorsal cranial suspensory ligament and the ventral ligament, which becomes the gubernaculum. During the 12th week, the testes descend to the inguinal canal and the suspensory ligament regresses. By the 28th week, there is an outgrowth from the gubernaculum that passes through the external ring and enters the scrotum. It is hollowed out by a tongue of peritoneum (processus vaginalis), that allows the testes to descend into the scrotum by 33 weeks. The peritoneal layer covering the testis becomes the visceral layer of the tunica vaginalis and the remainder of the peritoneal sac forms the parietal layer. The canal connecting the peritoneal cavity with the vaginal process becomes obliterated shortly before or after birth. The lack of testis determining factor in the female embryo causes the primitive sex cords to degenerate and the mesothelium of the genital ridge forms the secondary cortical sex cords. They invest the primordial germ cells to form ovarian follicles, which differentiate into oogonia and then primary oocytes that remain in this phase until puberty. The mesonephric ducts regress but the paramesonephric ducts develop into fallopian tubes, uterus and upper two-thirds of the vagina. The remainder of the inferior third of the vagina forms as a result of the caudal ends of the paramesonephric ducts, which come together and form a common channel (uterovaginal canal). It then fuses with the thickened tissue on the posterior urogenital sinus known as the sinovaginal bulb. The vaginal plate then develops at the inferior part of the canal, which then elongates and canalises between 12 and 20 weeks to form the inferior vaginal lumen.