Müllerian Duct

MÜLLERIAN DUCT, or duct; ductus Mülleri (named after the famous physiologist Johannes Müller, who first described it), is an embryonic structure that develops in the embryos of all vertebrates adjacent to the Wolffian duct (Figure 1) and gives rise to the efferent ducts of the female reproductive system: the Fallopian tubes, uterus, and vagina in mammals, and the oviducts in all other vertebrates. The Müllerian duct is formed in embryos of both sexes: in selachians by the splitting of the Wolffian duct, and in other vertebrates independently. The primordium of the Müllerian duct in the human embryo appears at the end of the 1st and beginning of the 2nd month: the epithelium covering the Wolffian body in its cranial part invaginates to form a longitudinally running groove; in the lower section, the edges of the groove close up, forming a tube lying lateral to the Wolffian duct (Figure 2); the upper section remains in the form of a slit; overall, this produces a funnel, embedded in the mesenchyme of the Wolffian body and ending blindly.

Further development of the Müllerian duct occurs through the growth of the blind end of the funnel in the caudal direction; initially, it runs lateral to the Wolffian duct, and at the end of the 2nd month, it turns inward, crosses the Wolffian duct, and continues to run further in the caudal direction, lying adjacent to the Müllerian duct of the opposite side. In this form, the Müllerian ducts reach the urogenital sinus and open into its posterior wall on the so-called Müllerian eminence, located between the openings of the Wolffian ducts (Figure 3). The mechanism of closure of the Müllerian ducts and their transition to the inner side of the Wolffian ducts is quite complex and is associated with the growth of the urogenital folds of the Wolffian body and the formation of the so-called genital cord.

In the 3rd month, the Müllerian ducts in male embryos begin to atrophy and resorb; only two terminal parts remain of them—the cranial and the caudal. The first is preserved in the form of the hydatid of Morgagni, appendix testis, and the second, during further development, becomes surrounded by the prostate gland and forms the prostatic vesicle, or utriculus masculinus. In female embryos at the end of the 3rd month, the adjacent parts of the Müllerian ducts begin to fuse into a single common canal; fusion begins in the middle third and spreads forward and backward from there. The uterovaginal canal formed in this way becomes wider in the 4th month, and the boundary between

Figure 2. Urogenital sinus and genital slit of a 25 mm human embryo: 1-urinary bladder; 2-ureter; 3-fused Müllerian ducts; 4-Wolffian ducts; 5-opening of the Müllerian ducts; 6-opening of the Wolffian ducts; 7-urogenital sinus. Fusion between the uterus and vagina of the upper sections of the uterus occurs only by the end of the 4th month; the boundary between the uterine sections and the tubes is outlined earlier at the site of attachment of the round ligament of the uterus (ligamentum teres, in male embryos gubernaculum Hunteri). Incomplete fusion of the Müllerian ducts explains various anomalies of the female reproductive tracts, ranging from a bicornuate uterus to complete doubling of the uterus and vagina.

Each of these anomalies corresponds to the normal development of the reproductive tracts in various representatives of mammals, since the degree of fusion of the Müllerian ducts is extremely diverse. Fusion does not occur at all in monotremes and marsupials, which is why they have

Figure 3. Scheme of the formation of internal genital organs (undifferentiated stage): 1-kidney; 2-ureter; 3 and 17-Müllerian duct; 4-adrenal gland; 6 and 8-mesonephros and its ligament; 7-Wolffian duct; 9-urinary bladder; 10-urachus; 11-opening of the ureter; 12-opening of the Wolffian duct into the urogenital sinus; 13-penis; 14-opening of the urogenital sinus; 15-anus; 16-urogenital sinus; 18-ligament to the diaphragm.

2 uteri and 2 vaginas; in placentals, with a single vagina, there may be 2 uteri (uterus duplex in elephants and some rodents), 1 uterus divided into two parts (uterus bipartitus in carnivores and pigs), or a uterus elongated into 2 horns (uterus bicornis in ungulates and insectivores). In birds, reptiles, and amphibians, fusion of the Müllerian duct does not occur, and each gives rise to an oviduct, whereas in birds the right Müllerian duct remains rudimentary (see also Vagina, Uterus, development).