Paramesonephric duct

Paramesonephric ducts (or Müllerian ducts) are paired ducts of the embryo that run down the lateral sides of the urogenital ridge and terminate at the sinus tubercle in the primitive urogenital sinus. In the female, they will develop to form the fallopian tubes, uterus, cervix, and the upper two third of the vagina. Lower 1/3 of vagina is derived from sinovaginal bulb derived from urogenital sinus; in the male, they are lost. These ducts are made of tissue of mesodermal origin.Enlarged view from the front of the left mesonephros before the establishment of the distinction of sex.Transverse section of human embryo eight and a half to nine weeks old. Paramesonephric ducts (or Müllerian ducts) are paired ducts of the embryo that run down the lateral sides of the urogenital ridge and terminate at the sinus tubercle in the primitive urogenital sinus. In the female, they will develop to form the fallopian tubes, uterus, cervix, and the upper two third of the vagina. Lower 1/3 of vagina is derived from sinovaginal bulb derived from urogenital sinus; in the male, they are lost. These ducts are made of tissue of mesodermal origin. The female reproductive system is composed of two embryological segments: the urogenital sinus and the paramesonephric ducts. The two are conjoined at the sinus tubercle. Paramesonephric ducts are present on the embryo of both sexes. Only in females do they develop into reproductive organs. They degenerate in males of certain species, but the adjoining mesonephric ducts develop into male reproductive organs. The sex based differences in the contributions of the paramesonephric ducts to reproductive organs is based on the presence, and degree of presence, of Müllerian inhibiting factor. During the formation of the reproductive system, the paramesonephric ducts are formed just lateral to the mesonephric ducts in both female and male embryos 6 weeks after fertilization. During this time primordial germ cells migrate from the yolk sac to the gonadal ridge; a region of mesenchyme arising from, and running parallel with, the mesonephros. The paramesonephric ducts are formed by the craniocaudal invagination of a ribbon of thickened coelomic epithelium that extends from the third thoracic segment caudally to the posterior wall of the urogenital sinus. The caudal parts of the paramesonephric ducts fuse into a single tube, known as the uterovaginal primordium, before flowing into the dorsal aspect of the urogenital sinus at the sinus tubercle directly medial to the mesonephric ducts. The development of the paramesonephric (Müllerian) ducts is controlled by the presence or absence of anti-Müllerian hormone (AMH; also known as Müllerian-inhibiting substance, 'MIF' for 'Müllerian-inhibiting factor', 'MIH' for 'Müllerian-inhibiting hormone', or 'APH' for anti-paramesonephric hormone). AMH is a glycoprotein hormone that is secreted by sustentacular cells (Sertoli cells) in males as they begin their morphologic differentiation in response to SRY expression. AMH begins to be secreted around week 8, which in turn causes the paramesonephric ducts to regress very rapidly between the 8th and 10th weeks. However, small paramesonephric ducts can still be identified, and the remnants can be detected in the adult male, located in the appendix testis, a small cap of tissue associated with the testis. Remnants of the paramesonephric ducts can also be found in the prostatic utricle, an expansion of the prostatic urethra at the center of the seminal colliculus. AMH receptor-type II (AMHR-II), also known as Misr-II, causes AMH to act indirectly on mesenchymal cells surrounding the paramesonephric ducts rather than acting directly on the epithelium of the duct. This receptor activation induces the ducts to regress. The importance of mesenchyme-to-epithelial signaling is to maintain AMHR-II expression in the mesenchyme. In the absence of the Wnta7a within the duct epithelium as the ducts regress, ductal AMHR-II expression is lost, and residual paramesonephric ducts would be retained in males, throwing off the urogenital system. Cryptorchidism (undescended testis) or ectopic testis with inguinal hernias have been identified in human males due to AMH and AMHR-II gene mutations. Studies have revealed another AMH receptor group, AMH receptor-type I (AMHR-I), based on the AMH being a TgfB/Bmp family member. Studies have shown that ALK2, Alk3 (or Bmpr 1a) and Alk6 all serve as AMHR-I receptors. When these receptors are blocked or knocked out in mice within the paramesonephric duct mesenchyme, AMH-induced paramesonephric duct regression is lost. In females, the paramesonephric ducts give rise to the uterine tubes, uterus, and upper portion of the vagina, while the mesonephric ducts degenerate due to the absence of male androgens. In contrast, the paramesonephric ducts begin to proliferate and differentiate in a cranial-caudal progression to form the aforementioned structures. During this time, the single-layered paramesonephric duct epithelium differentiates into other structures, ranging from the ciliated columnar epithelium in the uterine tube to stratified squamous epithelium in the vagina.