Embryology, Mullerian Ducts (Paramesonephric Ducts)

Development

There are 3 phases of the development of the Müllerian ducts: initiation, invagination, and elongation.[3] Throughout the initial phases of development, the Müllerian ducts are closely associated with the mesonephric or Wolffian ducts. Early in development, both males and females have 2 pairs of Müllerian and Wolffian ducts that play important roles in the development of the internal genitalia. In embryos with XY chromosomes, the Wolffian ducts typically give rise to the male reproductive tract, including the ductus deferentes, epididymides, and seminal vesicles. In embryos with XX chromosomes, the Wolffian ducts typically regress due to the lack of testis-produced androgens and may be actively promoted by COUP-TFII, as shown in mouse studies.[4] In embryos with XY chromosomes, the Müllerian ducts typically regress due to the presence of the testis-derived anti-Müllerian hormone (AMH) produced by the Sertoli cells of the functioning testis in utero. Without the presence of both testis-derived testosterone and AMH, the Müllerian ducts continue to develop into the female internal genitalia and produce the fallopian tubes, uterus, cervix, and upper third of the vagina.[1][3][5]

Both the Müllerian and Wolffian ducts develop on the mesonephric kidneys. Müllerian duct precursors are present on the mesonephros in a specific region of the coelomic epithelium referred to as the Müllerian ridge. The specification of this region is the first step of Müllerian duct development and begins at the cranial pole of the mesonephric kidney. The Müllerian ducts develop laterally and parallel to the Wolffian ducts at the cranial end. At the caudal end, the ducts cross ventrally, resulting in a location internal to the mesonephric duct. Next, the Müllerian surface epithelium (MSE) invaginates and proliferates caudally within the mesenchyme between the coelomic epithelium and the Wolffian ducts, forming the Müllerian duct mesenchyme (MDM). Then, the invaginating MSE fuses with the Wolffian ducts to form the Müllerian duct epithelium (MDE), a canalized tube that proliferates and migrates in a craniocaudal direction.[1][5]

Around 6 weeks post-fertilization, the terminal ends of both Müllerian ducts make contact and fuse to form the uterovaginal duct. This duct later develops into the fornix of the vagina. Later, the basement membrane of the 2 Müllerian ducts fuses. Around 10 weeks post-fertilization, the basement membranes of the 2 Müllerian ducts disappear, creating the uterus. At this time, a septum separates the primordial uterine cavity into 2, each derived from 1 of the paired Müllerian ducts. The uterine septum marks the site of fusion of the Müllerian ducts and is typically resorbed by apoptosis later in development. The BCL2 gene appears to regulate this process in humans. The distal end of the uterovaginal duct contacts the posterior wall of the urogenital sinus, resulting in the Müllerian tubercle. Further, after the fusion of the distal ducts, the broad ligament of the uterus is formed by an extension of peritoneal folds and connects the pelvic walls to the fused Müllerian ducts. The cranial end of the uterovaginal duct becomes the abdominal ostium of the fallopian tubes. The hymen appears to be formed by the urogenital sinus and the Müllerian ducts.[3][5][6][7][8] 

Cellular

In mice, immunohistochemical studies show that incipient Müllerian ducts initially form as mesoepithelial tubes that later differentiate into epithelial tubes.[3] They are derived from intermediate mesoderm and composed of both epithelial and mesenchymal cells. The Müllerian ducts develop as a result of the formation of 3 distinct tissue layers: Müllerian surface epithelium (MSE), Müllerian duct mesenchyme (MDM), and Müllerian duct epithelium (MDE). The Müllerian surface epithelium is the outermost layer. The Müllerian duct mesenchyme is the middle layer. The Müllerian duct epithelium is the innermost layer that lines the internal aspect of the duct. This internal epithelium expresses a mesenchymal marker early in development but does not express typical epithelial markers (eg, cadherin 1) until later in embryonic development.[1][5]

Molecular Level

The interaction between the epithelium and the mesenchyme regulates transcription factors and signaling molecules required for Müllerian duct development. Some transcription factors likely part of this human developmental process include EMX2, HOXA13, PAX2, LIM1, and WNT. Lim1 (Lhx1) and Pax2 are essential for specifying Müllerian duct precursor cells in mouse studies. Specifically, Lim1 is necessary for the formation of Müllerian duct epithelium, and in its absence Müllerian duct agenesis occurs in mice. Also, in mouse studies, loss of Pax2 results in kidney agenesis and agenesis of the reproductive tract due to loss of the Wolffian and Müllerian ducts. In studies of chickens, Bmp/PAX2 and Fdf/LIM1 signaling regulate the initiation phase of Müllerian duct formation and are also necessary for specification.[9] 

BMP signaling in the cranial mesonephros initiates PAX2 expression in the Müllerian duct mesenchyme. It appears that cells with PAX2 are pre-specified Müllerian duct precursor cells. Emx2 appears necessary for the formation of the Müllerian and Wolffian ducts in mouse studies.[10] Additionally, combination mutations in retinoic acid have produced malformations of the caudal portion of the reproductive tract. Invagination is also regulated by fibroblast growth factor (Fgf).[1][11]

WNT and HOX gene expression appear to be crucial for the development of the Müllerian ducts in humans. Animal studies have shown that a lack of Wnt7a leads to abnormal female reproductive tracts, as it is required for sexual dimorphic differentiation. Typical findings of Wnt7a dysfunction in animal studies include a small, thin uterus lacking typical uterine glands. In addition, the function of Wnt7a during embryonic development of the Müllerian ducts is to maintain the expression of Hoxa10 and Hoxa11. The expression of these 2 genes is critical for Müllerian duct development in animal studies. Hoxa10 also contributes to the development of the Müllerian ducts as it determines the borders of tissues of the reproductive tract. Differentiation of the Müllerian ducts into structures of the female genitalia is also regulated by Hoxa genes in animal studies, specifically Hoxa9, Hoxa10, Hoxa11, and Hoxa13. Wnt4 and Wnt5a have also been shown to be important for Müllerian duct development later in development. Moreover, the absence of Wnt4 causes abnormal uterine morphology, while the absence of Wnt5a causes disruptions in the differentiation of the Müllerian Ducts caudally.[1][11][12]

The precursor cells of the Müllerian ducts express Lim1, a homeodomain transcription factor. Lim1 is essential for the formation and maintenance of the female reproductive tract. Missense mutations in Lim1 have been shown to cause Müllerian duct agenesis in mouse studies. More recent studies have shown that Lim1 is essential for Müllerian duct elongation, and loss of this gene leads to shortened fallopian tubes, uterine aplasia, and infertility.[1][12]

The Wolffian ducts significantly influence the later development of the Müllerian ducts. The association of these 2 structures is required for the correct elongation of the Müllerian ducts, whereas specification and invagination are independent of them. The Wolffian ducts influence the elongation of the Müllerian ducts. This process occurs through the secretion of a local signaling molecule that induces Müllerian duct cell proliferation. WNT9B in humans is a Wolffian duct-derived factor considered necessary for the elongation of the Müllerian ducts. Further, researchers have observed Müllerian duct elongation driven by Müllerian duct epithelial proliferation and active migration. These proliferating cells are located at the tip of the Müllerian duct epithelium and have a high proliferation index. This elongation uses the phosphatidylinositol 3-kinase signaling pathway, which is typically activated by tyrosine-kinase receptors.[1]

Clinical Significance

Anomalies in the female reproductive tract are estimated to be present in 0.1 to 3.0% of live births. Because the Müllerian ducts originate from the same intermediate mesoderm as the mesonephros, any female reproductive tract anomaly should warrant investigation of renal anomalies. The typical investigation of Müllerian duct anomalies begins with a physical exam, which is often unrevealing. Imaging should also be evaluated with the initial imaging modality, in this case, pelvic ultrasound. Further investigation can be completed with an MRI, hysterosalpingography, or laparoscopy. Numerous uterine anomalies can be classified, in addition to conditions resulting from failure in developing the Müllerian ducts.[2][6][7][11]

Class I: Uterine Agenesis/Hypoplasia

Uterine agenesis and hypoplasia are due to early developmental dysfunction of the Müllerian ducts around 5 weeks of gestation. This anomaly accounts for 5% to 10% of all uterine abnormalities. Uterine agenesis is defined as no identifiable uterus or the presence of solely rudimentary tissue. This may present in an individual as primary amenorrhea with normal secondary sex characteristics during puberty due to fully developed ovaries. In uterine hypoplasia, the uterus is small but fully differentiated.[13]

The most common form of uterine agenesis is Mayer-Rokitansky-Küster-Hauser syndrome (MRKH). MRKH is an autosomal dominant condition with incomplete penetrance and variable expressivity. The incidence of MRKH has been estimated to be 1 in 4500 female births. It is defined as agenesis of the uterus, cervix, and upper 1/3 of the vagina. Among the agenesis of these parts of the reproductive tract, individuals with MRKH can also have skeletal, renal, cardiac, auditory, and digital anomalies. There are 2 types of MRKH. Type 1 has agenesis of the uterus with 2 rudimentary horns and normal fallopian tubes. Type 2 is defined by either symmetric or asymmetric hypoplasia of the uterus with aplasia of 1 of the 2 horns and fallopian tube malformations.[13][14]

Class II: Unicornuate Uterus

This condition is due to the arrest of the development of 1 of the Müllerian ducts. This anomaly accounts for 20% of all uterine anomalies. There are 4 different subtypes of the unicornuate uterus: absent rudimentary horn, non-cavitary rudimentary horn, cavitary communicating horn, and cavitary non-communicating rudimentary horn. The cavitary non-communicating rudimentary horn can cause obstruction, potentially resulting in abdominal pain and ultimately needing surgical intervention. Abnormal fetal presentation and intrauterine growth retardation are common obstetrical problems.[6][13]

Class III: Didelphys Uterus

Didelphys uterus is due to the failure of fusion of the Müllerian ducts to form the uterus and accounts for 5% of uterine anomalies. In a didelphic uterus, each of the uterine horns fully develops due to the complete non-fusion of the Müllerian ducts. Two crevices are present, and a deep fundal cleft is present. There can also be a transverse or longitudinal vaginal septum. There is no communication between the 2 uteruses. Spontaneous abortion and premature birth both increase due to this anomaly.[6][13]

Class IV: Bicornuate Uterus

A bicornuate uterus is due to the incomplete fusion of the Müllerian ducts. It is present in 10% of all uterine anomalies. This causes the central myometrium, which can extend to the internal or external cervical os. A septum that extends to make 2 cervixes is called a bicornuate bicollis uterus. The depth of the fundal cleft is greater than 1 cm. The horns are less functional in a bicornuate uterus than in a didelphic uterus. Studies have shown little effect of the bicornuate uterus on obstetrical outcomes; however, outcomes are better with a partial bicornuate uterus than with a complete one. Research has found that the bicornuate uterus has the highest cervical incompetence rate among the Müllerian duct anomalies. Problems may present at menarche if an obstructive uterus didelphys is present. Pelvic adhesions and endometriosis are more prevalent in obstructive anomalies.[2][6][13]

Class V: Septate Uterus

A septate uterus is the most common uterine anomaly, accounting for 55% of cases. It is due to defective resorption of the fibrous septum between the 2 Müllerian ducts. The uterus divides into 2 cavities because of this septum. The septum can be composed of muscle, fibrous tissue, or both. Septate uteruses have the poorest obstetrical outcomes, with spontaneous abortion rates of up to 94% and premature birth rates of up to 33%. The treatment for the septate uterus is a surgical intervention to remove the septum.[6][13]

Class VI: Arcuate Uterus

An arcuate uterus is due to the indentation of the endometrium into the uterine fundus. An arcuate uterus is due to the near-complete but not total resorption of the uterine septum. There is limited data on the effect of an arcuate uterus on the obstetrical outcome.[6][13]

Class VII: Diethylstilbestrol (DES) Exposure

DES is a nonsteroidal estrogen analog that causes altered Hox gene expression in the Müllerian Ducts. DES affects newborn girls after their mothers use DES to prevent miscarriage. However, the research found that DES caused uterine malformations and increased the risk of vaginal clear cell carcinoma. Studies have shown that 69% of women exposed to DES have uterine anomalies. The typical uterine malformations include a hypoplastic uterus, a T-shaped uterine cavity (31%), abnormal transverse ridges and hoods, and cervical anomalies (44%). There is an associated 2-times increased risk of spontaneous abortion and a 9-fold increased risk of ectopic pregnancy for women exposed to DES. Further, there is an association with DES and an increase in premature labor and cervical incompetence.[1][6][13]

Gartner Duct

The Gartner duct is a remnant of the Wolffian duct; it can form a cystic structure in the lower wall of the vagina, known as a Gartner cyst.[5]

Congenital Anomalies of the Fallopian Tubes

These rare anomalies are typically agenesis, hypoplasia, or segmental narrowing of the fallopian tubes.[6]

Transverse Vaginal Septum

A transverse vaginal septum is present when there is a defect in the canalization of the vaginal plaque at the location of the urogenital sinus meeting the Müllerian duct. Variations in this type of septum include perforation. A perforated transverse vaginal septum typically causes fewer symptoms as the patient can menstruate. The location of the septum determines the severity and treatment. The easiest to treat is the transverse septum, which is thin, located low, and perforated. The most difficult is a high and thick septum. This type can present with a rectovaginal fistula and require a hysterectomy.[7]

Hand-Foot-Genital Syndrome (HFG)

HFG is an autosomal-dominant condition caused by mutations in the Hoxa13 gene. Patients with this disorder have shortened thumbs, great toes, and a bicornuate or didelphys uterus.[11]

Persistent Müllerian Duct Syndrome (PMDS)

Alterations in AMH secretion, AMH gene, or AMH receptors result in retained Müllerian ducts. Complete gonadal dysgenesis in males results in no AMH secretion, whereas a mutation in the AMH gene or the AMH receptor leads to abnormal AMH function. The normal function of AMH in males is to bind to Type I and II receptors in the MDM, thereby activating signal transduction via SMAD phosphorylation. Activation of the receptors triggers apoptosis of MDE in males. PMDS is an autosomal recessive disorder in males caused by a mutation in the gene coding for the AMH type II receptor. There have been 38 identified mutations that can cause this syndrome. Some mutations cause changes to the receptor's C-terminus, rendering it biologically inactive. The non-functional AMH receptor leads to the retention of the Müllerian ducts in males. Normal findings at birth for a patient with PMDS include a normal phenotype and cryptorchidism (unilateral or bilateral). Persistent Müllerian structures are typically identified during surgical repair of cryptorchidism.[1][8][15][16]