Introduction

The Müllerian, or paramesonephric, ducts are structures that are critical in the development of the internal genital portions of the female reproductive system. The first observation of this formation was by JP Müller in 1830. The Müllerian ducts will ultimately form the uterine or fallopian tubes, uterus, cervix, and proximal third of the vagina in females. Initially, the Müllerian ducts are present in both sexes and appear as paired channels extending inferiorly along the lateral aspect of the embryo to the urogenital sinus at its sinus tubercle. These ducts typically regress in males under the influence of Anti-Müllerian Hormone (AMH), also known as Müllerian Inhibiting Substance (MIS), Müllerian Inhibiting Hormone (MIH), or Anti-Müllerian Factor (AMF). Anti-Müllerian hormone is produced by the Sertoli cells of the testes and inhibits the development of the female internal genitalia.

Without the influence of AMH, the Müllerian or paramesonephric ducts will develop in either sex into the uterus, uterine tubes, cervix, and the superior 1/3 of the vagina. The incipient development of the Müllerian ducts in both sexes and the development along a female path after sex determination is thought to be highly regulated by different signaling molecules and gene expression, including EMX2, HOXA13, PAX2, LIM1, and WNT. Disruption of these mechanisms can result in various developmental anomalies of the female internal genitalia, such as agenesis of the uterus or formation of a unicornuate, bicornuate, didelphic, septate, or arcuate uterus.[1][2]

Development

There are three 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 two pairs of Müllerian and Wolffian ducts that serve important roles in developing 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 based on 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 lateral 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 in the mesenchyme between the coelomic epithelium and Wolffian ducts; this forms 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 will proliferate and migrate in a craniocaudal direction.[1][5]

Around six weeks post-fertilization, the terminal ends from 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 two Müllerian ducts fuse. Around ten weeks post-fertilization, the basement membranes of the two Müllerian ducts disappear, creating the uterus. At this time, a septum separates the primordial uterine cavity into two, each derived from one of the paired Müllerian ducts. The uterine septum represents the fusion site of the Müllerian ducts and is typically resorbed through 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.[5][6][7][3][8] 

Cellular

Based on immunohistochemical studies of mice, the 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 three 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 or true epithelium markers (e.g., 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 necessary for developing the Müllerian ducts. Some transcription factors likely part of this human developmental process include EMX2, HOXA13, PAX2, LIM1, and WNT. Lim1(Lhx1) and Pax2 are essential in specifying the Müllerian duct precursor cells in mouse studies. Specifically, Lim1 is necessary for forming the Müllerian duct epithelium, and without Lim1, agenesis of the Müllerian ducts occurs in mice. Also, from mouse studies, loss of Pax2 results in the absence of the kidneys and agenesis of the reproductive tract due to the 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 occurs in the cranial mesonephros and begins the expression of PAX2 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 the development of a small, thin uterus that lacks 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 two genes is critical for developing the Müllerian ducts 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 shown importance to the Müllerian ducts during later development. Moreover, the absence of Wnt4 causes abnormal uterine morphology, while the absence of Wnt5a causes disruptions in the differential 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 of Lim1 have been shown to result in agenesis of the Müllerian ducts in mouse studies. More recent specific studies have indicated that Lim1 is essential for elongating the Müllerian ducts, and loss of this gene leads to shortened fallopian tubes, aplasia of the uterus, and infertility.[1][12]

The influence of Wolffian ducts significantly affects the later stage of development of the Müllerian ducts. The association of these two structures is required for the correct elongation of the Müllerian ducts, while specification and invagination are independent of these structures. The influence of the Wolffian ducts contributes to 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 the Müllerian duct to elongate through the proliferation and active migration of the Müllerian duct epithelium. These proliferating cells are located at the tip of the theMüllerian 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 starts with a physical exam, although this is often unrevealing. Imaging should also be evaluated with the initial imaging modality by 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.[11][6][7][2]

Class I: Uterine Agenesis/Hypoplasia

Uterine agenesis and hypoplasia are due to early developmental dysfunction of the Müllerian ducts around five 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, there is a small but fully differentiated uterus.[13]

The most common form of uterine agenesis is Mayer-Rokitansky-Kuster-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 two types of MRKH. Type 1 has agenesis of the uterus with two rudimentary horns and normal fallopian tubes. Type 2 is defined by either symmetric or asymmetric hypoplasia of the uterus with aplasia of one of the two horns and fallopian tube malformations.[13][14]

Class II: Unicornuate Uterus

This condition is due to the arrest of the development of one of the Müllerian ducts. This anomaly accounts for 20% of all uterine anomalies. There are four 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.[13][6]

Class III: Didelphys Uterus

Didelphys uterus is due to 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 there is a deep fundal cleft. There can also be a transverse or longitudinal vaginal septum. There is no communication between the two uteruses. Spontaneous abortion and premature birth both increase due to this anomaly.[13][6]

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 central myometrium, which can extend to the internal or external cervical os. A septum that extends to make two 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, there are better outcomes for a partial bicornuate uterus than a complete one. Research has found 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.[13][6][2]

Class V: Septate Uterus

Septate uterus is the most common uterine anomaly, comprising 55% of anomalies. It is due to defective resorption of the fibrous septum between the two Müllerian ducts. The uterus divides into two 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.[13][6]

Class VI: Arcuate Uterus

An arcuate uterus is due to the indention 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.[13][6]

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 two-times increased risk of spontaneous abortion and a nine-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][13][6]

Gartner Duct

The Gartner duct is a remnant of the Wolffian duct; this can become a cystic structure in the lower wall of the vagina and is known as the Gartner’s 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 are present in this type of septum, including 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 due to 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 causes no secretion of AMH, while a mutation in the AMH gene or the AMH receptor causes abnormal function of AMH. The normal function of AMH in males is to bind to Type I and II receptors present in the MDM; this causes signal transduction via SMAD phosphorylation. Activation of the receptors results in apoptosis of the MDE in males. PMDS is an autosomal recessive disorder in males caused by a mutation in the gene coding the AMH type II receptor. There have been 38 identified mutations that can cause this syndrome. Some of the mutations cause changes to the C terminal of the receptor rendering it bio-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 normal phenotype and cryptorchidism (unilateral or bilateral). The persistent Müllerian structures are typically discovered during surgical repair of the cryptorchidism.[1][15][16][8]