The complete knowledge of the development of the female genital tract is essential to understand various congenital anomalies of this area. The essential components of the female genital tract are ovaries, reproductive tract and external genitalia. These organs differentiate within the utero before the end of the first trimester. Human fetus has the capability to develop in either sex till first 7 weeks after conception. As the early genital tract development is similar in both female and male so this period is called as indifferent gonadal phase. The exact phenotype of human fetus is determined by the presence of sex chromosome XX or XY. In absence of Y chromosome the fetus differentiates as female. Under the influence of Y chromosome the testis develops and fetus develops as male (Figure 1.1). Key outline of female genital tract is shown in the Box 1.1.
Figure 1.1: Schematic diagram of overall development of ovary and testis in relation to sex chromosome
GONADAL DEVELOPMENT
The development of the gonads starts from the 5th week. The gonads develop in two phases:
- Initial indifferent phase and
- Later differentiated phase.
In the initial indifferent phase, the gonads are bipotential to develop male or female as the cells in the gonads can differentiate in either direction. Later phase of gonadal development is more differentiated and this is solely influenced by genes in the Y chromosome. The initial gonads develop from the gonadal ridges. The gonadal ridge develops in the medial side of the mesonephros as a bulge due to proliferation of the epithelial lining along with the underlying mesoderm (Figure 1.2). The epithelial cells invaginate within the mesoderm as multiple cords like structures known as primary sex cord. The primitive germ cells are noted in the yolk sac during the 4th week. During 6th week the primitive germ cells migrate by amoeboid movement and travels from original source of the yolk sac along the pathway of the dorsal mesentery of the hind gut to the ultimate destination of the gonadal ridge. At this time the epithelial cells of the genital ridge proliferate and invaginate into the mesenchymal tissue underneath it to form primary sex cord. The germ cells are incorporated into the primary sex cord. As the Y chromosome is absent so the primary sex cords in the female embryo break down as small clusters in 8th week and remain as rudimentary structure with the primitive germ cells within the medullary part of ovary (Figure 1.3). Later part these cords disappear and are replaced by blood vessels. Under the influence of X chromosome the secondary generation of cortical epithelial cells develop and invaginate within the mesoderm of the ovary to form secondary sex cords. These cortical cords increase in size and then break down in multiple small fragments. The single layer of cortical epithelial cells encircles the primordial germ cells to form primordial follicle.2
Figure 1.2: Schematic diagram shows development of genital ridge and migration of the germ cells from the yolk sac to the genital ridge
Figures 1.3A and B: (A) At first primary sex cord develops that regress in course of time; (B) the surface epithelium of the genital ridge proliferates and invaginates in the mesoderm to form primordial follicles
PRIMORDIAL GERM CELLS
The primitive germ cells (PGC) do not develop from the genital ridge. Their source of origin is totally different. PGCs develop in the yolk sac and travels to the genital ridge by the influence of stella, fragilis and BMP-4 genes. The PGCs characteristically express a special transcription factor OCT4. The PGC survives during migration by the complex interaction of its surface receptors c-kit and the stem cell factors liberated by the surrounding mesoderm.1 Once the PGCs arrive in the genital ridge, they become static and aggregates in the stroma of the genital ridge. These PGCs in the genital ridge proliferate by mitotic division into several million cells. The germ cells are encircled by cortical epithelial cells and form primordial follicles. The germ cell in the center of the follicle is known as oocyte. Once meiotic division of the oocyte starts the further mitosis is not possible. The oocytes remain frozen in the first phase of meiosis till the time of ovulation. In course of time the large number of the oocytes undergoes apoptosis and degenerate. At the time of menarche near about four lakhs primordial follicles remain in the ovary.3
REPRODUCTIVE TRACT DEVELOPMENT
The embryonic genitourinary tract is composed of three things: pronephros, mesonephros and metanephros. The pronephros and mesonephros both are transient structures and disappear in course of time. The mesonephric duct also known as Wolffian duct takes important role in the development of male reproductive system. However, the Wolffian duct slowly disappears in the female during embryonic development. The mesonephros and mesonephric duct remain as vestigial structures such as Gartner duct cyst, epoophoron and paraoophoron. The paramesonephric duct is also known as Müllerian duct. It generates from the longitudinal invagination of coelomic epithelium lateral to the Wolffian duct. It starts cranially from the abdominal cavity and extends caudally up to the pelvis. At first it runs laterally parallel to the Wolffian duct and then this duct crosses the Wolffian duct caudo-medially to meet with opposite paramesonephric duct (Figure 1.4). Uterus, cervix and vagina (upper 2/3rd) develops from the Müllerian ducts due to the fusion of the caudal end of the Müllerian ducts. The fusion of two Müllerian ducts occurs between 7th to 9th weeks. In this time fusion of the vertical parts of the two ducts may be incomplete and a midline septum remains (Figure 1.5). This midline septum of the uterus disappears around 20th week (Figure 1.6). The terminal caudal end of the fused Müllerian ducts comes in contact with the posterior part of urogenital sinus and induces the formation of the sinovaginal bulb. The sinovaginal bulb later on canalizes to generate lower 1/3rd of vagina (Figure 1.7). The proximal or cranial part of the Müllerian ducts does not fuse together and form the two fallopian tubes that open in the peritoneal cavity.
Figure 1.4: Schematic diagram of mesonephric duct along with paramesonephric duct. The paramesonephric duct runs parallel to the mesonephric duct
Figure 1.5: Mesonephric ducts regress and two paramesonephric duct fuse to form uterus, cervix and upper part of vagina
Figure 1.6: Midline septum separating the two paramesonephric ducts also slowly dissolves and single uterine, cervix and vaginal cavity develops
Figure 1.7: A protrusion known as sinovaginal bulb arises from the urogenital sinus and projects towards the vagina. Later on the upper 1/3rd of vagina and sinovaginal bulb fuses to form a complete vagina
4During the fusion of the Müllerian ducts the peritoneal fold is brought together that later on forms the broad ligament. Successively, the rectouterine and vesicouterine pouch form. The proliferation and differentiation of surrounding mesenchymal tissue around the uterus form parametrium.
MOLECULAR GENETICS IN THE DEVELOPMENT OF THE GONADS AND REPRODUCTIVE TRACT
The sex of the human fetus is determined by the presence of XX for female and XY for male. It was assumed long back that Y chromosome contains a gene known as testis determining factor (TDF). Later on, the gene of the TDF was isolated and identified from the short arm of Y chromosome. This part is labeled as sex determining region gene in the Y chromosome (SRY).2,3 The presence of SRY gene induces the development of male fetus. The SRY gene is situated in the short arm of chromosome Y. The SRY gene produces several nuclear proteins that specifically binds with DNA. This DNA-binding protein acts as a transcriptional activation factor of many other genes. The exact target gene of SRY is not known, however SRY gene products possibly act on SOX9 gene. By the influence of SRY gene the somatic cell populations of the gonads develop pre-sertoli cells followed by sertoli cells. This is an important event that stimulates the subsequent cascade of development of male fetus. As mentioned before, the female reproductive tract is produced in default and in the absence of SRY gene in the Y chromosome female reproductive system develops. In absence of SRY gene the primary germ cells proliferate and are surrounded by cortical epithelial cells to form primordial follicles. It has been proposed that several other genes are also responsible for the initial development of female genital tract such as LIM homeobox gene 9 (Lhx9), steroidogenic factor 1 (SF1), empty-spiracles homeobox gene 2(Emx2), paired-box gene 2(Pax2), Pax8 and Wnt7a.4–8 These genes are mainly responsible for Müllerian tract development and they act by an unknown complex interaction.
EXTERNAL GENITALIA DEVELOPMENT
In the initial period up to 7th week the external genitalia of both sexes are similar and indifferent as like gonads. This period is known as indifferent period of external genitalia. The distinct sexual characteristics of the external genitalia start from 9th week. At first the genital tubercle arises at the proximal end of the cloacal membrane due to proliferation of the mesenchyme. This occurs in the 4th week in both female and male fetus. This genital tubercle enlarges and forms the primordial phallus. On each side of the cloaca two folds develop: medially urogenital fold and laterally labioscrotal fold (Figure 1.8). The anterior fusion of labioscrotal fold is known as anterior labial commissure and the posterior fusion of the labioscrotal fold is labeled as posterior labial commissure. A ridge of mesenchyme (urorectal septum) develops in between the cloacal membrane and rectum that separates the genitourinary system from the rectum. The clitoris develops from the primordial phallus in case of female. The inner urogenital folds give rise to labia minora and the outer labioscrotal folds develop as labia minora.
Figure 1.8: Schematic diagram showing the development of external genitalia. Labia minora develop from the urogenital folds and labia majora develop from the labioscrotal swelling
DISORDERS OF GONADAL AND GENITAL TRACT DEVELOPMENT
The common causes of congenital abnormalities of female genital tract are:
- Environmental: Viral infections, ionizing radiations, etc.
- Genetic: Chromosomal disorders such as 45X in Turner syndrome, 47XXY in Klinefelter's syndrome, etc.
Gonadal Abnormalities
Ovarian Dysgenesis or Agenesis
The patients are 46XX and karyotypically normal. However, the primordial germ cells (PGC) do not develop or migrate from the yolk sac and so gonads are free of any PGC. The ovaries fail to develop in such cases. On histopathology, the ovaries show streak gonads.
Ovarian Hypoplasia
This is typically occurs in Turner syndrome with karyotypically 45X chromosome. In majority of the cases, the X chromosome is derived from the mother. These patients do not show any Barr bodies in buccal smear. In Turner 5syndrome, the PGC develops and migrates to the genital ridge to form gonads. Due to the absence of other extra X chromosome the PGCs do not sustain and fail to develop any primordial follicle. The ovaries do not develop and no ovarian sex hormones are produced. As there is no Y chromosome so SRY gene is absent and the Müllerian tract persists to form internal genitalia. Lack of ovarian sex hormone causes failure of development of female external genitalia. In some cases, the patients may show 45X/46XY karyotyping. Unlike classic 45X Turner syndrome, these patients are in higher risk of gonadoblastoma and dysgerminoma.
Histopathology: The ovaries are small and streak like. The outer cortex of the ovary is thin and composed of spindle shaped cells. The oocytes are characteristically absent. The hilum of the ovary may contain rete ovarii and hilar cells.
Testicular Feminization
The patients are genotypically male and show 46XY karyotyping. This disease is an X-linked recessive disorder. In this disease, there is lack of testosterone sensitivity in the receptor. The incidence of testicular feminization is about one in 60,000 male births.9 In this disease, SRY gene in the Y chromosome is retained. Due to the presence of SRY gene the Müllerian duct development is suppressed. Uterus and cervix are absent and the vagina terminates in a blind pouch. However, at puberty there is estradiol secretion from the testis because of lack of any negative feedback control. Therefore, the patients are phenotypically female and they show scanty pubic hair with normal breast development. The patients usually present with amenorrhea. The testes are cryptorchid and remains either in abdomen or in inguinal canal. The chances of malignancies are increased in higher age group and seminoma is the commonest malignant tumor.
Biology: Testosterone is synthesized in the testis under the influence of luteinizing hormone liberated from the pituitary gland. Dihydrotestosterone is formed by the enzymatic modification of testosterone in the target tissue with the help of 5 α reductase. Both the testosterone and dihydrotestosterone bind with their respective receptors on the nuclear membrane and the receptor-hormone complex enters into the nucleus and acts on the gene to produce necessary substances (Figure 1.9). Dihydrotestosterone, the metabolite of testosterone is more potent than testosterone and is responsible for the formation of various external male organ such as external genitalia, prostate and also urethra. In testicular feminization syndrome the following defects may happen: (1) Absence of 5 α reductase enzyme, (2) Defects of androgen receptors, (3) Receptor positive resistance.
Figure 1.9: The schematic diagram showing the causes of androgen insensitivity in testicular feminization
Histopathology: Microscopically, the testis show immature seminiferous tubules without any sign of spermatogenesis. The empty sertoli tubules may form nodule like structure. There is abundant interstitial stroma along with large sheet of Leydig cells.
Hermaphrodites (Intersex or Disorders of Sex Differentiation)
Hermaphrodite may be two types: True hermaphrodite and pseudo hermaphrodite.
True Hermaphrodite: The patient shows both male and female gonads and external genitalia.
Pseudohermaphrodite: The patient shows internal genitalia of the same genotype and external genitalia opposite to the genotype. The terminologies such as intersex, hermaphrodite, pseudohermaphrodite, etc. are often confusing and therefore, LWPES1/ESPEW2 consensus groups suggested the terminology as “disorder of sex development” (DSD).10 Chicago Consensus Conference,10 2005 also propsed to replace the term male pseudohermaphrodite as 46,XY DSD, female pseudohermaphrodite as 46, XX DSD, true hermaphrodite as ovotesticular DSD and XX male or XX reversal as 46, XX testicular DSD. The classification of the DSD was based primarily on the pathogenesis of DSD10 (Table 1.1).6
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Developmental Anomalies of Uterus and Cervix
The developmental anomalies of uterus and cervix may be due to:
- Complete or partial failure of the development of Müllerian duct
- Failure of fusion of two Müllerian ducts
- Incomplete removal of the septum of the fused Müllerian duct.
Complete Failure of the Development of Müllerian Duct
The complete agenesis of the Müllerian ducts may cause total failure of the development of fallopian tube to upper part of vagina. This is also known as Mayer-Rokitansky-Küster–Hauser (MRKH) syndrome. Skeletal and cardiac malformation may also be seen in MRKH syndrome. Till date no specific genetic abnormality is detected in this syndrome.
Failure of Fusion or Incomplete Removal of Septum (Figure 1.10)
The defective fusion of the two Müllerian ducts may be the cause of various abnormalities in the uterus, cervix and upper 1/3rd of vagina. Majority of the women with Müllerian duct anomalies do not experience any significant problem of conception or abnormality in menstruation. However, there are higher rates of spontaneous abortion, premature delivery or abnormal placement of fetus in uterus in Müllerian anomalies.11 The prevalence of Müllerian anomalies varies from 0.16 to 10%.11 The wide variation of incidence is due to the variability of the detection rate of such anomalies as many such Müllerian anomalies remain silent for long time till the specific investigations are carried out. According to American Fertility Society the Müllerian duct anomalies have been classified into six type depending on the degree of failure of fusion or regression of the septum.12 In addition the defects caused by diethylstilbestrol exposure in utero has been also included.
Class I: Segmental agenesis—Complete or partial agenesis of uterus, cervix, vagina.
Figure 1.10: The schematic diagram of various anomalies of the development of uterus; (A) Normal uterus; (B) Unicornuate uterus; (C) Didelphus uterus; (D) Bicornuate uterus; (E) Septate uterus; (F) Arcuate uterus
Class II: Unicornuate uterus—One rudimentray horn is present. This horn may or may not communicate with main uterine cavity.
Class III: Uterine didelphys—There is partial or complete failure of fusion of the Müllerian ducts resulting in double uterus, cervix or vagina.
Class IV: Bicornuate uterus—In this condition, the uterus has two horns. Bicornuate uterus is the commonest abnormality of the uterine malformation. This is caused by the failure of fusion of Müllerian duct at the apex of the uterus. The degree of the defect of fusion may vary and there may be two horns of uterus at the fundus or the fusion defect may be more extensive and may reach upto cervical canal.
Class V: Sepatate uterus—Uterine cavity is separated longitudinally by a septum. In case of septate uterus, the uterovaginal septum fails to dissolve after the fusion of the two Müllerian ducts. This is one of the commonest Müllerian duct anomalies. The septate uterus is frequently associated with spontaneous abortion. In case of Uterus subseptus, the uterine septum is partial or incomplete and does not reach up to the cervix.
Class VI: Arcuate uterus—Here the uterus is almost normal except a small notch in the fundus. This type of uterine anomaly is not related with any clinical side effect.
Anomalies of Vaginal Development
Vaginal agenesis: Here the vagina does not develop at all due to absence of Müllerian duct. In addition, the uterus and cervix also do not develop. This is the part of MRKH syndrome as described before.
Disorders of longitudinal fusion of vagina: This develops due to incomplete fusion of the two Müllerian ducts in the caudal end. The patient usually complains of dyspareunia or disorders in menstrual flow.
Disorders of transverse fusion of the vagina: This is due to the incomplete canalization of the vagina at the junction of Müllerian duct and sinovaginal bulb resulting in transverse vaginal septum or imperforate hymen. The patient complaints of primary amenorrhea. Retention of menstrual blood may cause hematocolpos and urinary retention.
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