Embryology, Genitourinary

Development

Urinary System

The urinary system begins forming during the 4th week of gestation. At the end of week 4, a nephrogenic cord has formed, giving rise to 3 kidney systems:

1. The pronephros in the cervical region.
2. Mesonephros in the thoracic and lumbar region.
3. The metanephros, which ultimately transforms into functional kidneys.

The most cranially positioned kidney system, the pronephros, is nonfunctional and regresses by the end of the 4th week of gestation as the more caudal kidney systems begin to form. The nephrotome is a vestigial excretory unit present with the pronephros. The mesonephros is the embryonic kidney in the lumbar region from weeks 4 to 8, and its remnants contribute to several structures of the male reproductive system.

The mesonephric cord forms vesicles from provascular cell masses that laterally contact the collecting system of the mesonephros, the mesonephric (Wolffian) duct. The fusion of these vesicles with the Wolffian duct and the acquisition of a lumen form the tubules of the nephron.[1] The functioning adult kidney, the metanephros, appears during week 5, is located in the sacral region, and becomes fully functional by the eleventh week; it is fully formed by week 32. The permanent kidneys develop from both the ureteric bud and the metanephric mesoderm (blastema), which give rise to the collecting system and excretory units, respectively. The ureteric bud, an outgrowth of the mesonephric duct near its junction with the cloaca, dilates to form the renal pelvis and major calyces after invading metanephric tissue.

After several generations of buds, absorption of the third and 4th generations forms the minor calyces and, eventually, the renal pyramid. The ureters and 1 to 3 million collecting tubules also originate from the ureteric bud. The cells of the metanephric tissue cap covering the collecting ducts form the renal vesicles, which give rise to S-shaped tubules that acquire capillaries to form glomeruli, the proximal end of which forms Bowman’s capsule. The excretory tubule lengthens to form the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule. With abdominal expansion and growth of the lumbar and sacral regions, the kidneys separate and ascend between weeks 6 and 9.

The cloaca is the shared component of the anorectal and urogenital passages at the 5th week of gestation that subdivides into 2 separate channels during the 6th and 7th weeks.[2] The cloaca splits into the urogenital sinus anteriorly and the anal canal posteriorly, separated by the urorectal septum, the tip of which forms the perineal body.[3] The anterior portion of the urogenital sinus becomes the urinary bladder, and the inferior portion develops into the pelvic and penile urethra in males. The pelvic part of the urogenital sinus develops into the membranous and prostatic urethra in males, while in females, it forms the urethra and vaginal vestibule. The cranial part of the urethra forms with paraurethral (Skene’s) glands in females, homologous to the male prostate gland, which is formed from the epithelium of the prostatic urethra that penetrates the surrounding mesenchyme. Bartholin’s glands, also known as the greater vestibular glands, are homologous to the bulbourethral (Cowper’s) glands in the male, both originating from the urogenital sinus.

The bladder begins development during weeks 4 to 7. Initially, the bladder is continuous with the allantois; when the lumen becomes obliterated, the urachus connects the bladder apex to the umbilicus. Involution of the cloaca and embryonic allantois forms the urachus, whose remnant is the median umbilical ligament.[4]

Genital System

All fetuses begin with undifferentiated gonads, which develop into either the ovaries in females or the testes in males. Although morphologically indistinguishable at this stage, unisex gonads are known as ‘bipotential’ because of the ability to transform into an ovary or testis, depending on whether the individual possesses an XX or XY chromosome, respectively.[5] The sex of the embryo is determined based on the combined genetic material from the sperm and egg that come together during fertilization. Still, fetal gonads do not acquire male or female morphological characteristics until week 7 of gestation.

The initial appearance of the gonads is the genital (gonadal) ridges formed by the proliferation of the epithelium and condensation of the underlying mesenchyme. The development of testes in males and ovaries in females relies on the induction by primordial germ cells from the yolk sac to the genital (gonadal) ridges during the 4th through 6th weeks. These primordial cells arrive at the primitive gonads at week 5 and invade the gonadal ridge during week 6.[6] The presence of the SRY gene (sex-determining region on the Y chromosome) on the short arm of the Y chromosome (Yp11) influences their development into testes, while the gonads become ovaries in the absence of the gene. Estrogen is essential for the formation of the external genitalia in females, whereas testosterone drives the development of the external genitalia in males. 

The formation of the gonads begins at embryonic day 10, and the expression of Sry protein, also known as the testes-determining factor, begins between days 10 and 11. The undifferentiated genital system consists of 2 pairs of ducts at week 7: the mesonephric (Wolffian) and paramesonephric (Müllerian) ducts. The paramesonephric ducts arise on the anterolateral surface of the urogenital ridge. In males, the paramesonephric ducts regress under the influence of anti-Mullerian hormone (AMH), also referred to as Mullerian inhibiting substance (MIS), which is produced by Sertoli cells beginning in weeks 8 to 10. Testosterone secreted by the Leydig cells beginning at week 9 causes the mesonephric ducts to differentiate into the epididymis, vas deferens, ejaculatory duct, and seminal vesicles.[7] Testosterone is also involved in the masculinization of the urogenital sinus and differentiation of the external genitalia into the penis and scrotum, after conversion to dihydrotestosterone catalyzed by the enzyme 5α-reductase.[6] In females, the absence of anti-Mullerian hormone (AMH) and the influence of estrogen lead to the formation of the Fallopian tubes from the cranial portion of the paramesonephric ducts, and the upper third of the vagina, cervix, and uterus from the caudal portion. These processes are complete by the end of the first trimester. The endometrium of the uterus arises from simple epithelium, whereas the myometrium and perimetrium that cover the uterus arise from the surrounding mesenchyme.[8]

The genital tubercle forms from a pile of mesenchymal cells from the cloacal folds and is positioned cranial to the opening of the urogenital sinus. The genital tubercle elongates under the influence of androgens to become the phallus in males and the clitoris in females in the absence of androgens.[6] The genital tubercle pulls together the urethral folds (from the cloacal folds) to form the urogenital folds, which fuse in the midline to become the shaft of the penis in males or the unfused labia minora in females. The scrotum in males is homologous to the labia majora in females, as they share a common origin, the labioscrotal swellings.

The labioscrotal swellings initially appear in week 4, lie lateral to the genital tubercle, and migrate caudally and medially during weeks 9 to 11 to form the scrotum by week 12. Estrogens are essential for the development of the clitoris, labia, and lower vagina. The processus vaginalis, from the evagination of the peritoneum of the abdominal cavity, forms the inguinal canal after protrusion into the scrotal swelling.[9] Descent of the testes is prompted by intra-abdominal pressure, causing their migration through the abdominal wall via the inguinal canal by 28 weeks of gestation. It should reach the scrotum by week 33. Coverings of the testis include the peritoneal layers derived from the processus vaginalis and derivatives of the anterior abdominal wall: the transversalis fascia (internal spermatic fascia), internal oblique muscle (cremasteric fascia and muscle), and external oblique muscle (external spermatic fascia).

Cellular

Interactions between epithelium and mesenchyme (embryonic connective tissue) are necessary for the development of all organs of the female and male urogenital systems, except for the gonads.[10] While the development of the genital organs (the epididymis, vas deferens, seminal vesicle, penis and prostate in males; the uterine tubes, uterus, cervix, vagina, and clitoris in females), relies on hormones, the development of urinary system structures (kidney, ureter, and bladder) is hormone-independent.[10]

The urogenital system is formed from intermediate mesoderm, while the lining of the urethra, urinary bladder, and reproductive system is composed of endoderm. Primordial germ cells that migrated from the epiblast through the primitive streak first appear in the primitive gonads at the beginning of week 5, then invade the genital ridges at week 6, as stated above.

Molecular Level

Bone morphogenetic proteins (BMPs) and their modifiers are essential for kidney development, particularly BMP4, expressed in the mesenchyme surrounding the nephric duct, and BMP7, expressed in the metanephric mesenchyme and ureteric buds.[11] The epithelia of the collecting duct derive from the ureteric bud, while the nephron epithelium comes from the metanephric mesenchyme.

The ureteric bud expresses Hoxb, and the metanephric mesenchyme expresses the transcription factors Six2 (a marker for self-renewing nephron progenitors) and Cited 1. FGF2 and BMP7 are essential for the proliferation of metanephric mesenchyme and for WT1 production. As such, deletions in the Fgfr2 receptor produce small kidneys, anomalous branching of the ureteric bud, scant nephrons, and defective stromal mesenchymal patterns.[12]

WT1, expressed by mesenchyme of the metanephric blastema, regulates the production of glial-derived neurotrophic factor (GDNF) and hepatocyte growth factor (HGF) by the mesenchyme, which then stimulates the branching and growth of the ureteric bud from the mesonephric duct. RET, the tyrosine kinase receptor for GDNF, and MET, the receptor for HGF, are synthesized by the epithelium of ureteric buds to stimulate their growth.

Upregulation of PAX2 and WNT4 by WNT9B and WNT6 promotes the condensation of the mesenchyme and epithelialization into tubules of condensed mesenchyme, respectively. Wnt signaling is essential for Müllerian duct development, differentiation, and regression, and WNT4 has been deemed the ovary-determining gene. In both males and females, WNT4 and the autosomal gene SOX9 are expressed in the gonadal ridges. 

SRY and/or SOX9 induce the secretion of FGF9 by the testes that promotes the penetration of the gonadal ridge by the tubules of the mesospheric duct. SRY upregulates SOX9 and activates SF1 (steroidogenesis factor 1) expression, thereby promoting the differentiation of Sertoli and Leydig cells.

The interaction between SOX9 and SF1 is essential for increasing AMH levels, thereby inducing regression of the Müllerian ducts. WNT4 upregulates DAX1, whose role is to suppress SF1 transcriptional activity, inhibiting the function of SOX9; thus, in females, ovaries develop from the inhibited expression of SOX9, and it is not just a passive ‘default’ process.[13] GATA4 and FOG2 are additional protein-coding genes that contribute to male gonadogenesis and the maintenance of SRY expression. Hox genes are responsible for the anterior-posterior orientation of the Müllerian duct system.

Lastly, sonic hedgehog (Shh) is expressed in the genital tubercle, and its signaling is implicated in the formation of the penis.[6] In its absence, there is a disruption of genital tubercle development and a persistent cloaca.

Function

The urinary system, composed of the kidneys, ureters, and urethra, is responsible for filtering waste, maintaining fluid balance, eliminating waste, and producing the hormones erythropoietin, renin, and calcitriol, which are necessary for red blood cell production, blood pressure regulation, and vitamin D regulation, respectively. Urine production begins at week 10 after the differentiation of glomerular capillaries. Urine production is necessary for maintaining amniotic fluid volume, which cushions the fetus and facilitates proper lung development.

The urinary bladder receives urine from the ureters and stores it before micturition. The reproductive system produces ova in females and sperm in males and transports them to unite during fertilization. A zygote is carried through the fallopian (uterine) tubes for implantation in the uterine cavity. Production of sex hormones, estrogen, and progesterone from the ovaries and testosterone from the testes, is an essential component of the hypothalamic-pituitary-gonadal axis.

Testing

Various imaging modalities exist to visualize the anatomy of the pelvis. Ultrasound is a cost-effective, minimally invasive imaging modality that can visualize genitourinary structures and highlight renal or gonadal abnormalities; it is the first choice for evaluating pediatric patients because it is painless, widely available, and immediate. Using prenatal imaging, 90% of fetal kidneys are identifiable by 17-20 weeks' gestation, and 95% by 22 weeks' gestation.[14] 

Ultrasound is essential in the diagnosis of horseshoe kidney, renal agenesis, Wilms’ tumor, polycystic kidney disease, and urachal anomalies. A computed tomography scan can also be performed, but at the expense of radiation exposure, and may involve contrast dye. MRI, though expensive, can also be employed to visualize genitourinary structures with greater detail and is the gold standard for evaluating uterine anomalies.[15] Measurement of the amniotic fluid index aids in evaluating fetal renal function, as too little amniotic fluid (oligohydramnios) is associated with poor lung development. In contrast, polyhydramnios, excessive amniotic fluid, indicates a swallowing disorder, such as esophageal atresia.

Pathophysiology

Urinary System

Wilms tumor (nephroblastoma) arises in the kidney due to a mutation in the WT1 gene on chromosome 11p13. The hypothesis is that persistent cells of the metanephric mesenchyme/blastema are precursors of 40% of Wilms tumor cases.[16] Wilms tumor is 1 component of WAGR syndrome caused by a microdeletion of WT1 and PAX6, which also features aniridia, genitourinary anomalies, and intellectual disability. Common genitourinary anomalies associated with WAGR syndrome include cryptorchidism, failure of at least 1 testis to descend, and hypospadias, an incomplete fusion of the urethral folds, resulting in an opening on the ventral aspect of the proximal penis in males. In females, it may present as streak ovaries, a hypoplastic uterus, or a septate vagina. Hypospadias commonly presents with chordee, involving the shortening and curving of the ventral penis.

Wilms tumor is also a feature of several other conditions, including Beckwith-Wiedemann syndrome, Bloom syndrome, Denys-Drash syndrome, and Li-Fraumeni syndrome.[17] Autosomal recessive polycystic kidney disease (ARPKD) presents with fluid-filled cysts in the collecting ducts, leading to renal failure in childhood. In contrast, autosomal dominant polycystic kidney disease (ADPKD) features cysts in any part of the nephron and renal failure in adulthood. 85% of cases of ADPKD result from mutations in the PKD1 gene, and the remaining 15% result from mutations in the PKD2 gene.[18]

Mutations in the GDNF-RET signaling pathway are implicated in the formation of a multicystic dysplastic kidney, in which multiple cysts are present and separated by parenchyma; when present bilaterally, it can cause impaired renal function since the collecting ducts within the nephrons fail to develop.

Oligohydramnios results from impaired renal function and is associated with conditions such as multicystic kidney disease, bilateral renal agenesis, and renal dysplasia. Too little amniotic fluid causes the fatal Potter sequence, resulting in a fetus with craniofacial abnormalities (cleft lip, flattened face, low-set ears, recessed chin), pulmonary hypoplasia, and clubbed feet, although a child with unilateral renal agenesis can survive via compensatory hypertrophy of the functioning kidney.[19] Urinary issues associated with unilateral renal agenesis include ureteropelvic junction obstruction and vesicoureteral reflux (VUR).

If the kidneys cannot ascend from the pelvis to their normal lumbar position, a pelvic kidney near the common iliac artery or a horseshoe kidney with fused lower poles may be present. The root of the inferior mesenteric artery is responsible for preventing proper ascent of the kidneys, which is commonly observed in Edwards syndrome (trisomy 18) and Turner syndrome.[20]

The bladder exstrophy-epispadias-cloacal exstrophy complex is a constellation of ventral wall defects resulting from a developmental abnormality that occurs 4 to 5 weeks after conception. Bladder exstrophy exists concomitantly with epispadias, a failure of the lateral body wall folds to close in the midline, leaving the urethral meatus on the dorsum of the penis. Cloacal malformations result from the failure of anorectal and urogenital channels to separate during weeks 6 to 7 of gestation and result in a single perineal opening.[2] The diagnosis is made clinically during a newborn examination, but can present as a hydrocolpos on prenatal ultrasound or MRI. Imaging is warranted, as up to 90% of patients with a cloaca also have a urologic anomaly such as renal agenesis, horseshoe kidney, hydronephrosis, or vesicoureteral reflux.[21] 

A patent urachus is the persistence of the embryological connection between the dome of the bladder and the umbilicus, resulting from failure of the allantois to break down. Urine can drain from the umbilicus if there is a urachal fistula present from the persistence of the lumen of the intraembryonic portion of the allantois, causing delayed cord stump healing and edema around the umbilicus of a newborn.

Several other urachal anomalies exist, including a urachal cyst (when a local remnant of the urachus persists), an umbilical-urachal sinus (resulting from failure of the umbilical end of the urachus to obliterate), and a vesicourachal diverticulum (caused by incomplete obliteration of the urachus on the bladder side).[4] Additionally, an ectopic ureter can develop if the ureteric bud undergoes abnormal migration during its insertion into the bladder.

Genital System

There are many steps during urogenital formation from which anomalies can arise.

• Failure of the paramesonephric ducts to fuse can occur at any point along the normal fusion pathway. It can result in uterine malformation, such as uterus arcuatus, a concavity at the uterine fundus due to failure of the fused midline segments to degenerate, and uterus didelphys, a complete duplication of the uterus from complete lack of fusion.[15] 
• Uterus bicornis is a uterine anomaly in which the uterus has 2 horns entering a common vagina as a result of only a partial fusion of the paramesonephric ducts.
• Failure of the sinovaginal bulbs to develop results in vaginal atresia, while the failure of them to fuse results in the presence of a double vagina.
• A hydrocele is the buildup of fluid around the testicle due to a remnant of tunica vaginalis, present in males as a painless scrotal swelling, and can be transilluminated.
• The appendix testis is a vestigial remnant of the Müllerian duct located at the anterior-superior pole of the testis, and the Gartner’s duct, epoophoron, and paraoophoron are the remnants of the Wolffian duct in females.
• A congenital indirect inguinal hernia results from the failure of the connection between the abdominal cavity and the processus vaginalis to close, allowing loops of the bowel to descend into the scrotum.

The most common cause of ambiguous genitalia is congenital adrenal hyperplasia, most frequently due to 21-hydroxylase deficiency (required for cortisol synthesis) caused by mutations or deletions in CYP21A, accounting for 90% of cases.[22][23] 17a-hydroxylase deficiency can also cause congenital adrenal hyperplasia, though less frequently, resulting in normal female internal and external anatomy at birth but primary amenorrhea and/or failure of secondary sex characteristics at puberty.[24] In males, small genitalia, undescended testes, or other lack of virilization at puberty may be the first sign.[25] 

In androgen insensitivity syndrome (AIS), receptors are unresponsive to androgens and ineffective in inducing differentiation of the male genitalia.[26] An XY male has phenotypically female genitalia, though Müllerian inhibiting substance (MIS) is present, causing the uterine tubes and uterus to be absent.[26] A micropenis, described as 2.5 standard deviations below the mean penis length when stretched, results from insufficient androgen stimulation or a defect in the HPG axis.[27] Inhibition or deficiency of the enzyme 5α-reductase results in the inability to convert testosterone to dihydrotestosterone, resulting in underdeveloped, albeit external male genitalia.[28] Lastly, XY female gonadal dysgenesis (Swyer syndrome) results from point mutations or deletions of the SRY gene. It presents in females as a lack of menstruation and/or secondary sex characteristics at puberty.[29][30]

Clinical Significance

Many congenital anomalies of the female reproductive system can go unnoticed until the child-bearing years, when infertility or amenorrhea becomes an issue. Gonadal dysgenesis, the presence of streak gonads instead of functional gonads, can be seen in individuals with Turner syndrome (45, XO) and is likely to go unnoticed until reproductive age, as is uterine malformation.[31] An undescended testis may be found at any point along the normal path of descent from the abdomen or ectopically.[32] Detection and proper management of cryptorchidism are crucial, as it is associated with infertility and an increased risk of developing testicular germ cell tumors, inguinal hernias, and testicular torsions.[32][33]

The presence of a cloaca or other anorectal malformation should prompt a more thorough exam, as it is 1 component of the VACTERL association (vertebral anomalies, cardiovascular anomalies, tracheoesophageal fistula, renal anomalies, and limb defects).[34] The failure to correctly diagnose a cloaca or significant anorectal defect in the newborn period could miss an associated obstructive uropathy that can progress to sepsis, acidosis, and renal failure. Surgery to create appropriate perineal openings for the gastrointestinal, gynecologic, and urologic tracts is the mainstay of treatment for anorectal malformations.[35] 

The most typical presentation of a Wilms tumor is an asymptomatic child younger than 5 years of age with an abdominal mass, which would prompt an initial evaluation with an ultrasound, computed tomography, or MRI.[36] When a Wilms tumor is first diagnosed, lung metastases are present in 10 to 20% of cases.[17] Horseshoe kidney is an anomaly that predisposes the individual to ureteropelvic junction obstruction, hydronephrosis, urinary tract infections, and tumors; one-third of these cases are found incidentally.[20] Complications stemming from urachal anomalies include urinary stasis, infection, stone formation, and, although rare, malignancy, necessitating a prompt diagnosis and surgical correction.[4] The clinical implications of all urogenital anomalies are too numerous for the scope of this discussion. With that said, the patient and family's emotional well-being and support system should always be addressed when discussing these diagnoses, as many of these conditions can carry severe emotional distress as well as physical morbidity and mortality.