Embryology and Congenital Anomalies of the Urinary System with Imaging Evaluation

PART I: EMBRYOLOGY OF THE URINARY SYSTEM

The Three Successive Kidneys

• Appears on day 22 and disappears by day 25 of gestation
• Entirely nonfunctional and transitory in mammals
• Serves as an inductive scaffold; the pronephric duct is retained and used by the mesonephros

• Appears on day 24 and regresses by week 16 of gestation
• Provides transient excretory function
• Its duct (the Wolffian/mesonephric duct) is critical for subsequent development:
  • In males: forms the epididymis, vas deferens, seminal vesicle, and ejaculatory duct
  • In females: regresses under the absence of androgen; the Müllerian ducts instead form the uterus, fallopian tubes, and proximal vagina
• Provides the Wolffian duct from which the ureteric bud sprouts

• Ureteric bud induction begins on days 28-32 of gestation
• Nephrogenesis begins by week 6-7; glomerulogenesis by weeks 8-9
• Nephrogenesis ceases by week 36 of gestation
• Results in approximately 1 million nephrons per kidney in humans (range is highly variable across populations)

Key Inductive Interactions in Metanephric Development

• An epithelial outgrowth from the distal Wolffian duct
• Invades the metanephric mesenchyme
• Undergoes repeated branching morphogenesis to give rise to:
  • Collecting ducts
  • Minor and major calyces
  • Renal pelvis
  • Ureter

• Condenses around the tips of the ureteric bud
• Undergoes mesenchyme-to-epithelial transition (MET)
• Forms the nephrons: glomerulus, proximal tubule, loop of Henle, distal tubule, and connecting tubule
• Nephrogenesis requires a balance between nephron progenitor self-renewal and epithelial differentiation (epigenetically regulated)

Ascent and Rotation of the Kidneys

• The metanephros initially develops in the sacral/pelvic region
• It ascends to the upper retroperitoneum (lumbar region) during weeks 6-9
• As it ascends, the kidneys rotate 90° medially so the hilus faces anteromedially
• Blood supply changes sequentially: originally from the common iliac, then aortic branches, and finally the definitive renal arteries from the abdominal aorta

Development of the Bladder and Urethra

• At week 5, the cloaca (terminal endoderm-lined yolk sac) is divided by the urorectal septum into:
  • Urogenital sinus (anteriorly) - becomes the vesicourethral canal
  • Anorectal canal (posteriorly)
• The mesonephric duct develops into the trigone of the urinary bladder (the posterior bladder wall)
• The allantois remnant forms the urachus, a fibrous cord connecting the bladder dome to the umbilicus
• The bladder is derived from the upper urogenital sinus

Fetal Renal Function

• Fetal urine production begins at 9 weeks of gestation
• By week 20, fetal urine is the primary source of amniotic fluid
• The fetal kidney is visualized on ultrasound from 12-15 weeks; adequate imaging of renal anatomy is possible by 16-20 weeks
• Corticomedullary differentiation is typically distinct by 25 weeks

PART II: CONGENITAL ANOMALIES OF THE KIDNEY AND URINARY TRACT (CAKUT)

A. ANOMALIES OF RENAL NUMBER

1. Bilateral Renal Agenesis

• Incidence: ~1 in 4,000 births
• Complete failure of ureteric bud induction or metanephric mesenchyme formation bilaterally
• Results in Potter sequence (formerly Potter syndrome):
  • Oligohydramnios (fetal urine is absent; no amniotic fluid after week 20)
  • Pulmonary hypoplasia (lethal - requires amniotic fluid for lung development)
  • Potter facies: pseudoepicanthal folds, recessed chin, posteriorly rotated flattened ears, flattened nose
  • Limb deformities: clubfoot, hip dislocation, joint contractures
  • Decreased fetal movement
• Fatal at birth due to pulmonary hypoplasia

2. Unilateral Renal Agenesis

• Incidence: 1 in 1,000-1,500
• More common in males, slightly more frequent on the left
• Ipsilateral ureter and hemitrigone absent in ~50% of cases
• Contralateral kidney undergoes compensatory hypertrophy but may be ectopic or malrotated
• Associated with:
  • Vesicoureteral reflux (VUR) on the contralateral side in ~30%
  • Genital anomalies (mullerian duct defects in females, Wolffian duct defects in males)
  • VACTERL syndrome (vertebral, anal, cardiac, trachea-esophageal, renal, limb defects)
  • Cardiovascular anomalies (30%), musculoskeletal (14%), adrenal gland (10%)

3. Supernumerary Kidney

• Extremely rare; a third distinct kidney with its own capsule, blood supply, and ureter
• Usually small and caudal to the normal kidney

B. ANOMALIES OF RENAL POSITION (ECTOPIA)

• In 30-50% of ectopic kidneys: varying degrees of hydronephrosis and VUR are present
• Ectopic kidneys are usually asymptomatic unless associated with ectopic ureter
• Adrenal glands develop independently and maintain their normal position even when the ipsilateral kidney is ectopic

C. ANOMALIES OF RENAL FORM/FUSION

Horseshoe Kidney

• Incidence: ~0.2% (most common renal fusion anomaly)
• The lower poles of both kidneys fuse at their caudal ends, connected by a fibrous band or functional renal parenchyma (isthmus)
• The isthmus becomes trapped below the inferior mesenteric artery during ascent, fixing the kidneys in a low, abnormal position
• The renal pelves face anteriorly (malrotation)
• Ureters pass anteriorly over the isthmus
• Complications:
  • Ureteropelvic junction obstruction (UPJO) in up to 50%
  • VUR in up to 50%
  • Nephrolithiasis (due to urinary stasis)
  • Increased risk of Wilms tumor and transitional cell carcinoma

D. ANOMALIES OF RENAL STRUCTURE

1. Renal Dysplasia

• Abnormal differentiation of metanephric tissue - the most common structural renal anomaly in infants
• Histologically: primitive tubules surrounded by undifferentiated mesenchyme, often with cartilage islands
• Types:
  • Multicystic dysplastic kidney (MCDK): multiple non-communicating cysts replacing the renal parenchyma; ureter atretic; kidney is non-functional
  • Segmental/focal dysplasia: associated with obstructed or ectopic upper pole ureter (duplex system)
• May be bilateral (incompatible with life)
• Unilateral MCDK: the contralateral kidney compensates; MCDK typically involutes over time

2. Renal Hypoplasia

• Reduced number of nephrons (fewer than 5 calyces); normal histology
• May be unilateral or bilateral
• Distinguished from dysplasia by absence of abnormal histology

3. Polycystic Kidney Disease

• Autosomal Recessive PKD (ARPKD) - mutation in PKHD1 (fibrocystin gene):
  • Neonatal and infantile presentation
  • Fusiform dilation of collecting ducts
  • Bilateral huge kidneys with tiny cysts
  • Associated with congenital hepatic fibrosis and Caroli disease
• Autosomal Dominant PKD (ADPKD) - mutations in PKD1 or PKD2 (polycystin 1/2):
  • Adult presentation typically; but can present in childhood
  • Progressive bilateral cyst formation; cysts arise from any nephron segment
  • Associated with hepatic cysts, intracranial aneurysms, mitral valve prolapse

E. ANOMALIES OF THE COLLECTING SYSTEM AND URETER

1. Ureteropelvic Junction Obstruction (UPJO)

• Most common congenital uropathy (~20% of all CAKUT)
• Obstruction at the junction of the renal pelvis and ureter
• Causes hydronephrosis without hydroureter
• Male predominance; left side more common; bilateral in 10-30%
• Intrinsic causes: aperistaltic ureteral segment; extrinsic causes: aberrant lower pole vessel

2. Ureteral Duplication

• Most common ureteral anomaly
• Incomplete duplication (bifid ureter): two ureters fuse before reaching the bladder (a Y-shaped ureter)
• Complete duplication: each moiety has its own ureter
• Governed by the Weigert-Meyer rule: the ureter draining the upper moiety inserts ectopically (below and medial to the lower moiety ureter); the ureter draining the lower moiety inserts in the normal trigonal position
  • Upper pole ureter: prone to ectopic insertion and obstruction
  • Lower pole ureter: prone to vesicoureteral reflux

3. Ectopic Ureter

• Ureteral orifice opens at a site other than the normal trigonal position
• In males: may open into the posterior urethra, seminal vesicles, vas deferens, or ejaculatory ducts (always above the external sphincter - continent)
• In females: may open into the vestibule or vagina (below the external sphincter - causes continuous dribbling incontinence despite normal voiding)

4. Ureterocele

• Cystic dilation of the intravesical submucosal ureter
• Orthotopic: orifice within the bladder trigone
• Ectopic: orifice outside the trigone (usually associated with complete duplication)
• Can cause obstruction and, in large ureteroceles, bladder outlet obstruction

5. Megaureter

• Ureter diameter >7 mm
• Primary (intrinsic): obstructed, refluxing, or non-refluxing/non-obstructed
• Secondary: due to posterior urethral valves, neurogenic bladder, or prune-belly syndrome
• Congenital obstructive megaureter is caused by an aperistaltic distal ureteral segment

F. BLADDER ANOMALIES

1. Bladder Exstrophy - Epispadias Complex

• Failure of cloacal membrane to break down at the appropriate time
• Spectrum: isolated epispadias -> classic bladder exstrophy -> cloacal exstrophy
• Results in anterior bladder wall defect with exposed posterior bladder mucosa; epispadias (dorsal urethral defect) is always present
• Pubic symphysis diastasis is characteristic
• Incidence of classic exstrophy: ~1 in 30,000-50,000

2. Patent Urachus and Urachal Remnants

• During fetal development, the urachus is the cephalad attachment of the cloaca to the allantois
• Normally becomes atretic, forming the median umbilical ligament
• Failure of obliteration results in:
  • Patent urachus: urine exits at umbilicus
  • Urachal sinus: opens at umbilicus but is blind-ended
  • Urachal cyst: entrapped fluid between two atretic ends
  • Vesicourachal diverticulum: opens into bladder only
• Asymptomatic remnants may become infected or rarely undergo malignant transformation (mucinous adenocarcinoma)

G. VESICOURETERAL REFLUX (VUR)

• Retrograde flow of urine from bladder into ureter and kidney
• Due to insufficient length of the intravesical submucosal ureter
• Graded I (ureter only) to V (gross dilation with calyceal clubbing)
• Most common uropathy in children
• Risk of renal scarring (reflux nephropathy) with recurrent pyelonephritis
• Most low-grade VUR (I-II) resolves spontaneously
• Familial tendency: increased VUR in offspring of affected parents

H. OBSTRUCTIVE UROPATHIES

Posterior Urethral Valves (PUV)

• Most common congenital obstructive uropathy leading to end-stage renal failure and renal replacement therapy
• Occurs only in males
• Anomalous folds of urethral mucosa arising near the verumontanum obstruct urine flow
• Can cause bilateral hydronephrosis, bladder dysfunction, and pulmonary hypoplasia (from oligohydramnios)
• Patients are at risk for developing bladder dysfunction over time and require lifelong urologic follow-up

I. PRUNE BELLY (Eagle-Barrett) Syndrome

• Triad of: deficient abdominal wall musculature + bilateral undescended testes + urinary tract anomalies (dilated ureters, large bladder, variable renal dysplasia)
• Occurs almost exclusively in males
• Associated with oligohydramnios and pulmonary hypoplasia in severe cases

PART III: ROLE OF IMAGING IN EVALUATION

1. Prenatal Ultrasound

• 16-20 weeks: recommended screening window; renal anatomy can be imaged with high definition; sensitivity for anomaly detection ~80%
• 12-15 weeks: fetal kidneys can first be visualized
• Corticomedullary differentiation: distinct by 25 weeks
• Normal fetal bladder: visualized from 13-15 weeks; normally thin-walled
• Ureters: not normally visualized; ureteral dilation suggests obstruction or VUR

2. Fetal MRI

• Used as an adjunct when ultrasound is limited (severe oligohydramnios, maternal obesity, unfavorable fetal position)
• Provides superior tissue characterization of renal parenchyma and associated anomalies
• Critical for evaluating:
  • Cloacal anomalies
  • Complex genitourinary malformations
  • Sacral defects and spinal dysraphism
  • Detailed anatomy in horseshoe kidneys or other fusion anomalies
• As stated in [Miller's Anesthesia, 10e](Miller's Anesthesia): "Fetal MRI should be considered in cases of severe oligohydramnios as an additional imaging technique to determine the presence of associated fetal anomalies"

3. Postnatal Renal Ultrasound

• Non-invasive, no radiation, readily available
• Evaluates: kidney size, position, echogenicity, corticomedullary differentiation, degree of hydronephrosis (SFU grading system), bladder wall thickness, post-void residual volume
• Limitations: does not assess renal function, misses VUR, limited for urethral pathology

4. Voiding Cystourethrogram (VCUG)

• VUR - defines grade (I-V), laterality, and morphology of the ureter
• Posterior urethral valves - shows the "sail sign" (dilated posterior urethra with abrupt narrowing at valves) and trabeculated bladder
• Ureterocele - filling defect at the bladder base
• Bladder anomalies - diverticula, exstrophy, urachal abnormalities
• Also evaluates bladder capacity and wall appearance

5. Radionuclide Scintigraphy

• Gold standard for assessment of differential renal function and cortical scarring
• Each functioning kidney should contribute ~50% of total renal function
• Uses: documenting renal scarring after pyelonephritis, evaluating differential function in unilateral uropathy (e.g., horseshoe kidney, ectopic kidney, renal dysplasia), confirming renal agenesis vs. ectopic kidney
• A DMSA-first approach ("top-down approach") is recommended by some guidelines for febrile UTI in children - obtain DMSA first, then VCUG only if DMSA is abnormal

• Dynamic study assessing renal perfusion, function, and drainage
• Essential for evaluating obstruction in hydronephrosis
• Lasix (furosemide) washout test differentiates true obstruction from dilated but non-obstructed collecting system
• T½ > 20 minutes after furosemide = obstructive pattern
• Essential in UPJO, megaureter, horseshoe kidney with hydronephrosis

• Less radiation than VCUG
• Useful for follow-up of known VUR (not the initial investigation due to lower resolution)

6. CT Urography (CTU)

• High spatial resolution; best anatomical delineation
• Particularly valuable for:
  • Complex anomalies (duplex systems, ectopic ureters, crossed fused ectopia)
  • Renal calculi complicating horseshoe kidney or ectopic ureter
  • Staging of Wilms tumor or other renal masses complicating CAKUT
  • 3D reconstructions for surgical planning
• Limitation: ionizing radiation (significant concern in children); requires iodinated contrast
• Low-dose protocols used in children; reserved for cases where ultrasound + scintigraphy are insufficient

7. MR Urography (MRU)

• No ionizing radiation - preferred over CTU in children for complex anomalies
• Static MRU: T2-weighted sequences showing dilated collecting systems (similar to MRCP concept)
• Dynamic contrast-enhanced MRU (functional MRU): provides both anatomical detail AND differential renal function data
• Excellent for:
  • Ectopic ureters (defining course and insertion)
  • Complex duplex systems
  • Horseshoe kidneys
  • Urachal anomalies
  • Planning surgical reconstruction

Imaging Algorithm Summary

Prenatal hydronephrosis detected
        ↓
Postnatal renal ultrasound (48-72h after birth)
        ↓
If hydronephrosis → VCUG (to exclude VUR, PUV)
        ↓
If obstruction suspected → MAG3 renogram (Lasix washout)
        ↓
If cortical function assessment needed → DMSA scan
        ↓
Complex anatomy / surgical planning → MR Urography

Summary Table: Imaging Modality and Primary Indications

• Bilateral CAKUT with severe oligohydramnios results in Potter sequence, a lethal condition due to pulmonary hypoplasia
• The most common CAKUT is UPJO (~20%); it is the commonest cause of antenatal hydronephrosis
• The Weigert-Meyer rule governs the ureteral insertion pattern in duplex kidneys (upper pole ureter = ectopic insertion; lower pole ureter = VUR-prone)
• The adrenal gland develops independently from the kidney and maintains its normal position even when the kidney is ectopic
• DMSA scanning is the gold standard for renal parenchymal evaluation; MAG3 renography for drainage/obstruction; VCUG for reflux

Embryology & Congenital Anomalies of the Urinary System

EMBRYOLOGY

Three Successive Kidneys

Metanephric Development (Key)

• Ureteric bud (from Wolffian duct) → collecting ducts, calyces, pelvis, ureter
• Metanephric mesenchyme → nephrons (glomerulus, tubules, loop of Henle)

Bladder & Urethra

• Cloaca is divided by the urorectal septum → urogenital sinus (bladder/urethra) + anorectal canal
• Allantois remnant → urachus (fibrous cord, bladder dome to umbilicus)
• Fetal urine is the primary source of amniotic fluid from week 20 onward

CONGENITAL ANOMALIES (CAKUT)

Anomalies of Number

• Bilateral renal agenesis → Potter sequence: oligohydramnios, pulmonary hypoplasia (lethal), Potter facies, limb deformities
• Unilateral agenesis (1:1,000-1,500): contralateral compensatory hypertrophy; associated with VACTERL syndrome, genital anomalies

Anomalies of Position (Ectopia)

• Pelvic kidney - failed ascent (1:3,000)
• Horseshoe kidney (0.2%) - lower poles fuse; isthmus trapped by inferior mesenteric artery; associated with UPJO and VUR in up to 50%
• Crossed renal ectopia - kidney crosses midline; 90% have parenchymal fusion

Anomalies of Structure

• Multicystic dysplastic kidney (MCDK): non-functional; multiple cysts; atretic ureter; involutes over time
• ARPKD: fusiform collecting duct dilation; associated with congenital hepatic fibrosis
• ADPKD: adult-onset; bilateral progressive cysts; associated with intracranial aneurysms

Collecting System Anomalies

• UPJO - most common CAKUT (~20%); hydronephrosis without hydroureter
• Duplex collecting system - governed by Weigert-Meyer rule: upper moiety ureter inserts ectopically (obstruction-prone); lower moiety ureter refluxes
• Ectopic ureter - in females: inserts below external sphincter → continuous dribbling incontinence
• Ureterocele - cystic dilation of intravesical ureter; can cause bladder outlet obstruction
• Megaureter - ureter >7 mm; obstructed, refluxing, or non-obstructed

Bladder Anomalies

• Bladder exstrophy - failure of cloacal membrane breakdown; exposed posterior bladder mucosa + epispadias + pubic diastasis
• Patent urachus - urine exits umbilicus; partial persistence = sinus, cyst, or diverticulum

Other

• VUR - retrograde urine flow; graded I-V; most low-grade resolves spontaneously; risk of reflux nephropathy
• Posterior urethral valves (PUV) - males only; most common obstructive uropathy causing end-stage renal failure; causes bilateral hydronephrosis + bladder dysfunction

ROLE OF IMAGING

Prenatal Ultrasound

• First-line screen; 16-20 weeks (sensitivity ~80%)
• Detects: hydronephrosis, MCDK, horseshoe kidney, absent kidneys, oligohydramnios, thick-walled bladder (→ PUV in males)
• Absent/non-visualized bladder + oligohydramnios after week 20 = bilateral renal failure

Fetal MRI

• Used when US is limited (oligohydramnios, poor window)
• Superior for complex anomalies, cloacal malformations, spinal dysraphism

Postnatal Ultrasound

• First postnatal study for all CAKUT (delay 48-72 hrs to avoid underestimation)
• Evaluates: kidney size, echogenicity, hydronephrosis grade, bladder wall thickness

VCUG (Voiding Cystourethrogram)

• Defines VUR grade (I-V)
• Diagnoses PUV ("sail sign" - dilated posterior urethra)
• Shows ureterocele as bladder base filling defect

DMSA Renal Scan

• Gold standard for differential renal function and cortical scarring
• Used after febrile UTI (top-down approach: DMSA first, VCUG only if abnormal)

MAG3 Renogram (± Lasix)

• Evaluates drainage and obstruction
• T½ >20 min after furosemide = obstructed pattern
• Essential for UPJO, megaureter assessment

CT/MR Urography

• CT: best anatomical detail; used for calculi, complex anatomy, tumor staging; avoided in young children (radiation)
• MRI: preferred in children; functional MRU gives anatomy + differential function; ideal for ectopic ureters, duplex systems, surgical planning

Quick Reference