Interkinetic nuclear movements promote apical expansion in pseudostratified epithelia at the expense of apicobasal elongation

Pseudostratified epithelia (PSE) are a common type of columnar epithelia found in a wealth of embryonic and adult tissues such as ectodermal placodes, the trachea, the ureter, the gut and the neuroepithelium. PSE are characterized by the choreographed displacement of cells’ nuclei along the apicobasal axis according to phases of their cell cycle. Such movements, called interkinetic movements (INM), have been proposed to influence tissue expansion and shape and suggested as culprit in several congenital diseases such as CAKUT (Congenital anomalies of kidney and urinary tract) and esophageal atresia. INM rely on cytoskeleton dynamics just as adhesion, contractility and mitosis do. Therefore, long term impairment of INM without affecting proliferation and adhesion is currently technically unachievable. Here we bypassed this hurdle by generating a 2D agent-based model of a proliferating PSE and compared its output to the growth of the chick neuroepithelium to assess the interplay between INM and these other important cell processes during growth of a PSE. We found that INM directly generates apical expansion and apical nuclear crowding. In addition, our data strongly suggest that apicobasal elongation of cells is not an emerging property of a proliferative PSE but rather requires a specific elongation program. We then discuss how such program might functionally link INM, tissue growth and differentiation.

Pseudostratified epithelia (PSE) are a common type of epithelia characterized by the choreographed displacement of cells’ nuclei along the apicobasal axis during proliferation. These so-called interkinetic movements (INM) were proposed to influence tissue expansion and suggested as culprit in several congenital diseases. INM rely on cytoskeleton dynamics. Therefore, longer term impairment of INM without affecting proliferation and adhesion is currently technically unachievable. We bypassed this hurdle by generating a mathematical model of PSE and compared it to the growth of an epithelium of reference. Our data show that INM drive expansion of the apical domain of the epithelium and suggest that apicobasal elongation of cells is not an emerging property of a proliferative PSE but might rather requires a specific elongation program.

Mutations of the SLIT2-ROBO2 pathway genes SLIT2 and SRGAP1 confer risk for congenital anomalies of the kidney and urinary tract

Congenital anomalies of the kidney and urinary tract (CAKUT) account for 40-50% of chronic kidney disease that manifests in the first two decades of life. Thus far, 31 monogenic causes of isolated CAKUT have been described, explaining ~12% of cases. To identify additional CAKUT-causing genes, we performed whole-exome sequencing followed by a genetic burden analysis in 26 genetically unsolved families with CAKUT. We identified two heterozygous mutations in SRGAP1 in 2 unrelated families. SRGAP1 is a small GTPase-activating protein in the SLIT2-ROBO2 signaling pathway, which is essential for development of the metanephric kidney. We then examined the pathway-derived candidate gene SLIT2 for mutations in cohort of 749 individuals with CAKUT and we identified 3 unrelated individuals with heterozygous mutations. The clinical phenotypes of individuals with mutations in SLIT2 or SRGAP1 were cystic dysplastic kidneys, unilateral renal agenesis, and duplicated collecting system. We show that SRGAP1 is expressed in early mouse nephrogenic mesenchyme and that it is coexpressed with ROBO2 in SIX2-positive nephron progenitor cells of the cap mesenchyme in developing rat kidney. We demonstrate that the newly identified mutations in SRGAP1 lead to an augmented inhibition of RAC1 in cultured human embryonic kidney cells and that the SLIT2 mutations compromise the ability of the SLIT2 ligand to inhibit cell migration. Thus, we report on two novel candidate genes for causing monogenic isolated CAKUT in humans.

Noninvasive assessment of antenatal hydronephrosis in mice reveals a critical role for Robo2 in maintaining anti-reflux mechanism

Antenatal hydronephrosis and vesicoureteral reflux (VUR) are common renal tract birth defects. We recently showed that disruption of the Robo2 gene is associated with VUR in humans and antenatal hydronephrosis in knockout mice. However, the natural history, causal relationship and developmental origins of these clinical conditions remain largely unclear. Although the hydronephrosis phenotype in Robo2 knockout mice has been attributed to the coexistence of ureteral reflux and obstruction in the same mice, this hypothesis has not been tested experimentally. Here we used noninvasive high-resolution micro-ultrasonography and pathological analysis to follow the progression of antenatal hydronephrosis in individual Robo2-deficient mice from embryo to adulthood. We found that hydronephrosis progressed continuously after birth with no spontaneous resolution. With the use of a microbubble ultrasound contrast agent and ultrasound-guided percutaneous aspiration, we demonstrated that antenatal hydronephrosis in Robo2-deficient mice is caused by high-grade VUR resulting from a dilated and incompetent ureterovesical junction rather than ureteral obstruction. We further documented Robo2 expression around the developing ureterovesical junction and identified early dilatation of ureteral orifice structures as a potential fetal origin of antenatal hydronephrosis and VUR. Our results thus demonstrate that Robo2 is crucial for the formation of a normal ureteral orifice and for the maintenance of an effective anti-reflux mechanism. This study also establishes a reproducible genetic mouse model of progressive antenatal hydronephrosis and primary high-grade VUR.

Vesicoureteral reflux and other urinary tract malformations in mice compound heterozygous for Pax2 and Emx2

Congenital anomalies of the kidney and urinary tract (CAKUT) are the most common cause of chronic kidney disease in children. This disease group includes a spectrum of urinary tract defects including vesicoureteral reflux, duplex kidneys and other developmental defects that can be found alone or in combination. To identify new regulators of CAKUT, we tested the genetic cooperativity between several key regulators of urogenital system development in mice. We found a high incidence of urinary tract anomalies in Pax2;Emx2 compound heterozygous mice that are not found in single heterozygous mice. Pax2^+/−;Emx2^+/− mice harbor duplex systems associated with urinary tract obstruction, bifid ureter and a high penetrance of vesicoureteral reflux. Remarkably, most compound heterozygous mice refluxed at low intravesical pressure. Early analysis of Pax2^+/−;Emx2^+/− embryos point to ureter budding defects as the primary cause of urinary tract anomalies. We additionally establish Pax2 as a direct regulator of Emx2 expression in the Wolffian duct. Together, these results identify a haploinsufficient genetic combination resulting in CAKUT-like phenotype, including a high sensitivity to vesicoureteral reflux. As both genes are located on human chromosome 10q, which is lost in a proportion of VUR patients, these findings may help understand VUR and CAKUT in humans.

A genome-wide scan for genes involved in primary vesicoureteric reflux

BACKGROUND

Vesicoureteric reflux (VUR) is the retrograde flow of urine from the bladder into the ureters. It is the most common urological anomaly in children, and a major cause of end-stage renal failure and hypertension in both children and adults. VUR is seen in approximately 1-2% of Caucasian newborns and is frequently familial.

OBJECTIVE AND METHODS

In order to search for genetic loci involved in VUR, we performed a genome-wide linkage scan using 4710 single-nucleotide polymorphisms (SNPs) in 609 individuals from 129 Irish families with >1 affected member.

RESULTS

Nonparametric linkage (NPL) analysis of the dataset yielded moderately suggestive linkage at chromosome 2q37 (NPL(max) = 2.67, p<0.001). Analysis of a subset without any additional features, such as duplex kidneys, yielded a maximum NPL score of 4.1 (p = 0.001), reaching levels of genome-wide statistical significance. Suggestive linkage was also seen at 10q26 and 6q27, and there were several smaller peaks.

CONCLUSION

Our results confirm the previous conclusion that VUR is genetically heterogeneous, and support the identification of several disease-associated regions indicated by smaller studies, as well as indicating new regions of interest for investigation.

Embryology and genetics of primary vesico-ureteric reflux and associated renal dysplasia

Congenital anomalies of the kidney and urinary tract, as well as primary vesico-ureteric reflux (VUR) and associated renal dysplasia, are the most relevant causes of end-stage renal failure in the pediatric population. In vivo and in vitro experimental studies have allowed the identification of several genes involved both in ureteric bud branching, ureteric elongation and insertion into the bladder, and in nephrogenesis. It has been proposed that both renal and ureteral abnormalities, as well as the associated renal hypo-dysplasia, may derive from a common mechanism as the result of a dysregulation of the normal developmental program. The large homologies between mice and the human genome suggest that the same genes could be involved both in rodent and human VUR. Furthermore, epidemiological observations suggest that not only syndromic but also isolated VUR is an inherited trait. Linkage analysis for homologous mouse genes in humans, genome-wide linkage studies in multigenerational families and association studies by polymorphisms support the hypothesis that VUR is genetically heterogeneous and is caused by a number of different genes acting with random environmental effects. The present teaching paper is an overview of the embryology and genetics of primary VUR and associated congenital reflux nephropathy.

Vesicoureteric reflux and renal malformations: a developmental problem

Vesicoureteric reflux (VUR) is a congenital urinary tract defect caused by the failure of the ureter to insert correctly into the bladder. It occurs in up to 1% of the general population and is associated with recurrent urinary tract infections and renal failure. Despite treatment of affected children for the past 40 years, the incidence of end-stage renal disease secondary to VUR has not decreased. Twin and family studies reveal that VUR has a genetic basis. Some of the gene candidates that have been identified regulate the position of ureteric budding, a critical step in both kidney and urinary tract development. Analysis of data from humans and mice suggests that some of the renal damage associated with VUR is congenital and is due to a kidney malformation. Therefore, in these cases, the association of VUR and renal failure may be caused by a genetic defect affecting the formation of the kidney and the urinary tract.

Variability in dilatation of the fetal renal pelvis during a bladder filling cycle

OBJECTIVE

To investigate the variation in the dimensions of the fetal renal pelvis in relation to the degree of bladder filling in fetuses with mild pyelectasis.

METHODS

Eighteen third-trimester pregnant women with mild uni- or bilateral fetal pyelectasis, defined as an anteroposterior (A-P) diameter of the renal pelvis between 5 and 10 mm, were recruited for the study. The women were examined for 2-3 h by ultrasound. The A-P and transverse dilatation of the renal pelvis and the bladder dimensions (to calculate fetal bladder volume) were measured at 2-3-min intervals. Postnatally, all infants were investigated by ultrasound at 3-4 months.

RESULTS

In 6/18 fetuses a consistent relationship between the size of the renal pelvis and bladder filling was found, with a mean difference in renal pelvic diameter before and after voiding of 6.7 mm and a largest observed difference of 14.3 mm. In 12/18 fetuses no such relationship was found. Postnatally, five infants were referred to a pediatric urologist. The investigations in these five infants could not confirm the hypothesis that variation in renal pelvic size in relation to bladder size may predict prenatal vesicoureteric reflux (VUR).

CONCLUSIONS

In mild pyelectasis the size of the renal pelvis is highly variable in one-third of cases. The association with bladder volume and micturition suggests evidence of VUR, but this could not be proven. If cut-off values are used to differentiate between normal and abnormal renal pelvic size then not only gestational age but also the degree of bladder filling at the time of measurement should be taken into account. Caution should be expressed when the diagnosis of a possible urological anomaly is based on a single measurement during just one investigation.

Overexpression of RET leads to vesicoureteric reflux in mice

RET, a tyrosine kinase receptor essential for kidney development, has recently been shown to be important for the formation of the urinary tract. When RET is overexpressed in the HoxB7/Ret transgenic mouse, kidneys are small and cystic, and in some of the mice, the ureters are grossly dilated. Here, we report that the observed ureteral dilatation is associated with the urinary tract abnormality vesicoureteric reflux (VUR), in which urine flows retrogradely from the bladder to the ureter. Reflux was determined in vitro by injecting methylene blue into the bladders of HoxB7/Ret and wild-type mice. At postnatal day 1, 30% of HoxB7/Ret mice had VUR compared with 4% of wild-type mice (P < 0.05). The length of the intravesical ureteral tunnel was shorter in HoxB7/Ret mice compared with wild-type mice, on both the right and the left sides (P < 0.05), suggesting a basis for the higher incidence of VUR in these mutants. At embryonic day 11, the ureteric bud was found to exit more caudally from the mesonephric duct in HoxB7/Ret mice, and this may predispose them to VUR (P < 0.05). Wild-type and HoxB7/Ret mice were tested for reflux at embryonic day 17, and both showed a high frequency of VUR (59 and 75%, respectively). These results suggest that VUR may occur transiently during normal urinary tract development before the ureter has completed its insertion into the bladder. In the HoxB7/Ret mouse, overexpression of RET appears to delay the maturation of the distal ureter, resulting in postnatal VUR. The HoxB7/Ret mouse is thus an important model in which to examine how vesicoureteric reflux arises during urinary tract development.

Infant vesicoureteral reflux: a comparison between patients presenting with a prenatal diagnosis and those presenting with a urinary tract infection

OBJECTIVES

To evaluate the severity, laterality, and gender distribution of infant vesicoureteral reflux (VUR) and its potential impact on renal outcome, we compared patients presenting fetally (FDR group) and those presenting with a urinary tract infection (INF group).

METHODS

A retrospective review of 202 patients with the diagnosis of VUR before 6 months of age was performed. The grade of VUR, gender, laterality, initial renal scarring, breakthrough urinary tract infections, new renal scarring, and surgical intervention were compared between the INF group (n = 146) and FDR group (n = 56).

RESULTS

The male/female ratio in the FDR group was 1.67:1 compared with 0.60:1 in the INF group. The FDR group had more unilateral VUR than the INF group (P <0.001), and no significant difference was found between the two groups in terms of VUR grade distribution (P = 0.13), percentage of initial damage (28% of FDR patients versus 23% of INF patients), or clinical course. In either group, boys and girls exhibited a very similar distribution of grade and renal damage.

CONCLUSIONS

Our findings do not support the commonly held belief that fetally diagnosed reflux is an overwhelmingly male, bilateral, and high-grade phenomenon. Few differences were observed between infants diagnosed fetally and those diagnosed subsequent to urinary tract infection. Once diagnosed, from either group, infant reflux has neither great morbidity nor a frequent need for surgery.

Greater reliability of neonatal ultrasonography in defining renal hypoplasia with antenatal hydronephrosis and vesicoureteral reflux

PURPOSE

Infants with history of antenatal hydronephrosis and neonatal vesicoureteral reflux may have detectable changes in renal scans before the advent of urinary tract infection. In cases of bilateral high-grade vesicoureteral reflux, differential renal function on renal scan may not reveal renal hypoplasia since comparison of relative function may be made between two abnormal kidneys. We tested the hypothesis that ultrasonography in the neonatal period may be accurate and complementary to renal scan in detecting renal hypoplasia at birth.

MATERIALS AND METHODS

Twenty-six infants who presented in the antenatal period with history of hydronephrosis and were noted to have neonatal vesicoureteral reflux postnatally were studied retrospectively. They had all been treated by a prospective protocol that included renal ultrasound and renal scans in the first 6 weeks of life. All had been placed on prophylactic antibiotics and had no urinary tract infection. Multiple sonographic parameters were analyzed including kidney length, echogenicity, calyceal blunting, parenchymal thinning and focal scars. We correlated the renal morphology on ultrasound, the renal function on renal scan and the degree of reflux seen on VCUG.

RESULTS

VCUG showed reflux in 44 renal units, grade of reflux was: I (2), II (7), III (12), IV (8), and V (15). A variety of nucleides were used including DMSA in 15, DTPA in 6 and MAGIII in 5. Renal scans identified global hypoplasia without focal scars (differential function less than 40%) in 10 of 44 refluxing renal units grades I (1), III (2), IV (4), and V (3). The sonographic finding of decreased renal length (<50th percentile for age) was present in 14 refluxing units of 44 refluxing renal units, grade I (1), grade III (4 bilaterally in 1), grade IV (4 bilaterally in 1), and grade V (5 bilaterally in 2). The sonographic finding of decreased renal length (<50th percentile) correlated strongly with renal hypoplasia on renal scans in refluxing renal units (p value <.005, sensitivity 80% and specificity 82%, positive predictive value 57%, and negative predictive value 93%).

CONCLUSION

Postnatal ultrasonography is a reliable measure of gross renal parenchyma, and in the presence of vesicoureteral reflux correlates with renal scintilligraphy. In addition, for cases of bilateral neonatal vesicoureteral reflux, ultrasound and renal scan are complimentary, each being able to detect the abnormalities that might be missed by the other.

The development of a pig model to study fetal vesico-ureteric reflux

OBJECTIVE

To develop a surgical protocol to induce vesico-ureteric reflux (VUR) in utero by ablating the ureteric tunnel in a fetal pig model.

MATERIALS AND METHODS

Fetal surgery was conducted on nine sows, which were divided into three groups according to changes in the surgical protocol. Sows in groups 2 and 3 received different anaesthetics and antibiotics, and the operating theatre temperature was increased. In all cases, the intramural part of the ureter was unroofed in the fetuses, which were then returned to the uterus. Upon delivery, cystograms were taken in the male piglets, and the urinary tracts removed for anatomical and histological examination.

RESULTS

All three sows in group 1 delivered healthy piglets, but the fetuses that had undergone surgery were mummified. In group 2 the animals survived the fetal intervention, as shown by ultrasonography after surgery, but the four sows aborted spontaneously within a week. In group 3, both sows delivered normally developed piglets, three of which had undergone ablation of the ureteric tunnel. VUR was present only in those renal units in which the ureteric tunnel was ablated, and this was associated with hydronephrosis, dilatation of the ureters and thinning of the renal parenchyma on gross pathological examination.

CONCLUSIONS

The fetal pig model of VUR not only appears to be feasible, but with similarities in renal anatomy and physiology also seems to be ideal for investigating fetal VUR.

Ureteric tunnel incision in the fetal pig: a model for non-obstructive prenatal vesicoureteric reflux

The understanding of vesicoureteric reflux (VUR) continues to improve, particularly as the renal tract can now be assessed prenatally. To further study the evolution of fetal renal changes, we studied two neonatal piglets that had undergone midgestation ureteric tunnel ablation. Dilatation of the pelvicalyceal system was seen radiologically and macroscopically in all three kidneys into which VUR had been created. Despite the study being marred by poor fetal survival, the results indicate that the model should be developed further to explore what appears to be an interrelationship between VUR and renal parenchymal changes in utero.

The origin of vesico-ureteric reflux in male newborns: further evidence in favour of a transient fetal urethral obstruction

OBJECTIVE

To present further arguments supporting a transient urethral obstruction occurring during early fetal development as an explanation of the origin of vesico-ureteric reflux (VUR) in baby boys.

PATIENTS AND METHODS

The findings of ultrasonography and voiding cysto-urethrography (VCUG) in 25 baby boys with VUR were reviewed, studying mainly bladder and urethral anomalies on VCUG and bladder-wall thickness on ultrasonography.

RESULTS

A bladder or urethral anomaly was found in 15 patients, comprising isolated posterior urethral dilatation in five, posterior urethral dilatation with a bladder anomaly in four, tubular appearance of the urethra in two and bladder neck hypercontractility with diverticulae in four. Bladder-wall thickness ranged from 1 to 8 mm (mean 3.7) and in eight patients, the thickness exceeded 5 mm.

CONCLUSION

The bladder and urethral anomalies found in this series of patients with neonatal VUR could be explained by a transient bladder outlet obstruction that possibly occurred in utero. These findings support the theory that a significant number of cases of VUR seen in baby boys result from a transient fetal urethral obstruction.

[Pathologic development of the kidney]

In the absence of firmly established views on the development of nephropathy, we describe in this paper the embryogenetic and clinical aspects of kidney disease. Congenital reductive nephropathy always arises in the ureteral bud and is determined by two factors, endogenous dysplasia and endogenous obstruction. The nine well-known patterns of disease that may result are described herein. The most important starting points are as follows: (a) A dysplastically disorganized and hence refluxive trigone of the bladder induces, via pyramidal-medullary deficiencies, a defect of the metanephros and thus what we term reflux nephropathy (III-V). BU and PN may supervene postnatally. (b) Similarly, obstruction of the ureteral outlet in the first trimester induces dysplastic ascending nephropathy. (c) The same obstruction beginning in the second trimester induces nondysplastic, purely obstructive nephropathy, characterized by glomerular hypogenesis and hemo-obliterative cirrhosis which varies considerably from stage to stage and from case to case and may go as far as complete loss of the parenchyma. (d) Obstruction of the pyeloureteral junction, occurring late in the embryonic phase and originating outside the urinary system, provides the clearest example of fully developed nondysplastic reductive nephropathy. The lesional process may come to a halt at any time. (e) Coincidence of early embryonic dysplastic-refluxive nephropathy and late embryonic infravesical obstruction (with no causal link) accounts for half the morbidity from valvular disease. The other half results from simple nondysplastic obstruction.

Effects of neurogenic bladder dysfunction in utero seen in neonates with myelodysplasia

Twenty neonates with myelodysplasia were prospectively evaluated during the first month of life with urodynamic studies, contrast voiding cystourethrograms (VCUG) and renal ultrasound scans. Ten of 20 infants had abnormal VCUGs--6 with vesicoureteric reflux (VUR) and 4 with bladder trabeculation alone. Only 2/20 neonates had abnormal ultrasounds--both with hydroureteronephrosis secondary to VUR. Elevated leak point pressure (LPP) and/or detrusor sphincter dyssynergia (DSD) were present in 10/20 infants--6 with elevated LPP and DSD, 3 with elevated LPP alone and 1 with DSD alone. Eight of 10 neonates with DSD or elevated LPP had abnormal VCUGs--4 with VUR and 4 with bladder trabeculation alone. Conversely, 8/10 neonates without DSD or elevated LPP had normal VCUGs. Contrast VCUG performed shortly after birth, therefore, is essential for the discovery of bladder trabeculation and reflux. There is a strong correlation between an abnormal VCUG and the presence of DSD and elevated LPP, suggesting that neurogenic bladder dysfunction in utero can lead to these structural abnormalities.

Embryogenesis of ureteral anomalies: a unifying theory

Extensive gross, microscopic and clinical studies of various ureteral anomalies have enabled investigators to set forth theories regarding the aetiologies of these anomalies consistent with observed fact. Synthesis of these observations allows for a simplified classification of mega-ureter (primary obstructed, reflux and non-obstructed, non-reflux mega-ureters), ureterocele, duplex ureters and ectopic ureters based upon some combination of mesenchymal differentiation anomalies and location anomalies. A defect early in mesenchymal differentiation would be expected to result in panureteral disease. A defect later in development would result in a focal abnormality anywhere along the course of the ureter. Influence upon ureteral bud mesenchyme by local expansion factors in the bladder base may result in various types of ureterocele. Position of the ureteral orifice on the trigone or in Wolffian duct derivatives would occur in accordance with the Weigert-Meyer principle and would correlate with upper tract dysplasias. Thus, an attempt has been made to systematize and trace the origins of mega-ureter, ureterocele, duplex ureters and ectopic ureters to defects of differentiation of the basic mesoblastic cell in aberrant locations of the ureteral bud.

Current concepts in congenital vesico-ureteral reflux

Primary vesico-ureteric reflux must be graded according to severity. Significant reflux up a dilated ureter appears to be an important factor in causing renal damage. There is evidence in favour of the theory that the renal deterioration seen in some cases of reflux can be explained on a congenital basis of site or origin of the ureteral bud and resultant dysplasia of the developing kidney. Surgery should be reserved for selected patients with significant reflux.

Abnormalities of the ureteral bud

The development and incorporation of the ureteric bud into the developing bladder is a complex process, and it is influenced by events occurring at the same time in the development of other systems in the perineum. Knowledge of the embryology of the ureteric bud and associated wolffian duct is the key to understanding the development of the entire genitourinary system. If the normal embryology is understood, the dysembryogenesis that results in many of the common anomalies of the lower urinary system is simplified. Many of the concepts proposed in this discussion are well substantiated, others are more difficult to prove conclusively. It is believed, however, that the embryologic approach provides a simplified understanding of many of the common problems encountered in pediatric urology. The key in clinical practice to the diagnosis and understanding of these defects is the cystoscopic examination. Careful assessment of the orifice's position, appearance, and tunnel length will allow the interpretation of how the abnormality came about, and also provides valuable information concerning the state of the associated renal segment. This in turn allow better therapeutic management of the problem based on the knowledge of the severity of the renal anomaly associated with the abnormality found in the bladder.

Development of the ureterovesical junction in human fetus: in consideration of the vesicoureteral reflux

The ureterovesical junctions in human fetuses from 11 to 27 weeks of age were studied histologically. Anatomical relationships and origins of the periureteral sheaths, intravesical ureteral muscles, and trigonal muscles were shown. The superficial periureteral sheath arising from the vesical wall was not the source of the trigonal muscles; the deep periureteral sheath arising from the intravesical ureteral wall formed the deep trigonal muscles. The muscles of the intravesical ureter were differentiated longitudinally and formed the superficial trigonal muscles. The development of the two trigonal muscles--superficial and deep--depends on the development of the deep periureteral sheath and the intravesical ureteral muscles. From the finding that both trigonal muscles are present in fetuses compared with both periureteral sheaths or intravesical ureteral muscles in the latest stage, the ability to maintain the valve mechanism in the ureterovesical junction in the human fetus is based on the periureteral sheaths and intravesical ureter--not on the trigone.

[The physiopathology of vesicoureteral reflux (author's transl)]

The tightness of the ureterovesical junction depends on all the structures composing the terminal and intra-mural ureter. The muscular, collagenic, and elastic fibers of the ureter constitute a mesh net which is stretched during bladder distention and closes the ureteric orifice as a valve. Congenital vesico-ureteric reflux results from a primary structural insufficiency of the terminal ureter or insufficiency of the bladder wall backing. As embryology shows primary reflux can be caused by a high ectopic implantation of the ureter. It results from the development of an ureteric bud appearing in a lower than normal position on the Wolffian duct. This results in a higher and more lateral opening of the ureteric orifice in the bladder which leads to a shorter intra-mural tunnel predisposing it to reflux. Secondary or acquired refluxes are associated with chronic obstructions (i.e. neurogenic bladder--lower urinary tract obstruction), and inflammatory lesions. Their pathogeneses are described and discussed. The maturation of the ureterovesical junction is considered a mechanism involving a reduced likelihood of secondary reflux. It may also, during the first years of life, palliate some minimal structural deficiencies of the intra-mural ureter, but growth and development are unlikely to normalize an ureter presenting at birth with a severe constitutional anomaly.

Embryologic basis for lower ureteral anomalies: a hypothesis

A hypothesis for the embryogenesis of lower ureteral anomalies invokes variations in location and number of ureteral buds on the mesonephric duct. Such vagaries determine the length of the common nephric duct and the time of meeting of the ureteral bud (or buds) with the urogenital sinus. These factors in turn affect the density of the mesenchymal tissue and its mass in relation to the adjoining structures and, ultimately, the musculature of the trigone as well as the level of the ureteral hiatus and the musculature of the bladder base. Primary reflux, ureteral ectopy, ureteral duplication, ureterocele, functional ureteral obstruction, and congenital strictures are viewed and possibly explained in the light of the embryonal variations mentioned.