Congenital variations in mucomuscular development of the ureter

Authors


Introduction

There are several apparently disparate conditions of the ureter for which there is little agreement about the cause. They have much in common, in that they all can be explained by variations in ureteric muscle development, with or without mucosal involvement. The functional significance of these conditions varies; lateral spurs and ureteric rings have no functional significance; ureteric valves produce a purely mechanical obstruction. Primary megaureter is of only functional significance, whereas PUJ obstruction is probably initially a functional obstruction with a mechanical obstruction developing subsequently (Table 1).

Table 1.  Congenital mucomuscular anomalies of the ureter
Common
terminology
Urographic
manifestation
Probable predominant
developmental anomaly
Functional
effect
Comments
Idiopathic PUJ
obstruction
Dilatation of the
pelvis and calyces
Overgrowth of the renal
pelvis and/or inappropriate
muscle composition or
configuration
Functional and
mechanical
obstruction
Anomalous or ineffectual muscle
configuration producing functional
obstruction, which progresses
to a mechanical obstruction
Lateral spursLateral indentation
of ureteric lumen
Failure of recanalization,
leaving residual spurs of
mucosa with or with no
muscular element
NoneAnomalous mucosal abnormality
of no functional significance
Ureteric ringsRing constriction
of the ureter
Anomalous accumulation
of circular muscle
NoneAnomalous muscle configuration
of no functional significance
Ureteric valvesCentral luminal
narrowing with
proximal obstruction
Failure of canalization,
leaving a mucosal diaphragm
with a muscle component
Mechanical
obstruction
Anomalous mucosal abnormality
producing a mechanical
obstruction
Primary megaureterDysfunctional
supravesical ureter
with proximal
dilatation
Inappropriate muscular
configuration. Probable
extension of the predominantly
proximal vertical muscle fibres
into the supravesical ureter
Functional
obstruction
Anomalous musculature
configuration producing a
functional obstruction

A brief review of the current understanding of these interesting conditions, including some relevant embryology and physiology, should help to clarify their cause and justify their being grouped together as ‘congenital variations in mucomuscular development’.

Ureteric embryology

The ureter has three histological coats; the mucosal epithelium, the smooth muscle and the adventitia. Between the fourth and fifth gestational weeks, the mesonephric duct just cephalad to the cloaca gives rise to the metanephric diverticulum or ureteric bud that will become the permanent ureter. The ureteric bud grows into undifferentiated mesenchyme that will form the definitive kidney. The proximal ureteric bud branches numerous times to form the calyces, papillary ducts and collecting tubules.

Early in the development of the ureteric epithelium, the mesenchymal arrangement in the ureteric bud is only one cell layer thick and has a circular arrangement. By the ninth week, portions of the epithelium become two layers. By the following week, the proximal ureter has five layers and the distal ureter two. At ≈ 12 weeks the final epithelium is formed. The surrounding mesenchyme gives rise to the lamina propria, the inner circular and outer longitudinal smooth muscle, and the outer adventitia. At 12 weeks, five strands of circular muscle begin to develop at the caudal end and extend cephalad to involve the entire ureter at ≈ 18 weeks [1]. The final arrangement of muscle fibres depends on whether the growth is primarily longitudinal or transverse [2]. If growth is primarily longitudinal, the fibres will assume a more longitudinal orientation (Fig. 1a). If the growth is primarily transverse or broadening, the fibres will retain most of their circular orientation (Fig. 1b). At the PUJ there is a broadening effect of the fibres in the renal pelvis and a lengthening effect in the upper ureter to produce a transitional zone (Fig. 2a,b). The transitional zone may be funnel-shaped so that the actual PUJ cannot be identified from its physical appearance and can only be accurately located by determining the level at which the pressure tracings characteristic of the renal pelvis change to those characteristic for the upper ureter (Fig. 3). Growth in simultaneous directions will result in fibres being stretched into a coiled or helical arrangement. Such a helical configuration is typical throughout much of the length of the ureter but, because lengthening is not completely uniform, the helix is irregular with fibres in all degrees of obliquity from vertical to transverse.

Figure 1.

The final arrangement of muscle fibres depends on whether the growth is primarily longitudinal or transverse. If growth is primarily longitudinal (a) , the fibres assume a more longitudinal orientation. If growth is predominantly transverse or broadening (b) , the fibres retain most of the circular orientation. From [ 2 ] with permission.

Figure 2.

A diagrammatic representation of how the transitional zone develops at the PUJ. a, The original orientation of the muscle fibres before differential growth, and b, the orientation of the muscle fibres in the renal pelvis and ureter as a result of different directions of growth. From [ 2 ] with permission.

Figure 3.

Characteristic pressure tracings of the renal pelvis and the ureter. The transition zone between the pelvis and the ureter may only be differentiated functionally, not anatomically.

Although the ureteric muscle should be viewed as a single muscle sheath, there is a tendency for layering, particularly in the proximal ureter where the inner fibres tend to be longitudinal and the outer fibres circular. As a purely circular arrangement of fibres is more likely to be found in the upper ureter, this may explain why ureteric rings are only seen in the upper ureter.

Physiology of the ureter

Transmission of urine from the renal pelvis into the upper ureter begins with relaxation of the PUJ and distension of the upper ureter to form a spindle or bolus of urine. In contradistinction to bowel, where the circular or annular muscle contracts and propels the fluid forward by a progressive ring action, the ureteric spiral muscles shorten and are drawn over the urine bolus. Contraction of the spiral muscles closes the lumen of the ureter behind the bolus. The initial relaxation of the ureter, which allows the hydrodynamic pressure of urine to distend the ureter, is the essential element that is missing in PUJ obstruction and primary megaureter.

PUJ obstruction

Explanations for idiopathic PUJ obstruction include ischaemic hypoplasia, focal developmental arrest and failure of recanalization during embryonic development [3–6]. An increased deposition of collagen and disordered muscle configuration has been noted on light and electron microscopic studies. When considering all of the possibilities, perhaps the most significant observation is that the PUJ usually permits the passage of a probe to distend the region to a normal calibre. Therefore, if this is not a mechanical obstruction, does the current understanding of the physiology of urine transport, as briefly discussed above, provide a sound theoretical basis for proposing a functional obstruction?

A funnel-shaped pelvis with oblique or helical fibres crossing the pelvi-ureteric region is an efficient configuration that will allow the initial relaxation and distension to form the urine bolus, and allow its onward propagation. The efficacy of this muscle configuration declines progressively as the fibres in the renal pelvis become more horizontal relative to the vertical fibres in the upper ureter, until a critical point is reached when relaxation of the muscle will not result in opening of the pelvi-ureteric junctional zone. During early fetal development, ballooning of the renal pelvis may reach this critical point when even a rising intrapelvic pressure cannot overcome the inability of the pelvi-ureteric musculature to open. This results in progressive distension of the renal pelvis, which exacerbates the muscle alignment. Thus, developmental overgrowth of the renal pelvis may itself result in a functional obstruction. Theoretically, an inappropriate muscular composition or configuration alone could also result in a functional obstruction or be a contributory factor. As a sac-like renal pelvis is the normal and common configuration, congenital overgrowth would seem to be a more likely explanation.

If such is the underlining mechanism, it is not difficult to understand why the aetiology of PUJ obstruction has been so elusive. Apart from the difficulty of histological reconstruction of the complex pelvi-ureteric musculature, the original muscular configuration may be obscured by pelvic over-distension, and this may be further obscured by secondary histological changes, e.g. interstitial fibrosis and collagen replacement of muscle fibre. Parenthetically, current computer software considerably simplifies three-dimensional reconstruction. Interestingly, a local accumulation of circular muscle, which is believed to be the basis for the ureteric ring, can occur exactly at the pelvi-ureteric region without producing obstruction (Fig. 4).

Figure 4.

A ureteric ring at the PUJ; ureteric rings are not associated with obstruction. From [ 11 ] with permission.

Congenital valves

This condition is in fact a circular mucosal diaphragm with a small central opening having no valvular function. It is believed to be a failure in canalization [7]. Although the diaphragm may contain bands of smooth muscle, it is presumed to be primarily a mucosal anomaly. It is always associated with obstruction. Radiographically, there is characteristically proximal dilatation with a normal ureter beyond the obstruction (Fig. 5).

Figure 5.

Congenital ureteric valves; obstruction with proximal dilatation of the ureter. From [ 8 ] with permission.

Primary megaureter

Described under various names [8], this condition is a congenital anomaly in which a short segment of the supravesical ureter fails to transmit peristaltic waves and results in a functional obstruction (Fig. 6). Like PUJ obstruction, the essential feature of primary megaureter is a failure of relaxation so that the initial bolus of urine cannot develop [9,10]. While this is also presumed to be caused by anomalies of muscle fibre arrangement, it differs from the pelvi-ureteric region. Theoretically, obstruction could be produced if the muscle fibres were entirely circular or longitudinal. As the muscle fibres in the intravesical ureter are entirely longitudinal, it is embryologically more likely for them to extend into the distal ureter to produce the functional obstruction known as primary megaureter. Like the pelvi-ureteric region, there is no evidence of a mucosal component as the ureter is capable of distension to a normal calibre.

Figure 6.

Primary megaureter; the ureter tapers smoothly to a narrowed juxtavesical segment which does not transmit peristaltic waves.

Like PUJ obstruction, the same difficulties apply in establishing the precise cause of primary megaureter. The muscles in the distal ureter extend up to and fuse with the bladder wall and trigone, terminating in the verumontanum. This has functional implications and has been described as the trigonal ‘tunnel’[2]. Failure in this region is related more to failure of bladder wall development and should not therefore be included with this group of ureteric anomalies.

The fetal ureter, kinks, bands, lateral indentations and ureteric rings

The ureter in infants and children may contain kinks, bands, and lateral indentations, as well as ureteric rings [11] (Fig. 7). All these normally disappear in the adult ureter, but the lateral indentation may persist either as a permanent indentation or may only appear when the ureter is relaxed (Fig. 8). Persistent spurs are presumed to involve both the mucosal and muscular layers, although histological data are lacking.

Figure 7.

Infantile ureters showing a ureteric ring and multiple lateral indentations. From [ 11 ] with permission.

Figure 8.

Lateral indentations may be seen in the relaxed adult ureter, which become effaced with distension of the ureter. From [ 11 ] with permission.

Authors

P. Dure-Smith, MD.

L. Lau, MD.

B. Khan, MD.

A. David, MD.

L. Lau, Loma Linda University Medical Center, 11234 Anderson Street, 2nd Floor, Loma Linda, CA 92354, USA.
e-mail: llau@ahs.llumc.edu

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