9
Embryology for Urologists
generally nothing more than a fibrous cord. In |
six times more common than bilateral involve- |
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hypoplasia it may be a thin-walled tube. These |
ment. It is definitely more common in females, |
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ureteral anomalies thus represent varying |
and because of associated symptoms of obstruc- |
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degrees of ureteral bud development which will |
tion or reflux it is generally diagnosed in child- |
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in turn determine the appearance of the vesical |
hood rather than in later life. |
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trigone, ureteral orifice, and kidneys. In other |
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words, the clinical appearance of the ureteral |
Incomplete Ureteral Duplication |
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orifice, trigone, and kidney normally parallel |
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the extent of ureteral development. These cor- |
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responding structures may thus be absent, |
The embryological explanation of incomplete |
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aplastic, or hypoplastic, as discussed earlier |
ureteral duplication is simple and differs totally |
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under renal development. |
from the explanation for complete duplication. |
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In incomplete duplication, premature bifurca- |
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Anomalies of Number |
tion or splitting of the ureteral bud occurs after |
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it arises from the mesonephric duct. If such |
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Ureteral anomalies of number are among the |
branching occurs just below the normal point of |
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most interesting from both a pathological and |
bifurcation as it enters into the metanephros, |
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embryological standpoint (see Fig.1.5).Whereas |
the condition is called “bifid pelvis.” Such pre- |
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ureteral triplication is extremely rare, duplica- |
mature splitting can occur anywhere along the |
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tion of one or both ureters is quite common, |
course of ureteral bud development. If it occurs |
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and in a large autopsy series the incidence was |
very early in its growth from the mesonephric |
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as high as one in 160 cases. Logically, duplica- |
duct, the point of bifurcation may actually be |
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tion is more often incomplete than complete as |
within the intravesical ureter that is within the |
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described below and unilateral involvement is |
developing bladder wall. Although such patients |
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may be diagnosed to have completely duplicated |
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ureters on radiography, cystoscopy would reveal |
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a single ureteral orifice within the bladder. |
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Complete Ureteral Duplication |
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As noted previously, the embryological explana- |
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tion of complete ureteral duplication differs |
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totally from that of incomplete duplication. |
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Specifically, two separate ureteral buds sprout |
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from the respective mesonephric duct (see |
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Fig. 1.6). Logically, the ureteral buds which sprout |
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from the higher level of the mesonephric duct |
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invaginate into the upper portion of the adja- |
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cent metanephros while the lower ureteral bud |
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invaginates into the lower portion of the meta- |
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nephros. One would thus expect the upper ure- |
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teral orifice to be associated with the ureter that |
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drains the upper renal pelvis while the lower |
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ureteral orifice in the bladder drains the ureter |
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of the lower renal pelvis. However, this is not so |
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and, in fact, the opposite is true. In reality, the |
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ureteral orifice serving the lower renal pelvis is |
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always situated cranially and laterally to the ure- |
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teral orifice serving the upper renal pelvis. |
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This is known as the Weigert-Meyer Law. Only |
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rare instances have been reported in which this |
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Figure 1.5. X-ray of incomplete ureteral triplication. |
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rule does not hold true. This is so because, as |
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10
Practical Urology: EssEntial PrinciPlEs and PracticE
Figure 1.6. Ureteral duplication. |
Mesonephric duct (Wolffian duct) |
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Upper bud |
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Lower bud |
U |
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Bladder |
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L |
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Rectum |
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Progressive development/absorbtion of ureteral buds into the developing urogenital sinus (bladder, urethra, etc.)
M
U M U
L L
Urogenital sinus (urethra)
mentioned previously, the portion of the urinary bladder which received the mesonephric duct with its attached ureteral bud or buds grows in an uneven manner so that the ureter, once absorbed into the bladder wall, migrates laterally and cranially, while the rest of the mesonephric duct with the attached upper bud grows medially and distally. It thus stands to reason that the lower ureteral bud, which is absorbed into the bladder first, assumes a more lateral and cephalad position than the upper ureteral bud which is carried distally and medially by the migrating mesonephric duct. Only as still more of the duct is absorbed into the bladder or urethra does this ureter attain its independent opening. It can, of course, be so high in position that it never leaves the mesonephric duct in which case it will empty into the ejaculatory duct or the higher structures to which of the embryological mesonephric duct gives rise or its embryological remnants in the female (Gartner’s Duct).
Clinically, because of their anatomic position, one or both of these duplicated ureteral orifices are often associated with either vesicoureteral reflux (ureter draining the lower renal segment), or ureteroceles and ureteral ectopia with stenosis and severe dilatation (ureter draining the upper renal segment). The paired ureters normally
course downward side by side to the level of the bony pelvis and, just prior to reaching the bladder, the ureter from the upper renal pelvis passes beneath its mate to enter the bladder at a lower and more medial position. Since this ureter draining the upper renal pelvis normally opens in the more dense trigonal tissue, it is often associated with stenosis and secondary ureteral dilatation or a ureterocele. It can, of course have an extra vesical entry; in which case it is generally much more obstructed and dilated.
Similarly, it is these ureters which are often associated with a ureterocele. On the other hand, the ureter draining the lower renal segment empties high and laterally into the bladder wall and is thus often off the trigone. It therefore has a relatively short segment, and by virtue of its lateral position is more subject to vesicoureteral reflux.
Clinical symptoms and signs associated with ureteral duplication are then generally related to obstruction or reflux, and these findings are best demonstrated by three classical radiographic findings which include (see Fig 1.7):
1.Non-visualization of a relatively large renal area (severely obstructed upper renal segment).
2.The classical “drooping lily” sign (from pressure above).
11
Embryology for Urologists
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ejaculatory duct, seminal vesicles, vas deferens |
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and epididymis in the male, ureteral ectopia |
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may occur at any of these sites – depending |
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upon how high the uretral bud was positioned |
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on the mesonephric duct. |
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Similarly, location of ectopic ureteral orifices |
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in the vesical neck, urethra or vestibule of the |
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female are explained on the basis of their |
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mesonephric duct origin and its contribution |
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to the urogenital sinus. Since vestigal remnants |
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of the mesonephric duct in the female may be |
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found along a line running from the hymen |
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(homologue of the verumontanum) through |
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the anterolateral walls of the vagina, the cervix, |
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the wall of the uterus, and between the layers |
Figure 1.7. X-ray of ureteral duplication with“drooping lily” sign. |
of the broad ligament to the ovaries, it is feasi- |
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ble that ectopic orifices could in fact drain into |
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any of these embryological remnants which |
3. The finding of a dilated, often refluxing later- |
include the epoöphoron, paroöphoron, and |
Gartner’s duct. Such a location would be |
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ally displaced ureter by it severely dilated |
extremely uncommon and of so, the obstruc- |
obstructed mate which drains the upper renal |
tion is usually so great that the renal segment |
segment. |
is non-functioning. |
Ureteral Ectopia
Ureteral orifices which drain other than in their normal position on the trigone are considered to be ectopic. The vast majority or 80% of these are associated with a duplicated renal collecting system as discussed above,and if this is the case,both could be ectopic. If only one is ectopic, it is generally the ureter draining the upper renal segment.
Embryology of Ectopia
The ectopic ureter can best be explained embryologically as follows. If the point of origin of the ureteral bud from the mesonephric duct is closer to the urogenital sinus (developing bladder) than normal, it will enter the bladder earlier and thus be positioned at a point more cephalad and lateral to the normal position on the trigone. On the other hand, if the point of origin of the ureteral bud is higher on the mesonephric duct than normal, it will be carried more medially and distally with the mesonephric duct and may, in fact, never enter the bladder proper.
Inasmuch as the mesonephric duct contributes to the formation of the trigone and posterior urethra and then gives rise to the entire
Clinical Correlation
If the ectopic orifice is outside the bladder, the location depends on the sex of the patient.
Location of Ectopic Ureteral Orifices – Male (in Descending Order According to Incidence)
Vesical neck and prostatic urethra
Seminal vesicles
Vas Deferens
Ejaculatory duct
Rarely in the rectum
Location of Ectopic Ureteral
Orifices – Female
In the female, the location of the ectopic ureteral orifices, are generally divided equally between the vesical neck and urethra, the vestibule, and the vagina.
Vesical neck and urethra Vestibule
Vagina
Cervix, uterus and rectum