Abnormal fusion of the müllerian system
Keywords: cervix, paramesonephric, müllerian
During early fetal development in females, the müllerian tubules (also known as paramesonephric tubules) are permitted to develop because of the absence of anti mullerian hormone (AMH). This is due to the absence of testicles and the Sertoli cells therein; the source of AMH.
Once mullerian tube development is underway, the most caudal sections of the tubules form to fuse the cranial segment of the vagina. When this does not occur, portions of the medial wall of the mullerian system can persist in the vagina into adulthood and cause dyspareunia or dystocia. In the author's experience, these remnants are most common in dogs and ruminants but they are well described in humans. These abnormalities should not be confused with persistence of the hymen which is the result of incomplete breakdown of the interface between the endoderm and ectoderm.
The degree of fusion of the müllerian tubes from caudal to cranial is a function of species in normal development. This can be seen in the image below. The degree of fusion in rabbits, opossums and other mammals with two cervixes is seen at left, progressing to greater and greater degrees of fusion with complete fusion in humans and higher primates at right.
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If fusion is abnormal and the species is bovine, a common manifestation is a double external os with immediate transition into a single cervical as shown here:
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The only abnormally un-fused portion of the mullerian system in the image above is the small transverse wall shown on the right side of the image, indicated by the light grey arrow. A similar case is shown below, where a finger was passed into the cervical os on one side, and out of the other. During calving, dystocia may arise if the extremities of the calf are separated by this defect. It can be transected without severe hemorrhage.
Notice the partial persistence of a hymen in this animal as well.
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In other cases, the degree of non fusion is more obvious, with two lumens progressing cranially for a short distance then fusing into a single canal. An example of this is shown below.
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In rare and extreme cases, there may be no fusion of the müllerian tubes at all (apart from fusion in the cranial vagina). This is a manifestation of the condition known as "uterus didelphys" from the Greek words di- (two) and didelphys (womb or uterus). The caudal portion of "uterus didelphys" in a cow is shown here:
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The author has had this image (above) for many years and cannot be sure of its origin. If you believe you know the author, please contact Dr Rob Lofstedt at lofstedt@upei.ca.
In a case of uterus didelphys with a double cervix as shown below,
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a cow could be inseminated repeatedly through a cervix that does not connect with the side of the uterus where ovulation occurs, preventing conception on those occasions.
Note: In cattle, double cervixes are found at a level of less than 1% to about 15%, depending on the report and there is good evidence to suggest that it is a heritable condition; recessive with partial penetrance. Interestingly, this author encountered many cases of double cervixes in the Midwest US and came to think of the condition as quite common. However, after moving to New England then to Eastern Canada, he encountered relatively few cases. This substantiates the contention that it is heritable and more common in some gene pools than others. Although double cervixes have been reported in horses, they are rare. Indeed, this is also true for other domestic animals.
One should recall that the mullerian system in the embryo extends to the interface of fusion between the endo- and ectoderm. In the fetus, that interface becomes the hymen, either perforate or imperforate (persistent). Cranial to that point, all structures are mullerian in origin. Therefore remnants of the divided mullerian system can persist caudal to the cervix and cranial to the hymen.
Persistence of the medial wall of the müllerian caudal to the cervix is not rare and can be a cause of dystocia with a fetal limb or head being trapped on one side of the structure and the rest of its body on the other. Remarkably, in some cases birth occurs normally. For example in the images below, the tract on the left was from a cow and that on the right from a heifer. Both animals had persistent medial walls of their müllerian systems. However, the uterus of the cow had undergone involution indicating that that she had calved despite the presence of the obstruction in her vagina.
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