Embryology of Urachus
John M. Hutson, MD,DSc, FAAP, Elizabeth C. Penington, MBBS,PhD, FRACS.
The Royal Children's Hospital, Melbourne, Australia.
The embryological origin of the urachus and the bladder in humans has been the subject of much debate over many decades. Opinions vary over how much the allantois and cloaca contribute to the formation of the urachus and the bladder. We sought evidence from comparative embryology to improve our understanding of bladder and urachal development.
Materials and Methods
Sprague-Dawley rat embryos were examined between 11 and 21 days’ gestation and compared with human embryo sections at similar stages held in collections in the UK, France and Holland. In addition other rat embryos treated with Adriamycin on days 6-9 of gestation (to produce the VACTERL group of anomalies) also were examined at 11- 21 days’ gestation. Embryos were examined by a combination of light microscopy, dissection and serial histological section.
Rat embryos at no stage had an endodermal allantois, their allantois being purely mesenchymal. In the youngest embryos examined the cloaca was one single cavity limited ventrally by cloacal membrane, which, at its maximum, extended superiorly to the umbilical stalk. There was no extension of the cloacal cavity through the umbilical ring at any stage in rat development. In contrast human embryos have a clearly defined allantoic diverticulum that extends off the cloacal cavity and through the umbilical ring into the umbilical cord. Rats did not have a urachus at any stage examined during development.
Rat embryos treated with Adriamycin almost universally had agenesis of the bladder. The cloacal cavity failed to develop a bladder diverticulum at the usual stage of development. While the remainder of the features of the rats closely resembled the VACTERL association, bladder agenesis is not a feature of VACTERL association in humans.
The results support the hypothesis that the human urachus is formed from the endodermal allantois diverticulum while the bladder itself develops for the cloaca. Bladder development in rat embryos appears to involve a process of budding from the cloaca while in humans the initial branch point is already present at the junction of the allantois and the cloaca.
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