Allantoic cysts are a type of true cyst of the umbilical cord.
The allantois forms from the part of the fetal yolk sac that eventually becomes the primitive hindgut (the cloaca). The cloaca divides into the hindgut posteriorly and the urogenital sinus anteriorly. The allantois remains connected to the urogenital sinus and extends into the base of the umbilical cord. The umbilical cord is formed from the fusion of the body stalk (containing the allantois) and the umbilical vessels and yolk stalk (containing the omphalomesenteric duct). The allantois usually regresses by the 14th week of gestation, but may occasionally persist and present as a cystic mass in the base of the umbilical cord.
- vesico-allantoic cyst: communicates with the bladder
May be seen as an anechoic cystic mass of the umbilical cord which is centrally located in the cord with the umbilical vessels located against the outer wall. It is typically located at the fetal end of the umbilical cord.
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