The urachus is an intra-abdominal fibrous remnant of the cloaca and the allantois.
During normal gestational development,
the urachus involutes and its lumen is obliterated,
becoming the median umbilical ligament.
The non-involution of the allantois can lead to urachal anomalies in later life.
Urachal anomalies are rare,
with higher prevalence in men than women.
EMBRYOLOGY (fig.1)
The urachus is a vestigial structure remnant of two embryologic structures: the cloaca and the allantois.
The ventral aspect of the cloaca elongates and forms four segments between the 4th and 7th weeks of gestation: an expanded distal portion,
the urogenital sinus; a tubular portion,
the primitive urethra; an upper dilatation,
the future bladder; and a tubular portion,
the urachus,
which is continuous with the extra embryonic allantoic stalk that appears early in the 3rd week of gestational life.[1]
Around the 4th to 5th gestational months,
involution of the allantois and urogenital sinus occurs as the bladder descends into the pelvis,
forming the urachus,
which elongates and becomes a thick fibrous cord connecting the apex of the bladder to the umbilicus.
Urachal remnants regress due to fibrosis with age,
eventually becoming the median umbilical ligament. [1,2]
ANATOMY (fig.2)
The urachus connects the bladder to the allantois during early embryonic development.
it extends from the anterosuperior surface of the bladder to the umbilicus and lies in the extraperitoneal space of Retzius between the transverse fascia anteriorly,
the parietal peritoneum posteriorly,
and the medial umbilical ligaments laterally,
which are the obliterated remnants of the umbilical arteries.
Its length ranges from 3 to 10 cm and it generally has an approximate diameter of 8–10 mm.
[2]
The urachus is a three-layered structure with a real or potential lumen most commonly lined by transitional epithelium.
Surrounding the epithelium there is a layer of connective tissue and an outermost layer of muscle that is continuous with the detrusor inferiorly. .
[1,3]