The urachus is a fibrous remnant of the allantois
Allantois is a part of a developing animal conceptus . It helps the embryo exchange gases and handle liquid waste....

, a canal that drains the urinary bladder
Urinary bladder
The urinary bladder is the organ that collects urine excreted by the kidneys before disposal by urination. A hollow muscular, and distensible organ, the bladder sits on the pelvic floor...

 of the fetus that joins and runs within the umbilical cord
Umbilical cord
In placental mammals, the umbilical cord is the connecting cord from the developing embryo or fetus to the placenta...

 . The fibrous remnant lies in the space of Retzius, between the transversalis fascia anteriorly and the peritoneum
The peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in amniotes and some invertebrates...



The vesico-urethral portion of the urogenital sinus
Urogenital sinus
The definitive urogenital sinus is a part of the human body only present in the development of the urinary and reproductive organs...

 absorbs the ends of the Wolffian ducts and the associated ends of the renal diverticula, and these give rise to the trigone of the bladder and part of the prostatic urethra
Prostatic urethra
The prostatic urethra, the widest and most dilatable part of the urethra canal, is about 3 cm. long.It runs almost vertically through the prostate from its base to its apex, lying nearer its anterior than its posterior surface; the form of the canal is spindle-shaped, being wider in the middle...

The remainder of the vesico-urethral portion forms the body of the bladder and part of the prostatic urethra; its apex is prolonged to the umbilicus
The navel is a scar on the abdomen caused when the umbilical cord is removed from a newborn baby...

 as a narrow canal, which later is obliterated as the urachus goes on to definitively form the median umbilical ligament
Median umbilical ligament
The median umbilical ligament is a structure in human anatomy. It is a shrivelled piece of tissue that represents the remnant of the embryonic urachus.It extends from the apex of the bladder to the umbilicus, on the deep surface of the anterior abdominal wall...



Failure for the lumen of the urachus to be filled in leaves a patent (open) urachus. The telltale sign is leakage of urine through the umbilicus. A patent urachus needs to be surgically removed. There are four anatomical cases
  • Urachal cyst
    Urachal cyst
    A urachal cyst is a sinus remaining from the allantois during embryogenesis. It is a cyst which occurs in the remnants between the umbilicus and bladder. This is a type of cyst occurring in a persistent portion of the urachus, presenting as an extraperitoneal mass in the umbilical region. It is...

    : there is no connection between the bladder and the umbilicus
  • Urachal fistula: there is free communication between the bladder and umbilicus
  • Urachal diverticulum [Vesicourachal diverticulum]: the bladder exhibits outpouching
  • Urachal sinus: the pouch opens toward the umbilicus (Guray, Sogut, et al. Urachal Cyst. Eastern Journal of Medicine 5 (2) : 76-78, 2000)

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