THE ALLANTOIS

DEVELOPMENT

The Allantois arises as a blind diverticulum from the caudal wall of the yolk sac, the part of which that forms the hind gut. The part of hindgut from which the allantois develops will then become the urinary bladder. Allantois grows into the connecting stalk, and shares in the formation of the primitive umbilical cord. The blood vessels on the wall of the allantois develop from the primary mesoderm of the connecting stalk, and they enlarge to become the umbilical blood vessels which consist of two arteries and one vein. As the urinary bladder enlarges during development of the embryo, the allantois obliterates to form a fibrous cord called the urachus which in adult is called median umbilical ligament that extends from the apex of the urinary bladder to the umbilicus.

STRUCTURE

The allantois is an endodermal evagination of the developing hindgut surrounded by splanchnopleuric primary mesoderm, where it is embedded in the connecting stalk.

Figure 1 Formation of allantois

FUNCTION

The allantois brings no function to human but in some mammals, it shares in the development of placenta. In human, mesodermal cells of the connecting stalk surrounding the allantois will form blood cells and vessels which will then develop to form the umbilical blood vessels.

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FATE

EXTRAEMBRYONIC PART

The part of allantois inside the umbilical cord will be removed with the umbilical cord during delivery

INTRAEMBRYONIC PART

The allantois will be obliterated and changed into a fibrous cord known as the urachus in the foetus and median umbilical ligament in the adult. It extends from the apex of the urinary bladder to the umbilicus.

ABNORMALITIES

One of the abnormalities in allantois formation is urachal fistula. In this defect, there is a fistula connecting the urinary bladder to the umbilicus. If there is obstruction in the urethra, the urine starts to drain through the umbilicus.

Another abnormality is urachal sinus. The proximal part of allantois is still patent forming a sinus which opens into the apex of the urinary bladder while the other part is obliterated into a ligament. This sinus can act as a source of infection to the urinary bladder. The distal part of the allantois may remain opened forming a sinus communicating with the umbilicus.

Urachal cyst is another defect in allantois abnormalities. The allantois is obliterated at the proximal and distal parts, but remains patent in the middle forming a cyst. If this cyst is infected, it enlarges due to accumulation of infected mucous secretion inside this cyst.

Treatment for these three abnormalities is surgical removal.

Figure 2 shows A. Urachal fistula B. Urachal cyst C. Urachal sinus 2

REFERENCES

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£ ¤ ¥ £ ¥¦ § ¥ § ¥ ¢ ¤ ¥ 1 W ¢ e (home e on the Inte net). S n F nc sco: W me Foundation, Inc. (updated 2010 July 9, cited 2010 Dec 24 ). Available from: http://en.wikipedia.org/wiki/Allantois

2. El-Gebaly, F. The Allantois. Essential of Human Embryology. Egypt: Faculty of

Medicine Ale ¨ andria University: 46 ʹ 47

3. Wikipedia (homepage on the Internet). San Francisco: Wikimedia Foundation, Inc. (updated 2007 Jan 123, cited 2010 Dec 24). Available from: http://en.wikipedia.org/wiki/File:Gray25.png

4. Scielo.org.za (homepage on the Internet). South Africa: SAMJ: South African Medical Journal.(updated 2010 Jan, cited 2010 Dec 24). Available from: http://www.scielo.org.za/scielo.php?pid=S0256-

95742010000100009&script=sci_artte ¨ t

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