Correspondence on: A Hyaluronic Acid-Rich Node and Duct System in Which Pluripotent Adult Stem Cells Circulate.

The origin of the wide spread node and duct system described by Rai et al. remains a mystery. The explanation came when another study on yolk sack hemopoiesis was compared with the "primo vascular system". It came out that the yolk sack hematogenic tissue, introduced to the embryo by the vitelline veins, does not disappear in adults, but continues to exist in form of nods and ducts along blood vessels and different organs.