Physiological and pathological morphology of the umbilical and placental circulation

Closure mechanisms of fetal vessels are essential after birth. They are physiological. The speedy and characteristic constriction is made possible by the special construction of the fetal vessels. The so-called "folds of Hoboken" in the umbilical artery initially form tapered constrictions which quickly extend to longer sections of the vessel. The contractions continue to include the chorion plate and the villi. Due to a lack of lamina elastica interna, a protrusion of pear shaped, expanded mediamyocytes and myofibroblasts is possible which reduces the cross-sectional area of these vessels. Similar, but lasting reductions in the cross-section of the villous arteries due to a fibrous-muscular media occur in the second half of pregnancy (IVth sign of maturity). Closure of the fetal vessels during intrauterine life leads to placental insufficiency of fetal death in utero. The endarteritis obliterans closes villous vessels by means of a connective tissue plug which starts on one side of the vessel and continues across the whole width of the lumen. The periphery dependent of these villi remains avascular, as the placenta is limited in its resorptive capabilities. In the case of intrauterine asphyxia, intravasal fibrinthrombi form as a sign of subacute insufficiency in the utero-placental circulation. A decompression collapse occurs in the fetal circulation, when fetal death occurs with continuing maternal circulation.