Histopathology and ultrastructure of human umbilical blood vessels.

To resolve controversy over umbilical vessels structure, a morphological review was undertaken of the histology of blood vessels in 130 fetal umbilical cords varying in gestational age and the ultrastructure of blood vessels in 6 umbilical cords. Arteries and veins were lined by endothelium. The internal elastic lamina was frequently interrupted when associated with intimal thickening of longitudinally orientated smooth muscle cells. Fragments of elastic laminae developed in the intima and inner media both of which were thicker in arteries than in vein. No external elastic laminae or distinct adventitia were found. Most notable was the accumulation of cell debris developed from blebs derived from polypoid cytoplasmic protrusions of smooth muscle cells of both arteries and veins. They underwent hydropic change and became detached and fragmented particularly after 20 weeks' gestation. Similar hydropic degeneration occurred in endothelial cells of arteries and veins, such changes being consistent with the destructive pattern of hemodynamic stresses.