Scanning electron microscopic investigations of the human umbilical artery intima. A new conception on postnatal arterial closure mechanism.

Umbilical cord arteries were investigated using a scanning electron microscope using different methods of preparation: Perfusion of one artery under pressure from a 100 cm water column caused artefacts and the preparatory work took at least 10 min after delivery. To shorten this time fully patent umbilical cords were double clamped and fixed immediately after birth. However, the removal of blood after fixation caused the endothelial layer to be lost. Therefore umbilical cords were double clamped, snap frozen and stored in liquid nitrogen until preparation. The endothelial lining of the fully patent umbilical artery at birth is composed of longitudinally arranged, spindle-shaped cells, connected by cellular junctions. The basement membrane contains numerous gaps. Because of these gaps postnatal vasoconstriction causes herniation of the subendothelial myofibroblasts forming subendothelial vacuoles. The vacuoles produce displacement of the endothelial cell cytoplasm towards the vascular lumina resulting in protuberances and blebs on the endothelial cells. Rupture of vacuoles leads to crater-like injuries. Beneath the basement membrane a thin layer of myofibroblasts is arranged longitudinally. Oblique or transversely ordered bundles of myofibroblasts are interposed at wide and irregular intervals. These transverse bundles are able to trigger localized contraction rings called "folds of Hoboken", the initial stage of postnatal arterial closure.