Structure and closure mechanism of the human umbilical artery.

The structure of the fully-patent umbical artery and rearrangement of its structural elements with postnatal closure were examined 10 centimeter long umbilical cord segments which were double-clamped at different time intervals after delivery. The fully-patent umbilical artery consists of two main layers: an outer layer of circularly arrange smooth muscle cells and an inner layer which shows rather irregularly and loosely arranged cells embedded in abundant metachromatic ground substance. No predominantly longitudinal arrangements of cells and fibers reported by earlier investigators could be identified in the inner layer. Closure of the umbilical arteries is initiated by numerous localized contractions which are mainly formed by muscle cells of the outer circular layer. Ultimate closure of larger segments of the umbilical arteries is also mainly produced by contractions of the outer layer. In contrast, the inner layer (which is rich in ground substance) seems to serve mainly as a plastic tissue which can easily be shifted in an axial direction and then folded into the narrowing lumen to complete closure. Electron microscopy reveals that the cells of this layer represent rather poorly differentiated smooth muscle cells which contain only a few tiny myofilaments and can therefore hardly contribute actively to the process of closure.