Umbilical cord diameter. This diameter decreases rapidly within the placenta. The fully patent umbilical artery has two main layers: an outer layer consisting of circularly arranged smooth muscle cells and an inner layer which shows rather irregularly and loosely arranged cells embedded in abundant ground substance staining metachromatic. The smooth muscle cells of the layer are rather poorly differentiated, contain only a few tiny myofilaments and are thereby unlikely to contribute actively to the process of post-natal closure. The umbilical cord contains Wharton's jelly, a gelatinous substance made largely from mucopolysaccharides which protects the blood vessels inside. It contains one vein, which
Umbilical ring Umbilical ring The umbilical ring is a dense fibrous ring surrounding the umbilicus at birth. At about the sixth week of embryological development, the midgut herniates through the umbilical ring; six weeks later it returns to the abdominal cavity and rotates around the superior mesenteric artery. Dense embryonic connective tissue encircles the attachment of the umbilical cord. It forms an umbilical ring of mesodermal condensation surrounding the coelomic portal, and is present in the 16 mm. embryo, but more emphatically so in the 23 mm. embryo. The compact tissue first appears in the stalk mesoderm, situated superficial to the allantoic