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Thrombi with newly formed vessels in thirteen human umbilical arteries were studied. Red blood cells formed masses which were covered by fibrin within twelve hours after birth. By the second day, the intima including endothelial cells was denuded. In the junctional area between the thrombi and the original intima on the sixth day, red blood cells were surrounded by endothelial-like cells, which displayed junctional complexes and did not show any connection with intact endothelial cells. These cells also exhibited filaments with fusiform densities, suggesting a probable origin from vascular smooth muscle cells. Examination of older specimens indicated that these were precursor structures of functional capillaries found in mature thrombi. ACTA PATHOL. JPN. 34: 529–536, 1984.

Intimal thickening and lipid accumulation, which occur in atherosclerosis, were observed in fifteen human umbilical vessels. Quantification of lipid laden cells was done electron-microscopically. Within 1 month after birth, lipid laden cells originating from macrophages were observed in the umbilical artery, while those originating from smooth muscle cells were seen in the umbilical artery and vein. Lipid droplet formation appeared to be independent and different in macrophages and smooth muscle cells; the latter cells were stimulated to produce lipid droplets by hypoxia and the former cells by blood components.