Preterm umbilical vein endothelial cells have fewer Weibel-Palade bodies than term cells. HUVECs were viewed by transmission electron microscopy for quantitative morphology. The mean numerical density (N_V) represents the number of Weibel-Palade bodies per cubic micrometer of nonnuclear cytoplasm. The difference between term and preterm endothelial cells is significant (P = .01).