Endothelial cell stores of P-selectin and von Willebrand factor increase with gestational age in human neonates. ELISAs were performed on HUVEC lysates and normalized to total cellular protein. These graphs summarize the data from 12 term infants (>38 weeks) and 15 preterm infants (18 to 31 weeks). (A) P-selectin. The bars represent the mean ± SD. The amount of P-selectin was significantly less in preterm HUVECs compared with term HUVECs (P = .003). When individual samples were examined, the amount of P-selectin generally correlated with gestational age, although there was variation from sample to sample. (B) von Willebrand factor. The bars represent the mean ± SD. The amount of vWf was significantly less in preterm HUVECs compared with term HUVECs (P = .005).