Expression of P-selectin is developmentally regulated in humans. Mesentery was collected during the autopsy of individuals of varying age and processed as in Fig 2. The sections were viewed with Nomarski differential interference contrast optics. All four panels are magnified 165×. (A and B) P-selectin expression. P-selectin expression was detected with the MoAb S12. (A) is from a term infant who died because of a congenital heart defect and (B) from a 22-week gestation infant delivered prematurely because of an incompetent cervix. There is immunostaining for P-selectin on mesenteric endothelial cells of the term infant (arrows), but no immunostaining was detected on the endothelial cells from the 22-week gestation infant. The arrow in (B) points to endothelial cells lining newborn mesenteric vessels that did not stain. (C and D) PECAM-1 expression: These sections are from the same tissue blocks as in (A) and (B). (C) is a section of mesentery from the term infant and (D) is from the 22-week gestation infant. The arrows in both figures indicate endothelial cells immunostained for PECAM-1.