Newborn rats have a defect in expression of P-selectin on endothelial cells after an inflammatory stimulus. The photomicrographs in this figure are of samples collected 30 minutes after an intraperitoneal injection of thioglycollate and are representative of the 11 rats studied in each group. Postcapillary mesenteric venules were identified by their morphology. The sections were viewed with Nomarski differential interference contrast optics. All four panels were magnified 500×. (A and B) P-selectin expression. P-selectin expression was detected with the MoAb PB1.3. (A) is representative of mesenteric venules in the adult rat. The arrows demonstrate positive brown immunostaining of endothelial cells lining the vessels. (B) is representative of the newborn rat. The arrow points to endothelial cells lining newborn mesenteric venules that did not stain. (C and D) PECAM-1 expression. (C) is a section of adult mesentery and (D) is a section of newborn mesentery. Positive brown immunostaining for PECAM-1 is demonstrated covering the full height of the cytoplasm of endothelial cells (arrows).