Platelet localization in Thbd^−/− placenta using P-selectin antibodies. Thbd-expressing (A) and -null (B) placentas were identified by in situ hybridization using antisense RNA probes. Blue color indicates Thbd expression. The null units express maternally derived Thbd in spiral arteries (C). Diffuse P-selectin stain (brown color) was observed at low magnification (E) (nonimmune IgG control in panel D), identified to correspond to platelets at higher magnification (F). Platelets are abundantly present in maternal blood spaces surrounding trophoblast giant cells. Platelet distribution was similar in Thbd^−/− (E,F) and Thbd-expressing (not shown) utero-placental units. Slides were counterstained with nuclear fast red (A-C) or hematoxylin (D-F), mounted in Permount (Fisher Scientific, Pittsburgh, PA) and examined under a Nikon Eclipse E600 microscope (Nikon, Melville, NY). Pictures were taken using a 10×/0.3 (A-E) or 100× oil/1.3 (F) objective with a spot camera (Diagnostic Instruments, Sterling Heights, MI). Solid arrows indicate trophoblast cells; dashed arrows indicate platelets; arrowheads indicate spiral arteries. Panel F corresponds to the boxed region in panel E. Scale bars represent 0.05 mm (A-C), 0.1 mm (D), and 0.01 mm (E,F).