Reduced P-selectin content in embryonic and fetal platelets correlates with reduced transcripts for P-selectin in fetal megakaryocytes. (A) Integrin (αIIbβ3) activation in unactivated and thrombin-activated primary platelets isolated from E12.5 and E15.5 murine embryos, P1 pups, and adult female dams. Mean ± standard error of the mean (SEM). (B) Simultaneous analysis of samples in panel A for surface expression of P-selectin (CD62P) in unactivated and thrombin-activated primary platelets. Significance was determined using a 2-way ANOVA followed by a Bonferroni posttest. *P < .001 vs activated maternal sample, n ≥ 3. (C) Immunofluorescence microscopy of fetal (E15.5) and adult platelets reveals fewer punctae of P-selectin–positive granules in fetal platelets relative to adult controls. Deconvolved merged maximum intensity projection images are shown. Scale bar, 5 μm. Similar patterns were observed in 3 independent experiments. (D) Transcript levels of P-selectin (Selp) relative to platelet factor 4 (Pf4) in primary megakaryocytes isolated from E15.5 fetal livers, and adult bone marrow. Extremely low levels of P-selectin transcripts were found in primary megakaryocytes harvested from E15.5 fetal liver. Significance was determined using an unpaired 2-tailed Student t test, *P < .05; n = 4. MFI, median fluorescence intensity.