Fetal/neonatal platelets contain a-granules and secrete PF4 on stimulation. (A) Immunofluorescence microscopy of VWF and VEGF protein reveals punctate patterns of expression in resting adult and fetal (E15.5) platelets. Deconvolved merged maximum intensity projection images are shown. Scale bar, 5 μm. Similar patterns were observed in 3 independent experiments. (B) Significant amounts of Pf4 were released after activation of P1 and adult platelets. Platelet activation with thrombin (0.5 U/mL) increases the concentration of Pf4 similarly in supernatants from P1 and adult platelets when evaluated by enzyme-linked immunosorbent assay. Significance was determined using a 2-way ANOVA followed by a Bonferroni posttest, *P < .05 between unactivated and activated samples per age group. Mean ± standard error of the mean shown; n = 4.