Large platelets circulate in the embryonic and fetal vasculature. (A) Platelet-enriched blood was stained with anti–GPV-FITC antibodies (E11.5 shown, left panel), which specifically label platelets but not red blood cells. Shown to the right are examples of platelets from E11.5, E15.5, and adult peripheral blood, depicting the broad face (top) and narrow side (bottom). Regardless of cell diameter, all platelets were narrow and biconvex. Scale bar equals 5 μm. (B) Mean platelet diameter (± SEM) in E11.5 to E15.5 fetuses, in neonates, and in adult mice. A minimum of 100 platelets was measured per peripheral blood sample and at least 3 independent samples were studied, except for E10.5, of which 2 pooled blood samples were examined and a total of 11 platelets was measured. (C) Transmission electron microscopic images of E11.5, E15.5, and adult platelets. All images taken at × 10 000. Open canalicular system (arrows), dense granules (arrowheads), and alpha granules (α) are evident.