Fetal platelets are highly reticulated. (A) Flow cytometric analysis of adult and fetal blood. Platelet fractions were identified by forward and side scatter parameters (left panels), lack of Ter119 and Hoechst staining (middle panels), and CD41 staining (right panels). Thiazole orange–positive platelets were gated as described in “Materials and methods.” (B) The mean percentage (+SEM) of reticulated platelets in E11.5 to E15.5 fetuses (▪), in maternal blood (⊡), in neonates (), and in nonpregnant adult mice (□). Fetal blood samples were analyzed from 3 littermates (E11.5, E14.5, and neonate), 3 fetuses from 3 separate experiments (E12.5), 4 embryos from 2 separate experiments (E13.5), or taken from a single fetus (E15.5). Adult blood was analyzed from a single sample at gestational days 13.5 to 15.5, in triplicate at gestational days 11.5 to 12.5, and from 5 independent nonpregnant adult samples. (C) Mean fluorescence (± SEM) of thiazole orange in untreated platelets and platelets treated with 10 μg/mL RNase A.