MARCKS heterozygous mice and KO MKs have impaired platelet formation. (A) Platelet counts were measured from adult, male MARCKS heterozygous (Het) and wild-type (WT) mice, as described (n = 15 mice per group; *P < .05, compared with WT). (B) Primary murine MKs were cultured as previously described after being isolated from MARCKS KO mice. Proplatelet formation was quantified 24 hours after MK gradient isolation (n = 5-6 fetal livers per condition; *P < .05, compared with WT). (C) Representative images of 24-hour time-point (original magnification ×20); scale bar represents 50 μm.