Altered thrombopoiesis in FlnA-null MKs. (A) Control and FlnA-null MK cultures with proplatelets on day 4 were visualized by immunofluorescence for β-tubulin. Images are representative of 4 different experiments. (B) The number of MKs extending proplatelets was counted 18 hours after BSA gradient (day 4 of culture) and expressed as the percentage of total MKs. Results are mean ± SD (n = 7). *P < .05. FlnA-null MKs have fewer proplatelets than control MKs. (C) Platelets in MK cultures were counted at 18 and 72 hours by flow cytometry. Data are expressed as ratio relative to control platelet counts and are mean ± SD (n = 4). (D) Control and FlnA-null MKs were subjected to time-lapse video microscopy and monitored for proplatelet formation. Frames presented were captured at 0, 6, 12, and 18 hours. Released platelets were observed in FlnA-null MK cultures (arrowheads). *Control cultures mostly contained proplatelet bearing MKs.