Primary, uncultured megakaryocytes can rapidly produce proplatelets on fibrinogen. (A) Bone marrow aspirates were prepared as described in “Materials and methods” and plated onto fibrinogen-coated 25-mm glass coverslips. The cells were allowed to adhere for 1 hour at 37°C, then the coverslips were rinsed and transferred to a microincubation chamber. The chamber was placed into a humidified microscope chamber at 37°C and 5% CO₂, and images were taken at the indicated time points (minutes). The enlarged area is from the box at time point 310:00 minutes. Branching points are indicated with an asterisk, and visible platelet swellings are labeled “ps.” Other cells, such as red blood cells and macrophages, are visible within the fields of view. The complete movie can be viewed on the Blood website (Video S1). (B) Proplatelets from primary megakaryocytes are α_IIb-positive. Cells were allowed to adhere to fibrinogen-coated coverslips for 4 hours at 37°C, then fixed and stained for α_IIb via alkaline phosphatase. Dark staining was observed only when the α_IIb-specific antibody was used (right panel), and not the IgG₁ control antibody (left panel).