Megakaryocyte adherence and proplatelet formation. (A-D) Representative images are shown for sorted CD41⁺c-kit⁺ megakaryocyte progenitors from Pf4/Srf WT and Pf4/Srf KO littermates that were cultured in TPO containing medium either on fibronectin (left) or in semisolid medium (methylcellulose, right). (E) Continuous videos were taken and analyzed for cell adherence when cultured on fibronectin (left) and for proplatelet formation by cells on fibronectin (middle) and in semisolid medium (right). The percentage of total cells analyzed over 88 hours is shown. **P < .05.