Intracellular mechanisms of proplatelet formation and elongation. (A) Transmission electron micrograph of a BM MK in contact with an endothelial cell. MK is outlined in black. Arrowspoint towards other BM cells interacting with the MK. Scale bar, 3 µm. (B) Platelet production from MKs is enabled through extensive cytoskeletal rearrangements leading to breaching of the endothelium through the formation of podosomes and proplatelet generation, during which proplatelets appear like “beads on a string” and function as essential intermediate structures. Initial polarization of the DMS is dependent on actin dynamics enabled by the Rho GTPase Cdc42, which induces LIMK1 activity and activates downstream effectors such as Cof1 and Twf1 and is further indispensable to enable proplatelet formation.^81,82,99 In addition to Cdc42 and PDK1,¹⁰² activity of another Rho GTPase, RhoA, which induces MLC2/NMIIA activation,^100,101 is critical in enabling F-actin rearrangements including the Arp2/3-dependent formation of podosomes, which are crucial to breach the endothelium, most likely though the secretion of MMPs.^78,79 Proplatelet elongation on the other hand relies on microtubule sliding within the proplatelet shaft,^117,127 which is dependent on the motor protein dynein.¹²⁸ Cytoskeletal cross talk is enabled through proteins linking the F-actin to the microtubule cytoskeleton, such as EBs, APC, and mDia1.^95,97