Cytoskeletal model of PLT production. Top left: immunofluorescence microscopy images of a proPLT producing MK, released barbell-proPLTs, and circular-prePLTs from a mouse fetal liver cell culture probed for β1-tubulin; rapid-freeze electron microscopy image of the PLT cytoskeleton. Bottom left: list of inherited thrombocytopenias affecting platelet size, grouped by underlying defect; list of common causes of acquired thrombocytopenia. Top right: model of the terminal stages of PLT production. Released proPLTs undergo successive rounds of fission along their midbody and at their ends to generate circular prePLTs and barbell-proPLTs. Circular prePLTs reversibly convert into barbell-proPLTs, from which PLTs are released after a final fission event at their midsection. PLTs may enlarge during culture and contribute to further PLT production. Bottom right: model of suspected errors in terminal PLT production that can account for phenotypes expressed in common inherited and acquired thrombocytopenias. For immunofluorescence microscopy, samples were fixed with 4% formaldehyde for 15 minutes and then permeabilized with 0.5% Triton X-100 and blocked in immunofluorescene blocking buffer (1% BSA, 0.05% sodium azide, and 10% FCS in PBS) overnight before antibody labeling. To demarcate permeabilized cells, samples were incubated with a rabbit polyclonal primary antibody for mouse tubulin generated against the C-terminal peptide sequence LEDSEEDAEEAEVEAEDKDH (Genemed Synthesis). Samples were treated with a secondary goat anti–rabbit antibody conjugated to an Alexa Fluor 488 (Invitrogen). Samples were examined with an Axiovert 200 microscope (Carl Zeiss) equipped with a 63× (numeric aperture, 1.4) PlanApoChromat oil immersion objective, and images were obtained using a charged coupled device camera (Hamamatsu). Images were analyzed using Metamorph image analysis Version 7.7.2.0 software (Molecular Devices) and ImageJ Version 1.45r software. Professional illustration by Alice Y. Chen.