Platelets originate from bone marrow megakaryocytes

Top panel: A large bone marrow megakaryocyte (thick arrow) adjacent to a bone marrow sinusoid has staining characteristics identical to those of the platelets (thin arrow) in the lumen of the sinusoid.
Middle panel: A megakaryocyte is extending a portion of its cytoplasm into the lumen of the sinusoid with the suggestion of platelets (arrow) budding off.
Bottom panel: An entire megakaryocyte (thick arrow) has entered the sinusoid with a portion of its cytoplasm streaming ahead and forming a long proplatelet extension from the end of which platelets (thin arrow) appear to be shedding.