Some articles on megakaryocytes:

P-selectin - Gene and Regulation
... P-selectin is constitutively expressed on megakaryocytes (the precursor of platelets) and endothelial cells ... First, P-selectin is synthesized by megakaryocytes and endothelial cells, where it is sorted into the membranes of secretory granules ... When megakaryocytes and endothelial cells are activated by the action of agonists such as thrombin, P-selectin rapidly translocated to the plasma membrane from granules ...