Extensions from megakaryocytes that extend through blood vessel walls in red marrow are sliced off from the cells by the force of blood flow. these extensions are

Proplatelets. Proplatelets are portions of megakaryocyte broken off to form a platelet by the force of blood flow. Megakaryocytes are large bone marrow cells responsible for producing blood platelets. Megakaryocytes form platelets either by breaking up explosively or by forming platelet ribbons. They produce blood platelets via intermediate cytoplasmic extensions known as proplatelets.