Megakaryocyte and Platelet Formation: A Scanning Electron Microscope Study in Mouse Spleen

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Abstract
Mouse spleen was examined by scanning electron microscopy after perfusion fixation and cryofracture. Megakaryocytes are recognized usually in the splenic cord near the sinus and postsinal venules but rarely in their lumen. They are usually fixed to the reticulum cells and sometimes both cells are connected to each other by cytoplasmic interdigitations. Immature megakaryocytes are round but mature ones are irregular in shape with many cytoplasmic protrusions. The cell surface reveals small pores and many microprocesses of 2 types: Type I microprocesses are blebs known to be involved in platelet liberation, while the significance of type II or villous microprocesses remains unknown. Surface pores represent the orifices of the platelet demarcation membrane system (DMS) in the cytoplasm. On the fracture surfaces of the cell, the DMS is recognized as a network of furrows and its development is confirmed to be initiated by invagination of the plasma membrane. Individual platelets formed by developing DMS become spined spheroid in shape. Four different modes of platelet release are also described.