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

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 two 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. Platelet release occurs in four different modes : 1) After the DMS has divided individual platelets in the intermediate zone of megakaryocyte, it invades the ectoplasm encircling small surface areas. Some ectoplasmic pieces thus formed are liberated from the cell like a cork being withdrawn from a bottle. Through these ectoplasmic defects the interior platelets are released either in masses or in beaded ribbons. Individual platelets liberated in the splenic cord migrate into the sinus lumen through the intracytoplasmic fenestration of the endothelium. 2) Megakaryocytes project larger pseudopods into the sinus lumen, which become fragmented into platelets. 3) Megakaryocytes themselves enter the circulation and release platelets in remote organs such as the lung. 4) The megakaryocytes project small ectoplasmic blebs (type I microprocesses) into the sinus lumen and then detach them. By this process, as well as by the fragmentation of the ectoplasm above mentioned, megakaryocytes may produce platelets lacking in organelles. © 1977, International Society of Histology and Cytology. All rights reserved.