The ultrastructure of megakaryocytes and blood platelets in the rat spleen Supported in part by research grants from the National Institutes of Health A-131 (C9) and from the Upjohn Company.

Abstract

The cytoplasm of the megakaryocyte in the rat spleen possesses three zones, the perinuclear, intermediate and marginal. The perinuclear zone is characterized by the presence of Golgi apparatus, ribosomes, endoplasmic reticulum, and mitochondria. These organelles are found also in the more voluminous intermediate zone which in addition exhibits platelet granules and an extensive development of vesicles from smooth-surfaced endoplasmic reticulum to form demarcation membranes by coalescence. The marginal zone is almost devoid of the organelles and inclusions present elsewhere. Shedding of platelets appears to occur by extension of a paired demarcation membrane from the intermediate zone to the cell membrane and subsequent separation of its lamellae so that all of the essential organelles and inclusions of the intermediate zone may be included within the platelet. In addition, platelets contain vesicles which are probably pinocytotic in nature. Platelets are sometimes engulfed by the cytoplasm of phagocytic cells.