Ultrastructure of rat megakaryocytes after prolonged thrombocytopenia

Abstract

Megakaryocyte (MK) differentiation was analyzed by electron microscopy in rats with prolonged (10 days) thrombocytopenia induced by either daily plateletphoresis or multiple injections of anti-platelet serum. Immature MK from thrombocytopenic animals were similar to those of normal rats, although the proportion of such cells was increased and they were slightly larger in average size. Maturing cells exhibited localized areas of hypertrophied and densely packed demarcation membranes and augmented rough endoplasmic reticulum (RER). The most striking new finding in maturing cells was the abundance of dense compartments (DC) in the demarcation membrane system (DMS). Staining with tannic acid indicated that, although deep within the cell, DC were in direct continuity with the extracellular space. A few were also identified in freeze-fractured MK. Intramembranous particle density in DC appeared to be less than that in the ordinary DMS. In stimulated animals, mature cells measured one-third larger than normal in diameter and often disclosed more nuclear lobes and persistent nucleoli. These changes were presumably mediated by the humoral agent, thrombopoietin.