Ultrastructure of megakaryocytes in the human bone marrow of patients with primary (essential) thrombocythemia.

An ultrastructural study was performed on bone marrow specimens in 10 patients (5 males/5 females, median age 53 years) with primary (essential) thrombocythemia (PTH) and an excessive elevation of the platelet count (1,625 +/- 783 x 10(9)/l). In contrast to a not severely altered neutrophilic granulo- and erythrocytopoiesis, megakaryocytes showed conspicuous large to giant forms. These were characterized by a highly lobulated nucleus containing several nucleoli and an extensive intermediate zone of the cytoplasm with many Golgi fields, numerous profiles of the so-called demarcation membrane system and an abundance of alpha-granules and some dense bodies. Our results demonstrate that ultrastructure of the megakaryocytes in PTH does not reveal gross abnormalities, but features which are compatible with an enforced thrombocytogenetic activity in accordance with the excessively elevated platelet count. Similar changes have been described in animal experiments with induced thrombocytopenia and stimulation of platelet shedding. Evaluation of thrombocytogenesis suggests that it may be mediated by a process of fragmentation with partitioning of the extensive intermediate zone into numerous prospective platelet territories followed by segregation.