Giant electron-dense chains, clusters and granules in megakaryocytes and platelets with normal dense bodies: an inherited thrombocytopenic disorder I. Megakaryocytes.

A woman and her male child were referred because of life-long thrombocytopenia, moderately increased platelet size, and absence of laboratory findings suggesting immune thrombocytopenia or defective platelet function. Evaluation of their platelets in the electron microscope revealed the presence of organelles never seen before in human cells. The present study has focused on megakaryocyte pathology to be sure the aberrant platelet organelles originated in the parent cell. There were two different types of organelles developing in megakaryocytes from our patients unrelated to the formed organelles in the large cells. No relationship could be identified between the aberrant structures and alpha granules, lysosomes or dense bodies. One of the large organelles was intensely electron opaque and appeared to arise from small dense fragments forming in segments of endoplasmic reticulum. The second, target-like organelle also appeared to develop in the endoplasmic reticulum. Its smallest precursors were hexagonal fragments with a granular layer inside a more electron-dense outer layer. Fusion of these elements resulted in formation of typical target-like organelles. Ultimately the target-like organelles became electron opaque and difficult to distinguish from the large dense bodies. Other features of these unusual structures were identified in platelets from the two patients and will be discussed in a subsequent report.