Ultrastructure of normal and ITP megakaryocytes. (A) Normal megakaryoblast (stage I megakaryocyte) showing a lobulated nucleus (N). In the cytoplasm, characteristic demarcation membrane system (asterisks) and normal mitochondria (arrowheads) can be found. Original magnification, × 3000. (B) Megakaryoblast of an ITP patient in the process of para-apoptosis; N indicates nucleus. The cell has an intact enlarged peripheral margin (large arrow). Original magnification, × 4500. (C) This inset of panel B shows a higher magnification with mitochondria, some of which are slightly swollen (open arrowheads) and others which have completely collapsed cristae, which appear as empty vacuoles (asterisks). The enlarged peripheral margin in the cytoplasm is free of organelles (open arrow). Original magnification, × 15 000.