Acute myelofibrosis. Immunohistochemical study of four cases and comparison with acute megakaryocytic leukemia

Four cases of acute myelofibrosis are contrasted to three cases of acute megakaryocytic leukemia. The cases were clinically indistinguishable. Light microscopic examination of hematoxylin and eosin-stained bone marrow biopsy specimens from the cases of acute myelofibrosis showed a trilinear proliferation of mature and immature hematopoetic cells, increased numbers of mature megakaryocytes, and marked fibrosis. In contrast, the blast cells in the cases of acute megakaryocytic leukemia showed a continuous spectrum of differentiation from small blasts to megakaryoblasts. Immunoperoxidase staining showed the blast cells in all three cases of acute megakaryocytic leukemia to be strongly positive for Factor VIII, a marker of megakaryocytic differentiation. These same blasts did not stain with markers of myelomonocytic (lysozyme), lymphocytic (Hle), and myeloid (leu-MI) differentiation. In contrasts, the blast cells in all four cases of acute myelofibrosis were Factor VIII, Hle, and Leu-MI negative but did occasionally stain with anti-lysozyme.