There are four major types of myeloproliferative disorders: chronic myeloid leukemia, chronic myelofibrosis, polycythemia vera, and essential thrombocythemia. Each has its own particular morphologic and clinical characteristics, and we’ll discuss them individually (we’ve already talked about chronic myeloid leukemia a little bit).

However, there are some similarities between all of them that bear mentioning.

1. All of the myeloproliferative disorders are characterized by a malignant, clonal proliferation of myeloid cells (hence the name myeloproliferative). “Myeloid” in this context means all the cell lines of the bone marrow: the neutrophilic, erythroid, and megakaryocytic series (and to a lesser extent, the monocytic, eosinophilic, and basophilic series) – basically, everything except the lymphoid series.

2. They all have a high white blood cell count with a left shift.

3. They all may present with splenomegaly (especially chronic myelofibrosis, in which extramedullary hematopoiesis can produce a truly massive spleen).

4. All of the myeloproliferative disorders are characterized by a hypercellular bone marrow (at least early on). This means that there are a lot more cells than usual – and, correspondingly, that the amount of fat (which normally increases with age) is decreased. Also, there are often many more megakaryocytes than usual, as seen in the above image (the megakaryocytes are the large cells with abundant, eosinophilic cytoplasm). In fact, if all you had was a bone marrow biopsy, it would be hard to tell the different myeloproliferative disorders apart in their early stages!

The differences between the myeloproliferative disorders lie primarily in the type of myeloid cell that is proliferating the most. In CML, the neutrophil series is the predominating cell line. Polycythemia vera is characterized by marked erythroid hyperplasia; essential thrombocythemia is characterized by marked megakaryocytic hyperplasia (resulting in tons of platelets in the blood); and chronic myelofibrosis is characterized by a proliferation of all myeloid cell lines early on (followed by progressive fibrosis of the marrow).