Bone marrow features of this condition are variable. Cellularity ranges from being increased to markedly reduced or near absent normal hematopoiesis. Fibrosis ranges from a focal increase of reticulin to coarse parallel reticulin, the presence of collagen, and osteogenesis or new bone formation (Figure 46.2). Dense fibrosis is also associated with dilated marrow sinusoids and intrasinusoidal hematopoiesis. Variable systems of reticulin grading are in routine use and a unified system is required to inform future studies. Megakaryocyte morphology is a key to identify the newly proposed diagnostic entity prefibrotic MF. In prefibrotic MF there is at most a borderline increase in reticulin, reduced erythropoisis, and increased granulocytic and megakaryocyte proliferation, and reactive lymphoid follicles may also be apparent. Significant megakaryocyte abnormalities (as discussed in ET)40 are a key to differentiating this entity from ET. Yet to be delineated is whether or not this is a distinct disease entity and whether hematopathologists can reliably establish this diagnosis. The spleen is a common site of extramedullary hematopoiesis in MF and variable patterns of infil tration by myeloid precursors are identified, from diffuse to nodular or a predominance of myeloid precursors.41

Blood film appearances are important in making the diagnosis of MF. The most characteristic include leucoerythroblastic features with teardrop poikilo-cytes; dysplastic features may be present in granulocytes and platelets. In the early stages of MF, these features may be either absent or not prominent.42 Increased CD34+ cells and an elevated LDH may also be demonstrated.

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