Introduction

The chronic myeloproliferative disorders (CMPD) are a clinically heterogeneous group of clonal proliferations of stem cell origin characterized, at least initially, by marrow hypercellularity with varying degrees of marrow fibrosis and an increase in the production of one or more terminally differentiated cell types (George & Arber, 2003). These differentiated elements may accumulate in the marrow, in peripheral blood, and in other organs (e.g., spleen). All types of CMPD have a variable tendency to undergo disease progression that may terminate in bone marrow failure due to myelofibrosis or in transformation to an acute leukemic phase, occasionally preceded by a brief myelodysplastic phase. In CMPD, a definite diagnosis usually cannot be made by morphologic examination alone (Anastasi & Vardiman, 2000). The evaluation of bone marrow histology, however, holds an important role in confirming the diagnosis and excluding unsuspected pathology. Expert opinion should be sought if bone marrow histology is to be used as a major diagnostic criterion, since the changes which are specifically associated with the various subtypes of CMPD (see later) are often subtle and difficult to recognize in morphologically less-than-optimal processed samples. This greatly reduces the value of bone marrow histopathology in inexperienced hands (Pearson, 2001).

Classification of these disorders benefits from the integration of morphologic features with clinical, hematologic, and cytogenetic findings (Harris et al., 1999). Of major importance is the presence or absence of the Philadelphia chromosome (BCR/ABL or translocation 9;22), the defining feature of chronic myelogenous leukemia (CML).