#00066561

A 63-year-old female with a history of essential thrombocythemia (ET) with progression to post-ET myelofibrosis presented for follow-up. A CBC with manual differential demonstrated normocytic anemia (hemoglobin 10.6 g/dL) with marked anisopoikilocytosis including dacryocytes (panel A, arrowhead; total magnification 1000x), polychromasia (panel A, arrow), and nucleated RBCs (panel B, arrowhead; total magnification 400x), left-shifted leukocytosis (WBC 16.5 K/µL) with 8% circulating blasts, and slightly increased platelets (413 K/µL). At the peripheral blood smear’s feathered edge, an intact circulating megakaryocyte with a multilobulated nucleus, coarse, condensed nuclear chromatin, and abundant granular cytoplasm was identified (panels C-D; total magnifications 400x [C] and 1000x [D]).

Megakaryocytes are the largest resident cells of the bone marrow and are rarely observed migrating within the periphery intact; this finding is compatible with significant marrow disruption. Myelofibrosis—marked by progressive reticulin and collagen marrow fibrosis—disrupts the marrow architecture, displacing hematopoietic elements to the spleen and liver, including mature megakaryocytes into circulation. The circulating immature myeloid cells, nucleated red blood cells, and deformed, teardrop-shaped dacryocytes, comprise the major findings of leukoerythroblastosis. Leukoerythroblastosis is one of the additional diagnostic criteria for post-ET myelofibrosis (WHO5 classification); the 8% circulating blasts warrant monitoring for progression towards blast phase transformation.