Bone marrow findings in myeloproliferative variants of hypereosinophilic syndrome (HES)

The bone marrow biopsy sections presented are from two representative patients with hypereosinophilic syndrome with (A to C) and without (D to F) elevated serum tryptase levels stained with hematoxylin and eosin (A, D), antitryptase antibody (B, E), and antireticulin antibody (C, F). Although the bone marrow biopsy specimens from both patients show a dramatic increase in eosinophils and myeloid precursors (A, D), only the specimen from the patient with elevated serum tryptase shows marked hypercellularity (A), atypical spindle-shaped mast cells (B), and reticulin fibrosis (C).