Electron-microscopic views of bone marrow from patients with idiopathic myelofibrosis.

(A) Several cells have entered the Mk. The host Mk shows ultrastructural alterations (vacuoles [V] and poorly delimited organelles [arrows]). Myeloperoxidase-positive PMN (PN) granules (arrowheads) are free in the Mk intracellular space (original magnification ×2760). (B) A damaged PMN cell is engulfed in the paranuclear area (N) of a Mk. The Mk cytoplasm has numerous vacuoles (V) and poorly delimited organelles, and myeloperoxidase-positive PMN granules are scattered in it. The engulfed PMN cell (surrounded by a dotted line; nucleus indicated by n) has plasma membrane alterations and apoptotic-like chromatin texture, suggesting its destruction when sequestrated in the host. Contact fibrosis (f) is present (original magnification ×3960). (C) Results of immunogold labeling for P-selectin done on frozen sections of human bone marrow. Labeling (arrowheads) is present in the Mk subcellular organelles in a distribution similar to that in samples from TPO mice: in the α-granule membrane (A), numerous small vacuolar structures (V), and occasionally along the demarcation membrane system (dm) (original magnification ×39 100).