Views of altered Mks and results with labeling for P-selectin.

(A) Myeloperoxidase-positive PMN granules (arrowheads) are free in the cytoplasm of an altered Mk (original magnification ×3135). (B) Myelofibrosis (f) develops at contact with an altered Mk. The Mk has a dysmorphic and heterogenous demarcation membrane system (dm). Some well-preserved α granules are scattered in the cytoplasm (arrows), as are swollen and disrupted α granules (arrowheads) (original magnification ×11 700). (C) Results of immunogold labeling for P-selectin done on frozen thin section from a TPO mouse Mk. P-selectin (arrowheads) is detected in the vicinity of a PMN cell (PN) entering the Mk demarcation membrane system (dm) by means of emperipolesis. Labeling appears in the α-granular (A) membrane, in cytoplasmic vacuolar structures (V), and along some demarcation membranes (dm) (original magnification ×38 750). The inset shows a lower-magnification view of the PMN cell (PN) in the Mk (original magnification ×12 400).