Examples of morphologic features scored on 370 diagnostic bone marrow trephine specimens from patients enrolled in prospective clinical trials of ET. (A) H&E-stained section (×400 magnification) showing a large, staghorn megakaryocyte in the center of the field. (B) H&E-stained section (×1000) showing a cloudlike megakaryocyte in a background of with increased cellularity. (C) A loose cluster of megakaryocytes showing other marrow cells between individual megakaryocytes (H&E stain ×400). (D) A tight cluster of megakaryocytes (H&E stain ×1000) showing molding of the juxtaposed cell surfaces between adjacent megakaryocytes, and no intervening marrow cells. (E-H) Reticulin stains (all ×400) showing increasing reticulin, graded 1 to 4, respectively.