Thin-section electron micrographs showing blebbing megakaryocytes (MKs) in the bone marrow. (A) Bone marrow MKs demonstrate a central organelle-rich area (thick arrows) surrounded by an organelle-deplete cyoplasmic rim (thin arrows). (B) MK blebs lack the characteristic contents and features of an assembling platelet. (C-D) Blebbing MKs are frequently observed in the marrow adjacent to blood vessels. Scale bars, 2 μm. (E) Comparison of α-granule content in MK blebs vs mature platelets. TEM images of MK blebs (n = 32) and mature platelets (n = 26; supplemental Figure 2) were analyzed for α-granule number and total area; α-granule density (α-granule per μm²) is shown as mean ± standard deviation. EC, endothelial cell; RBC, red blood cell.