Ultrastructural defects of Scl^–/Δ megakaryocytes. Transmission electron microscopic (TEM) images of representative megakaryocytes present in the bone marrow of Scl^+/Δ (A) and Scl^–/Δ (B) mice treated for 4 days with 2 μg PEG-rhTPO and analyzed on day 5. Panels C and D show magnified regions of panels A and B, respectively, as indicated by the boxed regions. Note the presence of alpha granules (g) in the cytoplasm of the Scl^+/Δ megakaryocyte in panel C and depleted granules (g) in the cytoplasm of the Scl^–/Δ megakaryocyte in panel D. (E) Example of a megakaryocyte from a TPO-treated Scl^–/Δ mouse showing emperipolesis (3 cells contained within the cytoplasm). Nuclei (n) and demarcation membrane systems (dms) are indicated. Scale bars represent 10 μM (A-B, E) and 2 μM (C-D).