Representative electron micrographs of platelets produced by megakaryocytes deficient in GATA-1. Normal mouse platelets (A) have a uniform appearance and show the typical discoid shape with a normal complement of organelles, including platelet-specific granules. In contrast, GATA-1–deficient platelets (B and C) are large, invariably round, heterogeneous, and show a paucity of organelles, especially platelet-specific granules. In particular, these platelets have an excess of rough endoplasmic reticulum (C), similar to the megakaryocytes from which they are derived. Bars in (A) and (B) are 5 μm; in (C), 1 μm.