TPO mRNA expression assessed by in situ hybridization on paraffin-embedded human BM trephine sections. BM of a subject with normal platelet count (A) showing weak signal, perhaps in stromal cells but negative staining in hematopoietic precursor cells. BM trephine of a patient with severe aplastic anemia (B and C) and a patient with ITP (D) showing strong hybridization signal in stromal cells (Str) and weak signal in endothelial cells (En). The macrophages (Mac) with golden brown hemosiderin granules show negative staining (D). BM of a subject with secondary thrombocytosis (E) showing no visible difference in staining intensity compared to that of the subject with a normal platelet count. Trephine section of aplastic anemia patient (F ) probed with plasmid vector pBR322 as a negative control. Original magnifications: (B, E, and F ) ×400; (A, C, and D) ×1,000.