Histology of mice reconstituted with bone marrow–expressing TpoR variants. Bone marrow cells were infected with empty vector pMEGIX, or pMEGIX viruses coding for wild-type TpoR (TpoRwt), Δ5TpoR, or TpoRW515A mutants. (A) Splenomegaly was detected in mice expressing the active TpoR mutants compared with control empty vector or TpoRwt mice. All mice present more than 95% GFP-positive cells at 3 weeks after reconstitution. (B) Histology of spleen sections of mice reconstituted with the indicated constructs. Spleen sections are stained with hematoxylin-eosin (HE; 3× or 40× magnification), silver stain (SL; 20× magnification), or trichrome (TB; 20× magnification). Spleen sections were performed 45 days after transplantation. Microscopic section of the spleen shows expansion of the red pulp (HE, 3×). The splenic enlargement is due mainly to extramedullary hematopoiesis (HE, 40×) as well as fibrosis (SL, 20× and TC, 20×) that occur in the splenic red pulp sinusoids. Megakaryocytes are prominent, but erythropoiesis and granulopoiesis are present as well in the splenic sinuses. Both Δ5TpoR and TpoRW515A induce some degree of early fibrosis with reticulin (see silver staining: SL, 20×) and collagen deposition stained in blue by the Masson trichrome (TC, 20×). Images were obtained using a Mirax Digital Slide System.