Histopathologic characterization of the transplanted animals. (A-B) Hematoxylin and eosin stains of the spleens (original magnification ×40 and 500) obtained from vector (control) and JAK2WT expressing mice show normal architecture and minimal extramedullary hematopoiesis. Spleens of JAK2V617F-expressing mice show marked distortion of the splenic architecture with an attenuation of the white pulp and expansion of the red pulp containing abundant extramedullary hematopoiesis, including megakaryocytes and erythroid and granulocyte precursors. JAK2V617F/Y201F-expressing mice show minimal alteration of the architecture with only mild extramedullary hematopoiesis. (C) BM sections (hematoxylin and eosin stain; original magnification ×500) from JAK2V617F-expressing mice show hypercellularity with increased and dysplastic megakaryocytes and increase in granulopoiesis. JAK2V617F/Y201F mouse BM was essentially normal and resembles that of control animals. (D) Liver sections (hematoxylin and eosin stain; original magnification ×500) from JAK2V617F-expressing mice show extramedullary hematopoiesis, whereas livers of JAK2V617F/Y201F mice appear identical to those of control animals. Arrows indicate megakaryocytes and granulocytes in the liver of JAK2V617F mice.