JAK2^V617F transgenic mice display megakaryocytic hyperplasia and develop fibrosis. Hematoxylin and eosin (A) and reticulin (B) staining of paraffin-embedded sections of spleen and femur (for bone marrow) from control and transgenic mice. Photos in panel A are representative results obtained with all 3 lines of transgenic mice, whereas those in panel B represent selected cases of line A mice that developed fibrosis. Arrows point to typical megakaryocytes, which are not present in the spleen of control mice. Pictures were taken with a 40× objective.