Histopathologic analysis. (A) Blood smears show decreased RBCs, leukocytes, and PLTs in induced MxCre;Jak2V617F/+;Stat5fl/fl mice compared with MxCre; Jak2V617F/+ mice. Arrows point to reticulocyte and nucleated immature RBC. (B) BM sections from induced MxCre;Jak2V617F/+ mice display hypercellularity with trilineage hyperplasia. In contrast, BM sections from induced MxCre;Jak2V617F/+;Stat5fl/fl mice are indistinguishable from the control BM. (C) Spleen sections from induced MxCre;Jak2V617F/+ mice exhibited a marked expansion of red pulp with clusters of megakaryocytes and immature erythroid progenitors. Noticeably, spleens from induced MxCre;Jak2V617F/+;Stat5fl/fl mice were normal and identical to control spleens. Reticulin staining on the BM (D) and spleen (E) sections showed fibrosis in the BM and red pulp of the spleens of older Jak2V617F knockin mice (24 weeks after induction), whereas Stat5-deficiency prevented the development of fibrosis in the BM and spleens of MxCre;Jak2V617F/+;Stat5fl/fl mice. Scale bars represent 20 μm in all images.