Histopathology and analysis of erythroid precursors in Cdk6-deficient JAK2^V617Ftransgenic mice. Mice were euthanized at 8 weeks of age. (A) Summary of histopathologic features of 8-week-old mice with indicated genotypes. (i) Light microscopically discernible presence of erythroid and myeloid cells in the splenic red pulp indicative of an active extramedullary hematopoietic process. (ii) Nonquantitative assessment of the histologic presence of erythroid and myeloid cells. (iii) Variably increased representation of myeloid cells. (iv) Variable degree of resolution of the neoplastic process and features of dyspoiesis (modest in some samples and much more in some others). (v) Variable restoration of splenic follicular architecture (marginal to modest in some samples and much more in some others). Total numbers of CD41⁺ megakaryocytes (B) or total numbers of Ter119⁺ erythrocytes (C) per 1 × 10⁶ cells are shown for BM and spleen. Error bars indicate + SD *P < .05; **P < .01; ***P < .001; ****P < .0001. (D-E) Total numbers of erythroid precursors per 1 × 10⁶ cells are shown for BM (D) and spleen (E). Ter119^medCD71^high (proerythroblasts), Ter119^highCD71^high (basophilic erythroblasts), Ter119^highCD71^med (late basophilic and polychromatophilic erythroblasts), and Ter119^highCD71^low (orthochromatophilic erythroblasts) represent the regions R1-R4, respectively. Statistical analysis was performed with one-way ANOVA with subsequent Bonferroni posttest, and significance is indicated (n ≥ 10 per genotype). *P < .05; **P < .01; ***P < .001; ****P < .0001.