The spleen is a transient erythropoietic organ in the neonate C57BL/6 mouse. (A) Terminal erythroid differentiation in the spleen for the first 8 weeks after birth was measured by flow cytometry using CD44 vs FSC as markers. (B) The total number of erythroblasts in the spleen for the first 12 weeks was determined by flow cytometry using Ter119⁺ CD44^hi populations of erythroid cells (red gate is shown in panel A). (C) Absolute number of splenocytes for the first 12 weeks. (D) Evolution of the mouse body weight (blue solid lines) vs spleen weight (red solid line) from week 1 to 12. (E) Erythropoietic activity in the spleen was determined by immunohistochemistry using Ter119 staining 3 weeks (upper panels) and 7 weeks (bottom panels) after birth (left panels, original magnification ×20; right panels, original magnification ×40). n = 4-13 animals per day; data are presented as mean ± SEM. *P < .05, **P < .01, ***P < .001, indicated week vs week 1 (ANOVA with Tukey’s post hoc test with corrections for multiple comparisons).