Dynamic changes in murine erythropoiesis from birth to adulthood: implications for the study of murine models of anemia.

Liver, spleen, and bone marrow are 3 key erythropoietic tissues in mammals. In the mouse, the liver is the predominant site of erythropoiesis during fetal development, the spleen responds to stress erythropoiesis, and the bone marrow is involved in maintaining homeostatic erythropoiesis in adults. However, the dynamic changes and respective contributions of the erythropoietic activity of these tissues from birth to adulthood are incompletely defined. Using C57BL/6 mice, we systematically examined the age-dependent changes in liver, spleen, and bone marrow erythropoiesis following birth. In addition to bone marrow, the liver and spleen of newborn mice sustain an active erythropoietic activity that is gradually lost during first few weeks of life. While the erythropoietic activity of the liver is lost 1 week after birth, that of the spleen is maintained for 7 weeks until the erythropoietic activity of the bone marrow is sufficient to sustain steady-state adult erythropoiesis. Measurement of the red cell parameters demonstrates that these postnatal dynamic changes are reflected by varying indices of circulating red cells. While the red cell numbers, hemoglobin concentration, and hematocrit progressively increase after birth and reach steady-state levels by week 7, reticulocyte counts decrease during this time period. Mean cell volume and mean cell hemoglobin progressively decrease and reach steady state by week 3. Our findings provide comprehensive insights into developmental changes of murine erythropoiesis postnatally and have significant implications for the appropriate interpretation of findings from the variety of murine models used in the study of normal and disordered erythropoiesis.