Morphometric analysis of postnatal erythropoiesis in the spleen and bone marrow of the mouse.

Postnatal erythropoiesis in the spleen and bone marrow was morphometrically examined by electron microscopy in female mice. In the splenic red pulp, erythroblasts increase in number in early life, particularly until 20 days of age. On the basis of a cytometric analysis, erythroblasts can be classified into four types: small, medium, large and extra-large. In the neonatal spleen, small erythroblasts constitute 16% of all the erythroblasts; medium 55%; large 23%; and extra-large 6%, respectively. In mice older than 20 days, medium erythroblasts are reduced in proportion, and small erythroblasts account for more than 50% of all the erythroblasts. In the bone marrow, small erythroblasts represent the largest population until 40 days of age. At 60 days, however, medium erythroblasts are increased, being largest in proportion. The erythroblast types in the marrow at 60 days are similar in proportion to those in the neonatal spleen. This finding seems to suggest an active state in erythrocytopoietic activity. The population pattern of erythroblasts is thought to serve to evaluate erythrocytopoietic activity.