Erythropoiesis in the neonatal mouse

An electronic cell sizing and counting system was employed for the first time to study developmental changes in the peripheral blood picture of neonatal mice. Among the parameters studied were erythrocyte volume and number, hemoglobin level, mean corpuscular hemoglobin, and mean corpuscular hemoglobin concentration. The system used determined several of these blood values simultaneously and permitted much more rapid and accurate measurements than have previously been obtained using standard hemocytometric techniques. The erythrocytes of the neonatal period were macrocytic and hypo­chromic. The mean volume and mean corpuscular hemoglobin decreased after birth, while the erythrocyte count, hemoglobin content and mean corpuscular hemoglobin concentration rose. Adult values for each of these parameters were reached by the 41st day after birth. The eryth­rocyte population gradually became more homogeneous as the largest cells were eliminated with time, when supplemental iron was administered to female mice during or just before pregnancy,the erythrocyte distributions of the offspring were the same as in normal newborn mice, but the erythrocyte number was higher. On the basis of evidence reported in this study together with that obtained by other investigators, a model for the production of hypochromic macrocytes in neonatal mice is described. It is suggested that the immediate cause of the macrocytosis is the skipping of cell divisions during erythrocyte production.