Age-related hematological changes in normal F344 rats: during the neonatal period

In order to clarify age-related changes in hematological values of normal rats after birth, blood samples from neonatal F344 rats of both sexes were examined periodically during the period from 0 to 40 days postpartum. The erythrocyte count (RBC) increased with time after birth as a function of age. In contrast, the reticulocyte count (Retics) continuously decreased with time after birth. Hemoglobin (Hb), hematocrit (Ht), and the mean corpuscular hemoglobin concentration (MCHC) tended to decrease after birth until weaning (about 21 days postpartum), but they began to increase after weaning. Mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH) also gradually decreased after birth until weaning, but they were unchanged thereafter. The platelet count (PLT) gradually increased after birth and reached a plateau at weaning. Microscopic examination of blood smears revealed that erythrocytes at birth had characteristic morphological features such as anisocytosis, polychromasia, basophilic stippling, Howell-Jolly body, and erythroblastosis. These characteristic features, however, disappeared by 30 days after birth. The total leukocyte count (WBC) gradually increased with time after birth, due to an increase in the number of lymphocytes. The lymphocyte count started to rapidly increase within several days after birth and the increase continued thereafter. Other differential leukocyte counts also showed various characteristic patterns of changes during the neonatal period. There were no apparent differences between males and females in these changes in hematological values.