Red cell size heterogeneity during ontogeny

To determine the manner in which erythrocyte changes occur during ontogeny, several red cell parameters were analyzed in fetuses, newborn infants, children, and adults. Although mean cell volume (MCV) and hemoglobin F (HbF) levels decreased as expected during in utero development, the coefficient of variation of red cell size (%CV), or red cell distribution width (RDW), increased from fetuses to newborn infants. In normal adults, the %CV was 15 (RDW was 13). The %CV in fetuses at 18-24 weeks gestation was 18, and it was 21 at term birth. High values for %CV or RDW indicate significant anisocytosis. Erythropoiesis at the time of birth is not a steady-state condition. Erythrocytes of a wide variety of sizes are present, with the appearance of new small cells on a background of older, larger red cells. This increased anisocytosis suggests that these new erythrocytes do not appear to be due to a smooth, continuous evolution of red cell size, but rather to discrete, perhaps clonal, changes.