Gestational age-dependent changes in circulating hematopoietic stem cells in newborn infants.

In the fetus, hematopoietic stem cells originate in the yolk sac and are believed to be transferred to all other hematopoietic organs via the circulation. In humans, the time course of this transfer has not been systematically evaluated in viable premature infants. We examined the cord blood of 13 preterm (25 to 36 weeks of gestation) and 10 term (38 to 42 weeks of gestation) infants for pluripotent (mixed colony-forming unit-granulocyte, erythrocyte, macrophage, megakaryocyte), erythroid (burst-forming unit-erythroid, colony-forming unit-erythroid) and myeloid (colony-forming unit-granulocyte, macrophage) stem cells. A gestational age-dependent decrease in all lineages of circulating hematopoietic stem cells was noted (p less than 0.001). By 34 weeks of gestation, preterm infant cord blood had a similar concentration of circulating stem cells compared with that of term infants. This gestational age-dependent decrease in hematopoietic stem cells of all lineages supports the hypothesis of a blood-borne transfer of hematopoiesis that appears largely complete by 34 weeks of gestation. Infants born after less than 32 weeks of gestation have high levels of circulating hematopoietic stem cells that may reflect the active transfer of hematopoiesis from liver to bone marrow.