Fate of Fetal Nucleated Erythrocytes Circulating in Maternal Blood: Apoptosis Is Induced by Maternal Oxygen Concentration

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Abstract
The fetal circulation contains abundant nucleated erythrocytes (NRBCs). The Po2 of fetal blood is an estimated one-third to one-fourth of the Po2 of adult blood, and fetal hypoxemia stimulates fetal erythropoietin production and consequently increases the number of hematopoietic progenitor cells or NRBCs in fetal circulation ( 1). Severe hypoxia stimulates the proliferation of hematopoietic progenitor cells ( 2). When neonates are born with abundant NRBCs in their peripheral blood, the number of NRBCs rapidly decreases, and NRBCs become undetectable within 1 week after birth. Together, these facts suggest that differences in oxygen concentration between fetal and maternal blood play an important role in the clearance of NRBCs from maternal circulation.