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During embryogenesis, hematopoiesis occurs in spatially and temporally distinct sites, including the extraembryonic yolk sac, the fetal liver, and the preterm marrow. The development of primitive erythroblasts in the yolk sac is critical for embryonic survival. Nucleated primitive erythroblasts differentiate within the vascular network rather than in the extravascular space and ultimately enucleate upon terminal differentiation. Macrophage and multipotential myeloid progenitors initially arise in the early yolk sac and provide specialized tissue-resident macrophage populations that persist into adulthood in multiple organs, including the brain, liver, and lungs. Definitive erythroid progenitors also arise in the yolk sac and seed the early fetal liver to generate red cells until the establishment of blood cell production from hematopoietic stem cells. Hematopoietic stem cells emerge from major arterial vessels at 5 weeks of gestation and seed the liver and ultimately the marrow to provide lifelong hematopoiesis. The liver serves as the primary source of red cells from the 9th to the 24th week of gestation. Hematopoiesis in the fetal liver consists not only of definitive erythroid, megakaryocyte, and multiple myeloid lineages but also lymphoid lineages. B-cell progenitors are present in the fetal liver beginning at 7 weeks of gestation and expand rapidly in the bone marrow from 11–20 weeks of gestation. Lymphopoiesis is also present in the lymph plexuses and the thymus beginning at 9 weeks of gestation. Hemoglobin (Hgb) Gower-1 (ζ2ε2) is the major hemoglobin in embryos younger than 5 weeks. Hgb F (α2γ2) is the major hemoglobin of fetal life. The fetal hemoglobin concentration in blood decreases after birth by approximately 3% per week and is generally less than 2% to 3% of the total hemoglobin by 6 months of age. The mean hemoglobin level in cord blood at term is 16.8 g/dL, with 95% of the values falling between 13.7 and 20.1 g/dL. The red cells of a newborn are macrocytic, with a mean cell volume in excess of 110 fL. The red cell, hemoglobin, and hematocrit values decrease only slightly during the first week after birth but decline more rapidly in the following 5–8 weeks, producing the physiologic anemia of the newborn. The absolute number of neutrophils in the blood of term and premature infants is usually greater than that found in older children. Segmented neutrophils are the predominant leukocytes in the first few days after birth. As their number decreases, the lymphocyte becomes the most numerous cell type and remains so during the first 4 years of postnatal life. Phagocytosis of bacteria by neutrophils from premature and term infants is normal. Bactericidal activity varies according to the conditions of testing and the clinical status of the neonate. The platelet counts in term and preterm infants are between 150 and 400 × 109/L, comparable to adult values. The absolute number of lymphocytes in newborns is equivalent to that in older children, with lower values in premature infants at birth. The absolute ...

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