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Because birth represents one temporal event in the development of the infant, the interpretation of the blood picture in the newborn requires an understanding of the maturational processes that precede it.

Frank A. Oski, MD1

Yolk Sac

The anatomic sites of hematopoiesis change during the development of the embryo and fetus (Fig. 429-1). Red blood cell (RBC) formation first occurs within the yolk sac at 2 weeks’ gestation. In this primitive phase of hematopoiesis, clusters of cells called blood islands form in the yolk sac. The peripheral cells of these blood islands differentiate to become the walls of blood vessels, whereas the central cells become primitive blood cells or hemocytoblasts. Similar blood islands are seen throughout the body stalk at 4 weeks’ gestation. This primitive, intravascular phase of hematopoiesis declines by 6 weeks’ gestation and is no longer observed after the third month.

Figure 429-1.

Production of blood and hemoglobin throughout development. Top: Changing sites of erythropoiesis throughout development. RBC formation starts in the yolk sac in the embryo. Erythropoiesis then transitions to the liver and, to a lesser extent, the spleen during fetal life. The bone marrow is the sole normal site of erythropoiesis from shortly after birth and throughout the remainder of life. Bottom: Simultaneous changes in globin production that occur as the site of blood formation changes. The main Hb of the fetus is Hb F (α2γ2). The predominant Hb after about 3 months of life is Hb A (α2β2).

Liver and Spleen

Blood formation begins to shift to the sinusoids of the liver at the fifth week of gestation. The liver is the first site of definitive hematopoiesis where leukocyte precursors and megakaryocytes (platelet precursors) are also produced. Erythroid elements are the predominant cell type, however. The liver is the main site of hematopoiesis from the third to the fifth month of fetal life (Fig. 429-1) and declines thereafter, although continuing to produce blood cells until the first week of postnatal life. A smaller amount of definitive hematopoiesis also occurs in the spleen during this phase.

Bone Marrow

Hematopoiesis in the bone marrow begins in the third and fourth months of gestation, and it becomes the predominant site of hematopoiesis by the sixth month and remains so thereafter—throughout the rest of gestation and extrauterine life (Fig. 429-1). The bone marrow is also the site where significant myeloid hematopoiesis (production of white blood cells [WBCs] and platelets) begins. During the last trimester, most blood cells are produced in the marrow. The cellularity of the bone marrow becomes maximal at 30 weeks’ gestation. In the newborn and during early infancy, red or hematopoietic marrow fills the bony cavities ...

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