Chapter 429. Hematopoeisis

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.

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 of the entire axial skeleton, the long bones, and many membranous bones. With advancing postnatal age, hematopoietic tissue gradually moves to the vertebrae, sternum, pelvis, scapulae, and proximal ends of the long bones. The rest of the marrow space becomes fatty.

In hemolytic anemias, such as hereditary spherocytosis, the normal rate of RBC production can increase 8-fold. In thalassemia major, the erythropoietic tissue may expand 30-fold. In these pathologic states, the hematopoietic or red marrow extends from the ends of the long bones toward the middle of the shafts, replacing the fatty or yellow marrow. This expansion causes the loss of trabeculae and cortical thinning. Hematopoiesis may also extend into extramedullary sites, such as the liver, spleen, or paraspinous masses in severe hemolytic anemia, states of ineffective erythropoiesis, and myelofibrosis.

RBC production in the bone marrow is regulated by the hormone erythropoietin. During fetal life and early in postnatal life, erythropoietin is produced mainly in the liver, but its production switches to the kidney after birth. Erythropoietin binds to a high-affinity receptor expressed on the surface of erythroid progenitors called burst-forming units–erythroid (BFU-E) and colony-forming units–erythroid (CFU-E) to induce their division and differentiation. The production of erythropoietin is regulated by the degree of tissue oxygenation. Decreasing tissue oxygenation increases erythropoietin production in compensation.

As the site of hematopoiesis changes from the yolk sac to the liver and to the bone marrow, there are simultaneous changes in the types of hemoglobin (Hb) that are produced by RBC precursors (Fig. 429-1). Hb is the oxygen-carrying protein within RBCs. It accounts for more than 95% of the total protein and about 90% of the dry weight of the RBC. Hb is composed of four globular protein subunits, called globins, and four oxygen-binding heme groups, which are attached to each globin. The two main types of globins are the α-globins and the β-globins, which are made in essentially equivalent amounts in precursors of RBCs. Normal adult Hb (Hb A) has two α-globins and two β-globins (α2β2). There are also distinct embryonic, fetal, and minor adult analogs of the α-globins and β-globins, all of which are encoded by separate genes. The α-globin and its embryonic analog, the ζ-globin, are encoded in a cluster of genes on chromosome 16 (Fig. 429-2). The β-globin and its embryonic, fetal, and minor adult analogs—the ε-, γ-, and δ-globins, respectively—are encoded in a cluster of genes on chromosome 11 (Fig. 429-2).

The earliest Hbs to be produced are the embryonic Hbs, Hb Gower-1 (ζ2ε2), Hb Gower-2 (α2ε2), and Hb Portland (ζ2γ2). These Hbs predominate in embryos from 4 to 8 weeks of gestation, but disappear by 8 to 12 weeks. Fetal Hb (Hb F) (α2γ2) quickly replaces these embryonic Hbs as ζ- and ε-globin production rapidly falls. After the eighth week of gestation, Hb F is the main Hb, and at 6 months, it accounts for about 90% of the total. Thereafter, the percentage of Hb F decreases slowly as Hb A (α2β2) synthesis progressively increases. Small amounts of Hb A can be detected even in embryonic life. By the sixth month of gestation, there is about 5% to 10% Hb A, and it steadily increases to reach about 30% at term. At birth, the proportion of Hb F is about 70%.

At the time of birth, there is an abrupt decrease in γ-globin synthesis. Consequently, the proportion of Hb F (α2γ2) falls to less than 20% by 4 months of age and less than 2% at 1 year of age, which is the normal proportion in older children and adults. The rate of decline in Hb F is slower in individuals with β-hemoglobinopathies and β-thalassemias (see Chapter 434), in which higher proportions of Hb F may persist throughout life. Other conditions in which Hb F is elevated include hereditary persistence of fetal Hb (HPFH) (see Chapter 434) and the bone marrow failure syndromes (see Chapter 432).

The production of the minor adult Hb, Hb A2 (α2δ2), begins in late gestation and makes up only a small fraction of the total Hb, even in adult life. Hb A2 accounts for about 2% to 3% of total Hb after the first few months of postnatal life. Hb A2 is elevated in β-thalassemia trait (see Chapter 434).

Hb is an oxygen-transport protein. It must have a high enough affinity to oxygen to bind it in the lungs, but also a low enough affinity to oxygen to release it to the tissues. This reversible binding of oxygen is described by the oxygen-Hb dissociation curve (see Fig. 434-4). This curve describes the relationship between the percent oxygen saturation of Hb within a suspension of RBCs and the oxygen tension (pO2) of the solution. The p50 is the oxygen tension (pO2) at which Hb is 50% saturated, and it is a measure of the oxygen affinity of Hb. In the normal adult, the p50 is approximately 27 mm Hg. An increased p50 indicates a lower oxygen affinity, which shifts the oxygen-Hb dissociation curve to the right (see Fig. 434-4). A reduced p50 indicates higher oxygen affinity, which shifts the curve to the left. The affinity of Hb to oxygen is a function of the particular Hb (eg, Hb A or Hb F), but it is also regulated by physiologic feedback mechanisms, including the production of the organic phosphate 2,3-bisphosphoglycerate (2,3-BPG). Binding of 2,3-BPG to Hb decreases its oxygen affinity. These developmental and physiologic dynamics allow the body to adapt to changing environments. A key environmental change occurs at the time of birth when the body transitions from the relatively low-oxygen environment of the uterus to a high-oxygen extrauterine environment.

The p50 of fetal blood is about 20 mm Hg. This high oxygen affinity, relative to blood of older children and adults, allows the extraction of oxygen from the maternal circulation. The high affinity is largely a function of Hb F, which binds less avidly to 2,3-BPG than Hb A. The relatively high total Hb concentration of the fetus, which increases total oxygen-carrying capacity, helps maintains oxygen delivery to the tissues despite the high oxygen affinity of the Hb. Although the high oxygen affinity of fetal blood suits the conditions of the intrauterine environment, it limits the proportional release of oxygen to tissues after birth.

The extrauterine environment is relatively oxygen rich, so it is not necessary to maintain the high oxygen affinity needed for the intrauterine environment. Therefore, the p50 decreases appropriately after birth mainly as a result of the concomitant fall in Hb F concentration and rise in Hb A concentration. Relative to Hb F, Hb A is more sensitive to the effects of 2,3-BPG, so p50 increases as Hb A production increases. The postnatal decline in the p50 of infants to normal adult values takes 2 to 3 months.

In term newborn infants, the average blood volume is 85 mL/kg. Premature newborns have an average blood volume of 90 mL/kg body weight. The blood volume increases to a mean of 105 mL/kg during the first few days of life and then decreases during the first few months. After these initial postnatal adjustments, blood volume maintains a relatively constant relationship to body weight throughout the remainder of life. The average blood volume of infants after 6 months of age is 75 to 77 mL/kg, similar to older children and adults.

Embryo and Fetus

The mean Hb concentration progressively increases throughout gestation. It is 6 to 9 g/dL at 10 weeks’ gestation, 14 g/dL at 24 weeks, and 16.5 g/dL at term. The RBC count increases simultaneously from about 0.5 to 1.5 × 1012/L at 10 weeks’ gestation to 4.7 × 1012/L at term. Embryonic and fetal RBCs are much larger than those from newborns, children, and adults. The mean corpuscular volume (MCV) decreases from 250 fL at 10 weeks’ gestation to 107 fL at term. Thus, in neonates, the normal MCV is much higher than in older individuals. Moreover, many nucleated red blood cells (NRBCs) and reticulocytes are normally present in the peripheral blood of embryos and fetuses.

Newborn and Infant

Red Blood Cells

At the time of delivery, cord blood has, on average, an Hb concentration of 16.8 g/dL, an MCV of 107 fL, 5% reticulocytes, and 500 NRBCs/mm3. A postnatal decrease in plasma volume occurs between birth and 3 days of age that increases the Hb, hematocrit, and number of RBCs. At 3 days of age, the Hb concentration averages 17.8 g/dL, the MCV 100 fL, the reticulocyte count 2%, and 5 NRBCs/mm3. NRBCs disappear from the peripheral circulation after 3 days of age, and their presence beyond then should be considered abnormal. The Hb concentration then falls and returns to the cord blood value by about 2 weeks of age. During the first few days after birth, there are significant differences in the Hb concentrations between venous and capillary blood. Capillary Hb is about 2 g/dL higher than venous Hb because of loss of plasma from the capillaries and relative hemoconcentration.

The mean Hb of the newborn is about 17 g/dL. This relative polycythemia is the result of the low arterial oxygen tension in utero and the increased oxygen affinity of Hb F. These factors stimulate erythropoietin production and erythropoiesis in the fetus. The arterial oxygen tension abruptly increases on birth in the oxygen-rich extrauterine environment, so erythropoietin concentration and the rate of erythropoiesis decrease. Consequently, erythroid progenitors in the bone marrow decrease from an average of 35% to 15% of total nucleated cells after 1 week, NRBCs disappear from the peripheral circulation after 3 days of age, and the reticulocyte count falls to near zero for about 2 months after birth. The RBC lifespan in newborns is about 90 days, compared with 120 days in older children and adults. So, due to decreased production and a slightly shorter RBC lifespan, Hb values in term infants decrease steadily during the first 8 to 10 weeks of life to reach a mean nadir of 11 g/dL. This is called the physiologic anemia of infancy (Fig. 429-3). The lower limit of normal for Hb concentration at the physiologic nadir is approximately 9.5 g/dL. Afterward, RBC production resumes, as reflected in a rise in reticulocyte count and an increase in Hb (Fig. 429-3). The rate of production thereafter is sufficient to maintain a stable mean Hb concentration of 11 to 13 g/dL, despite the threefold increase in blood volume and weight that occurs during the first year of life.

In the preterm infant, the Hb concentration falls lower and more quickly than the term infant, even with optimal nutrition and support. The smallest infants have the greatest decline in Hb concentration. By 6 to 8 weeks of age, the Hb falls to a mean of about 9.5 g/L in preterm infants with birth weights between 1500 and 2000 g to 9 g/dL in those with birth weights between 1000 and 1500 g, and even lower in very-low-birth-weight infants (Fig. 429-3). This decline in Hb concentration is called the anemia of prematurity. The lower limit of normal for Hb concentration at the nadir of the anemia of prematurity is 7 to 7.5 g/dL. This exaggerated physiologic anemia has several causes. The concentration of erythropoietin in premature infants is inappropriately low for the degree of anemia, perhaps because hepatic production of erythropoietin is less sensitive to tissue oxygenation than the kidney. Other factors include a large expansion in blood volume because of rapid growth, a shorter RBC survival than term neonates, and phlebotomy for laboratory tests, especially in sick infants.

White Blood Cells and Platelets

The normal leukocyte and neutrophil counts vary tremendously, even by the hour of life, in the first few days after birth. Therefore, it is necessary to consult appropriate nomograms in the immediate neonatal period to properly interpret leukocyte counts. Outside the neonatal period, the normal absolute neutrophil count has a lower limit of normal of approximately 1000/mm3, except in African American infants, where it may normally be quite lower. The absolute neutrophil count is the product of the total leukocyte count and the percentage of neutrophils (segmented neutrophils and band forms). The normal platelet count is 150,000 to 500,000/mm3, which is the same as children and adults. Thrombocytosis (platelet counts > 500,000/mm3) may occur in thriving preterm infants without adverse consequences.

Children and Adolescents

Red Blood Cells

At 1 year of age, the mean concentration for Hb is about 12.5 g/dL for both boys and girls, with a lower limit of normal of about 11 g/dL (Table 429-1).2 Thereafter, the mean Hb concentration increases steadily with age. RBC indices such as the MCV also change with age. Values in boys and girls first begin to diverge significantly with the onset of puberty, after which boys have higher Hb concentrations and MCVs than girls (Fig. 429-4). The erythroid-stimulating effects of androgenic hormones account for this difference between sexes. Thus, it is always important to refer to age- and sex-specific normal values when interpreting the blood counts of children (Table 429-1). It is also necessary to consider a child’s sexual development. For example, a 14-year-old boy who is Tanner stage IV or V will have a higher Hb concentration than one who is Tanner stage I or II.

White Blood Cells and Platelets

The lower limit of normal for the absolute neutrophil count is approximately 1000/mm3 in the first 1 to 2 years of age and 1500/mm3 thereafter. African Americans tend to have total leukocyte counts and absolute neutrophil counts that are lower than that of Caucasians. The absolute neutrophil count may be as low as 700 to 800/mm3 in normal African Americans. This has been called “ethnic pseudoneutropenia,” an arcane name unnecessarily ascribed to a biologically normal state. The normal platelet count is 150,000 to 500,000/mm3 in children and adults.