Overview
Hematopoiesis is a complex process encompassing the continuous generation of specialized, mature blood cells from pluripotent hematopoietic stem cells (HSCs). The hematopoietic system is not fully developed at birth. The proportion of bone marrow (BM) components and normal hematologic values for neonates, infants, older children, and adults are different as a result of the unique characteristics of embryonal and fetal development of the hematopoietic system, which continues to evolve after birth [Reference Proytcheva and Proytcheva1]. Knowledge of these differences is essential to distinguish normal development from a pathologic process when evaluating blood and BM in pediatric patients.
Development of Hematopoiesis
Embryonic and Fetal Hematopoiesis
In humans, hematopoiesis (Fig. 1.1) begins in the yolk sac with the generation of angioblastic foci or “blood islands” during the third week of gestation [Reference Proytcheva and Proytcheva1]. The blood islands contain primitive erythroblasts. The erythroblasts are large and nucleated, containing embryonic hemoglobin [Reference Proytcheva and Proytcheva1,Reference Hudnall, Hematopoiesis and Hudnall2]. Embryonic erythropoiesis in the yolk sac is followed by the appearance of non-erythroid progenitors in both the yolk sac and aorta-gonadal-mesonephros (AGM) region (four to five weeks) [Reference Proytcheva and Proytcheva1–Reference Carlson3]. The AGM region contains pluripotent HSCs [Reference Proytcheva and Proytcheva1]. At the sixth week of gestation, the HSCs from the AGM and yolk sac colonize the fetal liver, spleen, and BM [Reference Proytcheva and Proytcheva1,Reference Hudnall, Hematopoiesis and Hudnall2].
Figure 1.1 Schematic representation of developmental time frame of the site shift in human hematopoiesis.
The fetal liver becomes the major site of hematopoiesis between 11 and 24 weeks of gestation [Reference Proytcheva and Proytcheva1]. The early stage of fetal hepatic hematopoiesis consists of predominant erythroid progenitors. Unlike yolk sac erythropoiesis, hepatic erythropoiesis is definitive and resembles that found in postnatal life; for example, it generates nucleated red blood cells (NRBCs) (Fig. 1.2) [Reference Proytcheva and Proytcheva1]. Megakaryocytes appear during the 12th week of gestation, and mature neutrophils appear after the 16th week. The spleen does not normally function as an active site of hematopoiesis.
Figure 1.2 Fetal liver, 18 weeks of gestation, showing numerous erythroid progenitors (H&E, 400x). NRBC, nucleated red blood cell.
The BM starts hematopoiesis during the 16th week of gestation but does not become a primary hematopoiesis site until the 25th or 26th week of gestation [Reference Proytcheva and Proytcheva1]. Fetal BM hematopoiesis is multi-lineal and generates definitive NRBCs containing fetal hemoglobin (HbF) and hemoglobin A (HbA), as well as myeloid and lymphoid progenitors [Reference Proytcheva and Proytcheva1].
Neonatal and Childhood Hematopoiesis
In a full-term infant, hepatic hematopoiesis has ceased except in scattered small foci that become inactive soon after birth. The BM becomes the primary site of hematopoiesis. In neonates (Fig. 1.3) and young children (see Fig. 1.4), hematopoiesis takes place throughout the entire BM space, including the long bones [Reference Hudnall, Hematopoiesis and Hudnall2,Reference Parveen, Farhi, Chai and Edelman4]. BM cavities are filled with hematopoietic elements and fat cells. Fat cells gradually increase as age increases, beginning with the digits and advancing toward the axial skeleton [Reference Parveen, Farhi, Chai and Edelman4].
Figure 1.3 Bone marrow biopsy showing >90% cellularity in a 14-day-old neonate (H&E, 400x).
Figure 1.4 Bone marrow biopsy revealing 60% cellularity in a 17-year-old male (H&E, 400x).
Adult Hematopoiesis
Hematopoiesis is limited to the BM of the flat bones (skull, ribs, sternum, vertebrae, scapulae, clavicles, pelvis, upper half of the sacrum) and the proximal portions of the long bones (femur, humerus). The remaining BM cavity is occupied by fat cells [Reference Parveen, Farhi, Chai and Edelman4]. As age increases, fat cells increase. In the elderly, fatty tissue fills most of the BM space. However, under stressful conditions, hematopoietic elements can replace fat cells [Reference Proytcheva and Proytcheva1,Reference Parveen, Farhi, Chai and Edelman4].
The iliac crest is the standard site for BM biopsy in all age groups because it is a non-weight-bearing structure and there are no close vital organs. The iliac crest can retain hematopoietic activity into the ninth decade of life and beyond.
Hematopoietic Regulation
Hematopoiesis is a complex, dynamic process. The growth, differentiation, and maturation of hematopoietic cells occur in a sequential order and are regulated by cell-to-cell interaction and cytokine activities in the BM microenvironment.
Cytokines are cell-signaling molecules that aid cell-to-cell communication and regulate hematopoiesis. Cytokines exist in peptides, proteins, and glycoproteins. They are produced by a broad range of cells. Cytokines act on primitive stem cells and lineage-committed progenitor cells [Reference Parveen, Farhi, Chai and Edelman4]. This process is mediated through specific receptors that transmit a sequence of intracellular signals. Cytokines may act locally at their production site or travel in the blood. Particular cytokines can elicit more than one activity due to actions on different target cells. Not all cytokines stimulate the growth and differentiation of BM cells. Some cytokines inhibit hematopoiesis [Reference Proytcheva and Proytcheva1,Reference Hudnall, Hematopoiesis and Hudnall2,Reference Parveen, Farhi, Chai and Edelman4].
The stimulatory cytokines are produced by both BM stromal cells and non-stromal cells [Reference Hudnall, Hematopoiesis and Hudnall2]. Stromal cells generate stem cell factors, Fms-like tyrosine kinase 3 ligand, interleukin (IL)-6 and IL-11, granulocyte colony-stimulating factor, and monocyte colony-stimulating factor [Reference Hudnall, Hematopoiesis and Hudnall2,Reference Parveen, Farhi, Chai and Edelman4]. Non-stromal cells produce IL-1 (by monocytes, granulocytes), IL-3 (by T-cells), IL-5 (by T-cells), granulocyte-monocyte colony-stimulating factor (by T-cells), erythropoietin (by renal peritubular cells), and thrombopoietin (by hepatocytes) [Reference Hudnall, Hematopoiesis and Hudnall2]. These molecules primarily act synergistically to regulate the self-renewal of hematopoietic stem cells, the proliferation and differentiation of lineage-committed progenitor cells, and the function of mature hematopoietic cells [Reference Parveen, Farhi, Chai and Edelman4].
The inhibitory cytokines are manufactured by BM stromal cells and macrophages. Examples include tumor necrosis factor, transforming growth factor β, interferon gamma (IFNγ), macrophage inflammatory protein 1α, tetrapeptides, and pentapeptides [Reference Hudnall, Hematopoiesis and Hudnall2,Reference Parveen, Farhi, Chai and Edelman4]. These pro-inflammatory cytokines contribute to the marrow suppression seen in chronic inflammatory conditions [Reference Hudnall, Hematopoiesis and Hudnall2].
Pathologic conditions are usually caused by imbalances in hematopoietic cytokine production, such as anemia of renal failure due to erythropoietin deficiency, aplastic anemia due to IFNγ excess, and thrombocytopenia of hepatic failure due to thrombopoietin deficiency [Reference Hudnall, Hematopoiesis and Hudnall2].
Adhesion molecules also participate in hematopoiesis regulation. This process is mediated by binding of the adhesion molecules with their receptors present on the surface of target cells. As a result, these adhesion molecules promote the attachment of various hematopoietic cells to each other, to stromal cells, and to the extracellular matrix, affecting the generation, differentiation, and function of hematopoietic cells and regulating the retention and release of hematopoietic cells in the BM [Reference Parveen, Farhi, Chai and Edelman4]. The important adhesion molecules include adhesion molecules of the immunoglobulin family, integrins, and selectin [Reference Parveen, Farhi, Chai and Edelman4].
At the gene level, the GATA1 and PU.1 genes primarily regulate primitive erythroid and myeloid development [Reference Jagannathan-Bogdan and Zon5]. The RUNX1 and GATA2 genes are regulators for myelopoiesis [Reference Jagannathan-Bogdan and Zon5,Reference Antoniani, Romano and Miccio6]. The RUNX1 gene also regulates lymphoid and erythroid development. The HOX gene family is expressed in hematopoietic stem cells and progenitors and is involved in the regulation of early development, as well as lineage and stage differentiation [Reference Alharbi, Pettengell and Pandha7]. The Cyclin D3 gene is engaged in the regulation of erythroid number and size [Reference Antoniani, Romano and Miccio6].
Normal Bone Marrow in Children
The BM is a functionally dynamic structure, and if the need for leukocyte, erythrocyte, or platelet production increases, hematopoiesis expands, and the fat is replaced by red BM elements. In young children, an increase in hematopoiesis is accommodated by a reduction in sinusoids.
The BM is located between the bone trabeculae. The different hematopoietic components are not randomly distributed. The early myeloid progenitors are localized in the paratrabecular areas. Normally, there are no more than two or three layers of maturing myeloid elements. With maturation, the cells migrate to the intertrabecular space. The erythroid progenitors mature and differentiate in erythroblastic islands (a central macrophage surrounded by erythroblasts). As the erythroblasts become more differentiated, the erythroid islands migrate toward sinusoids. Megakaryocytes reside near marrow sinusoids, allowing for platelets to shed directly into the circulation [Reference Proytcheva8].
The cellularity and composition of the BM are dependent on age (Table 1.1) [Reference Proytcheva8]. At birth, the BM is almost devoid of fat and contains only hematopoietic elements. With advances in age, the red hematopoietic marrow is gradually replaced by fat. The BM cellularity is ≥80% in normal infants and approximately 60% during the first five years of life; it remains relatively constant, comparable with that of other age groups, after that (Fig. 1.4) [Reference Proytcheva8].
Table 1.1 Bone Marrow Cellularity and Cellular Composition in Normal Children of Various Ages
| Age | Cellularity | Main cellular composition | ||||
|---|---|---|---|---|---|---|
| Myeloid lineage (M) | Erythroid lineage (E) | Megakaryocytes | Lymphocytes | Others | ||
| Newborn (right after birth) | 90–100% | ↑, left shift | <5% blasts | |||
| Neonate (≤30 days) | 90% | ↓ in first 2 weeks | ↓ | Monolobated small forms | Gradually ↑, most B-cells | |
| Infant (1 month to <24 months) | 80–90% | Reach a steady level (30–35%) | Initial ↓, ↑ in the 2nd month, stabilization at 3–4 months | Monolobated small forms | About 50% after the 1st month, high number of B-cells and hematogones | Absent iron store, M:E = 5–12:1 |
| 2–5 years | 60–80% | ↑ | ↑ | ↓ B-cells, ↓ hematogones, slightly ↑ T-cells | Detectable stainable iron after age 4–5 years | |
| 6–12 years | 50–70% | <20% lymphocytes, T-cells > B-cells |
| |||
| >12 years | 40–60% | <20% lymphocytes, T-cells > B-cells |
| |||
At birth, the BM has a predominance of myeloid progenitors and a relatively low number of erythroid progenitors and lymphocytes. During the first month of life, myeloid and erythroid progenitors start to decrease, and lymphocytes increase. Myeloid components reach a steady level after the first month. Erythroid elements become stabilized at four months of life. The number of megakaryocytes in children is comparable with that of adults. Another significant difference in the BM composition between children and adults is the presence of a higher number of lymphocytes, with a higher ratio of B-lymphocytes:T-lymphocytes, in young children compared with adults. The number of B-cells gradually decreases with the increase in T-lymphocytes after the age of four years [Reference Proytcheva8].
