Structure and Function of the Bone Marrow

During embryogenesis and fetal development, hematopoiesis shifts from the yolk sac to the liver and, after the twentieth week of gestation, to the bone marrow.¹ Although hepatic erythropoiesis may persist for several weeks after birth, in the term infant hematopoiesis takes place almost entirely in the bone marrow. A defining feature of hematopoietic stem cells is their ability to home to the bone marrow. This homing occurs via chemoattractants such as stromal cell‑derived factor-1 and its cognate chemokine receptor on the stem cell, as well as interaction between a variety of stem cell adhesion molecules and their ligands on stroma and endothelial cells. The hematopoietic stem cell may be identified by the presence of cell surface markers, such as CD34 or by properties to exclude Hoeschst dye 33342, via a multidrug transporter.²

Grossly, two types of bone marrow can be recognized in normal individuals: yellow marrow, so called because of the predominance of fat cells, and red marrow, in which blood cells predominate. White marrow, consisting predominantly of stromal cells and intercellular matrix, may result from atrophy or starvation. Red marrow proportionately is a much greater component of body weight and volume in the infant than in the adult (Figure 79-1).³ Early in life, it is contained in the medullary cavities of the long bones, which gradually fill with fat such that by late puberty, the adult distribution of hematopoiesis (sternum, pelvis, vertebrae, cranium, ribs, epiphyses of long bones) is achieved. That the degree of hematopoiesis in a given bone may vary with age is an important consideration in selecting a site for bone marrow aspiration or biopsy; for example, although the anterior and posterior iliac crests can be used at any age, the tibia can be used only until the age of 2 years. In disease states characterized by excessive destruction of blood cells, such as some severe hemolytic anemias, hematopoiesis may increase twofold to eightfold. Active sites of hematopoiesis may expand, and extramedullary hematopoiesis may be found, particularly in the liver and spleen.

Figure 79–1 Comparison of active red marrow–bearing areas in the child and the adult. Note the almost identical amount of active red marrow in the child and adult despite a fivefold discrepancy in body weight.

(Modified from MacFarlane RC, Robb-Smith AHT: Functions of the blood, Oxford, 1961, Blackwell.)

Microscopically, the marrow is a network of vascular channels (sinuses) separating islands of fat, hematopoietic cells, and rare osteoblasts and osteoclasts (which are important for bone remodeling).^4,5 The vasculature and cells are joined by a reticulin (fiber) network or scaffolding. By light microscopy, bone marrow aspirate specimens demonstrate hematopoietic elements; however, a bone marrow biopsy provides a more accurate measure of cellularity. Reticulin can be seen by light microscopy when special histochemical stains are used.

The blood vessels that feed the marrow are branches of those that feed the surrounding bone.⁶ Large central arteries run longitudinally within the marrow and send radial branches that penetrate the endosteum and form capillaries in the Haversian and Volkmann’s canals of the bony cortex.⁷ These capillary systems drain into the bone marrow sinuses, which in turn drain into a central sinus or vein. Because the marrow circulation interconnects with the general circulation in this fashion, fluids and medication injected in bone marrow are absorbed as rapidly as through intravenous routes. Unlike peripheral veins, intramedullary vessels supported by their bony shell do not collapse in shock; therefore, intraosseous (either tibial or iliac crest) infusion is appropriate when standard intravenous access is not available.⁸ It is also noteworthy that the interconnection between the marrow and general circulation provides the mechanism by which bone marrow may embolize to the lung after osseous trauma or fracture. This has not been demonstrated to be of clinical significance in the case of intraosseous infusion.⁹

The concept of the stem cell has expanded recently,¹⁰ and its ex vivo manipulation has raised much interest by a wide group of physicians and scientists studying nonhematologic systems. One essential feature of the stem cell is its plasticity.¹¹ Provocative studies have shown that stem cells isolated from the bone marrow can be driven to differentiate to muscle, liver, cardiac, or neuronal tissue. This raises the possibility that the marrow may be a convenient and ethically less challenging source of stem cells for stem cell engineering and tissue replacement. Although the clinical applications to nonhematologic tissues remain distant, the use of hematopoietic stem cells to replace a diseased marrow is a common practice (i.e., stem cell transplantation). Alternative sources of hematopoietic stem cells have been found peripheral blood following growth factor−induced mobilization or from placental cord blood.

Hematopoiesis

Cells within the hematopoietic island include the RBCs, granulocytes (neutrophils, eosinophils, basophils), monocytes and macrophages, platelets, lymphocytes, and their precursors. The earliest precursors, or stem cells, are thought to look like small lymphocytes and are not usually distinguishable from them by microscopy. Their existence was best confirmed by in vitro culture assays in which nucleated cells from bone marrow aspirate specimens plated onto tissue culture dishes layered with methylcellulose generate colonies (aggregates of cells) of one or more lineages. Now, immunophenotyping provides rapid identification of cell lineage and stage of development.¹² The first morphologically identifiable precursor cells are the proerythroblast, myeloblast, monoblast, megakaryoblast, and lymphoblast. These committed precursors and their terminally differentiated counterparts sit within the hematopoietic islands. Megakaryocytes (which make up <1% of hematopoietic cells) generally are located next to marrow sinusoids and shed platelets (fragments of megakaryocyte cytoplasm) directly into the lumen. Erythroblasts also are produced near the walls of the vascular sinuses in clusters with macrophages called erythroblast islets. As the erythroblasts develop, they extrude their nuclei, which are phagocytosed by the macrophages. In contrast, granulocytes (most numerous of the hematopoietic cells), monocytes, and lymphocytes are produced throughout the marrow away from vascular sinuses. The mature white blood cells (WBCs) are motile and migrate to the sinuses.

Within the peripheral circulation, the number of cells of each type is maintained in a narrow range in the normal individual. Adults and post-pubertal adolescents have approximately 5000 granulocytes, 2000 lymphocytes, 500 monocytes, 5 × 10⁶ RBCs, and 150,000 to 300,000 platelets per microliter of whole blood. Age-dependent values for younger children are shown in Table 79-1.¹³ To a lesser extent, values also are a function of race and sex, so that African Americans (especially males) normally may have granulocyte counts less than 1500/μL. Normative values for Latinos are less clear, but have been reported to be closer to those of Caucasians.¹⁴ As summarized elsewhere, under normal conditions the rate of production of each cell type equals the rate of destruction. Because the life span of mature RBCs in adults is 100 to 120 days, 5 × 10⁴ RBCs/μL are produced daily. The average platelet life span is 7 to 10 days so that approximately 2 × 10⁴ platelets/μL are produced daily. With less than a 12-hour life span, granulocytes production occurs at a rate of 10⁴ cells/μL. The very slow rate of production of lymphocytes reflects their long life span, measured in years.

Table 79–1 Normal Values for Hematology

The mechanisms that regulate this steady state are incompletely understood. However, evidence strongly suggests the existence of a pluripotent stem cell that is capable of self-renewal, from which progenitor cells committed to hematopoiesis (RBCs, granulocytes, megakaryocytes, and monocytes) and to lymphopoiesis develop (Figure 79-2).¹³ The “trilineage myeloid” stem cell has been designated colony forming unit-stem (CFU-S) on the basis of bone marrow culture assays and experiments in which the spleens of lethally irradiated mice infused with donor marrow cells are found to contain colonies each consisting of precursors of RBCs, granulocytes, monocytes, and megakaryocytes.¹⁶ The existence of CFU-S in human beings is further deduced from chromosomal studies in myeloproliferative disorders. Lymphoid development appears to arise from a separate progenitor. Although the CFU-S is found predominantly in the bone marrow, there probably are small numbers of circulating pluripotent stem cells because marrow of lethally irradiated animals can be reconstituted by using peripheral blood.³

Figure 79–2 A schematic outline of the progenitor basis of hematopoiesis. Not shown in this outline is the process of self-renewal of fractions of the progenitor cell populations, particularly the immature progenitors. Also not shown is the progressive amplification of progenitors and precursors as they mature and differentiate. The bipotential erythroid-megakaryocyte progenitor shown in this drawing has been demonstrated in the mouse, but not definitively in humans. Dotted arrows indicate an alternative scheme.

(Modified from Nathan DG: Introduction: hematologic diseases. In Wyngaarden JB, Smith JH, editors: Cecil’s textbook of medicine, Philadelphia, 1988, WB Saunders.)

The numbers of committed progenitor cells that differentiate in any time period is dependent on feedback from humoral regulators that are produced within the marrow microenvironment and by extramedullary sources, including T cells, macrophages, endothelial cells, and fibroblasts. These hematopoietic growth factors are cytokines, known by a variety of names. Many of the cytokines have overlapping functions. However, gene targeting in the mouse (“knockout mouse”) of cytokines or their receptors has identified the essential nonredundant functions for several hematopoietic growth factors. Mice deficient in erythropoietin (Epo), thrombopoietin (Tpo), and granulocyte colony-stimulating factor (G-CSF) suffer from severe anemia, thrombocytopenia, or neutropenia, respectively.

Although the list of cytokines and small molecules that regulate hematopoiesis continues to grow, clinical application has been chiefly limited to Epo and G-CSF. Chemical modification of Epo and G-CSF has resulted in two longer-acting forms, darbepoetin alpha, and pegfilgrastim, respectively. Their primary advantage is longer half-lives. Recently, two platelet-stimulating agents, romiplostim and eltrombopag, have been Food and Drug Administration−approved for use in adults with chronic immune thrombocytopenic purpura (ITP).¹⁵ Characteristics of specific hematopoietic growth factors, discussed in the following sections, are summarized in Table 79-2.

Table 79–2 Recombinant Hematopoietic Growth Factors

∗ Thrombopoietin-like peptides have been approved.