NORMAL ANATOMY AND 1 HISTOLOGY OF BONE MARROW FETAL HEMATOPOIESIS regulatory factors. This tightly regulated process In the developing embryo and fetus, hema- involves self-renewal of hematopoietic stem topoiesis is first detected in the yolk sac and cells and the production of lineage-committed sequentially in the dorsal aorta (aorta-gonad- progenitor cells; the latter undergo a cascade of mesonephros), liver, spleen, and finally the differentiation resulting in the diverse mature bone marrow. In the yolk sac, hematopoiesis hematolymphoid elements that are released occurs in the 2nd and 3rd embryonic weeks into the blood (15,28). and is characterized by the sequential forma- The hematopoietic specificity of bone mar- tion of waves of hematopoietic progenitor cells row–derived stem cells has been challenged, with limited reconstitution capacity (2,8). This and a “plasticity” of these stem cells has been site is restricted to the exclusive production of postulated (18). Although controversial, some nucleated erythroid cells within blood islands; evidence suggests that bone marrow–derived these primitive nucleated erythrocytes are the stem cells may be induced to give rise to various first circulating cells of the developing embryo nonhematolymphoid lineages such as muscle, (2,4,6,8). The earliest multilineage hematopoi- cartilage, and nerve (18). The recent detection of etic precursor cells are found in the embryonic a second category of stem cell, the mesenchymal dorsal aorta (aorta-gonad-mesonephros/para- stem cell, has prompted reconsideration of this aortic splanchnopleura) (1,2,4,8). Specialized hypothesis. Consequently, current investigators adhesion molecules produced by stromal cells propose that mesenchymal stem cells are the within both the yolk sac and dorsal aorta progenitors of nonhematopoietic lineages, not promote hematopoiesis and angiogenesis (5). the hematopoietic stem cells (13,21,22). These Indeed, a common in utero precursor cell for mesenchymal stem cells are a normal constitu- hematopoietic and endothelial cells has been ent of the bone marrow microenvironment and proposed and termed the hemangioblast (3,8). give rise to mesenchymal bone marrow stromal Stem cell migration to the fetal liver results constituents including fibroblasts, adipose cells, in hepatic hematopoiesis by about 6 weeks of endothelial cells, osteoblasts/osteocytes, and gestation, followed shortly thereafter (at about chondroblasts/chondrocytes (33). 7 weeks of gestation) by seeding of the fetal Hematopoietic stem cells are not specifi- spleen (1,6,7). Hematopoiesis is detected in cally identifiable by the traditional morpho- fetal bone marrow by 14 weeks of gestation, logic review of bone marrow aspirate smears, and bone marrow is the primary site of hema- although a resemblance to small lymphoid cells topoiesis throughout the rest of gestation. The is suggested. Instead, stem cells are defined by vertebral column is the most active site of fetal functional properties or immunophenotypic bone marrow hematopoiesis followed by femur, profile. Hematopoietic stem cells are capable of pelvis, fibula/tibia, and humerus (7). both self-renewal and multilineage differentia- tion, a proposed unique property of asymetric HEMATOPOIESIS cell division which results in one new stem cell and one daughter cell capable of differentiation Stem Cells (12,16). Immunophenotypic features of hema- The production of all hematolymphoid lin- topoietic stem cells include expression of CD34, eages within the bone marrow is achieved by c-kit, and Thy1, while both CD38 and specific lin- the coordinated interaction of hematopoietic eage markers are typically absent (18,20,27,28,35). stem cells, the bone marrow microenviron- Expression of CD34, other adhesion molecules, ment, and various stimulatory and inhibitory and chemokine and cytokine receptors is linked 1 Non-Neoplastic Disorders of Bone Marrow to the localization (homing) of hematopoietic matopoietic stem cell activity are influenced by stem cells in highly specific bone marrow mi- these bone marrow microenvironment niches croenvironment niches (33). (16,19,24,30,33,36). T cells within the bone The homing of hematopoietic stem cells and marrow microenvironment play a key role in progenitor cells is regulated by cytokines, che- hematopoietic regulation by stimulating vari- mokines, and adhesion molecules (14,24,30). ous target cells to produce either stimulatory or Stromal cells provide survival and regulatory inhibitory regulatory factors. Similarly, mono- signals by direct contact with stem/progenitor cytes/macrophages within the microenviron- cells within a microenvironmental niche (24). ment produce many cytokines such as interleu- Hematopoietic stem cells localize to the endos- kin (IL)1, IL6, granulocyte colony-stimulating teal region of bone marrow; key signaling inter- factor (G-CSF), and monocyte–colony-stimulat- actions between hematopoietic stem cells and ing factor (M-CSF), all of which interact directly subendosteal osteoblasts account for this unique with hematopoietic stem cells and progenitor localization (12a,16,23,26,30,33). Recent studies cells (19). These regulatory factors may be se- suggest a major role for osteoblasts in support- creted into the extracellular matrix or remain ing hematopoiesis and regulating the size of the cell surface associated (11,33). hematopoietic stem cell pool (12a,16,30,33). Regulatory Factors The selective paratrabecular localization of im- mature granulocytic cells can be appreciated Numerous cytokines play a critical role in by morphologic and immunohistochemical as- hematopoiesis (Table 1-1). The majority of these sessment. In contrast, both lineage-committed cytokines are produced within the bone marrow progenitor cells and terminally-differentiated microenvironment by T cells, macrophages, cells tend to localize to central regions of the osteoblasts, and various other mesenchymal medullary cavity (29). stromal cells (33,34). By binding to specific mem- The transition from the undifferentiated sta- brane receptors on target cells, these cytokines tus of hematopoietic stem cells to committed initiate a cascade of intracellular genetic events lineage-specific differentiation is characterized that promote proliferation and maturation (9). by the regulation of expression of lineage-spe- Steady state hematopoiesis is maintained by both cific genes (27). The expression of these lineage- stimulatory and inhibitory regulatory factor pro- specific genes allows the committed progenitor duction as listed in Table 1-1. Some cytokines act cells to respond to stimuli generated from bone on the earliest hematopoietic stem cells, while marrow microenvironment elements, ultimate- others control the proliferation and maturation ly resulting in the production of various mature of lineage-committed progenitor cells. progeny (27). Lineage commitment is associated Stem cell factor (SCF) is a prototypic early with the loss of the self-renewal capability. acting cytokine that is critical not only for sustaining hematopoiesis but also for stem cell Bone Marrow Microenvironment migration and adhesiveness (10). In addition to The functional unit of hematopoiesis, the its action on hematopoietic stem cells through bone marrow microenvironment, includes the interaction with its cognate receptor c-kit, SCF capillary-venous sinus, the surrounding extra- is also critical for mast cell proliferation, matura- cellular matrix, and stromal elements. This is tion, and survival, accounting for its alternate a highly organized, three-dimensional matrix name of mast cell growth factor. Other early composed of structural scaffolding produced acting cytokines include FLT3 (FMS-like tyrosine by bone marrow stromal cells (fibroblasts, kinase 3) ligand and IL1, IL3, IL6, and IL11. macrophages, endothelial cells, adipocytes, Granulocyte-monocyte–colony-stimulating and reticular cells). The extracellular matrix factor (GM-CSF) is also a multilineage cytokine includes regulatory factors as well as fibronectin, whose major action is on granulocytic and collagen, laminin, and various proteoglycans erythroid colony forming units in cell culture (25,29,33). Interactions with the bone marrow assays. Prototypic late-acting cytokines include microenvironment determine the fate of he- G-CSF, erythropoietin, and thrombopoietin, matopoietic stem cells, since all aspects of he- which stimulate lineage-committed progenitor 2 Normal Anatomy and Histology of Bone Marrow Table 1-1 HEMATOPOIETIC REGULATORY GROWTH FACTORSa Regulatory Factor Comments Early Acting Cytokines; Multilineage, Pleiotropic or Synergistic Growth Factors Stem cell factor Ligand for c-kit receptor In receptor tyrosine kinase superfamily Many other names: kit ligand, steel factor, mast cell growth factor Influences stem cell migration, adhesiveness, proliferation Required to support hematopoiesis Synergistic with other regulatory factors Granulocyte/monocyte colony-stimulating factor Multilineage cytokine (GM-CSF) Promotes cell survival by suppressing apoptosis Interleukins (IL) 1, 3, 4, 6, 7, 11, 12 Multilineage (including B and T cells) cytokines Synergistic with other interleukins; IL-6 critical for plasma cell production IL3 Multi-CSF, multilineage cytokine Promotes cell survival by suppressing apoptosis FLK2/FLT3 (FMS-like tyrosine kinase 3) ligand Multilineage growth factor in receptor tyrosine kinase superfamily Lineage-Specific Cytokines Granulocyte colony-stimulating factor (G-CSF) Proliferation, maturation of neutrophilic lineage Monocyte colony-stimulating