The structure of the bone marrow

A trephine biopsy allows a two-dimensional view of the bone marrow down the light microscope (Fig 1.2). Haematopoietic cells of varying lineage and maturity are packed between fat spaces and bony trabeculae. Ultrastructural studies reveal clusters of haematopoietic cells surrounding vascular sinuses which allow eventual discharge of mature cells into the blood. Different lineages are compartmentalised; for example, the most immature myeloid precursors lie deep in the marrow parenchyma while more mature forms migrate towards the sinus wall. Lymphocytes tend to surround small radial arteries while erythrocytes form islands around the sinus walls.

Blood precursor cells in the marrow exist in close proximity to stromal cells. Stromal cells are those cells which do not mature into the three main types of peripheral blood cells – thus they include macrophages, fat cells, endothelial cells and reticulum cells.

Immature blood cells are attached to these stromal cells by multiple cellular adhesion molecules (e.g. fibronectin and collagen). Adhesive molecules have specific receptors on stromal and haematopoietic cells. As blood cells mature, the receptors down-regulate and the cells become less adherent and commence the journey through the sinus wall and into the bloodstream.

Haematopoiesis: the stem cell hierarchy

Haematopoiesis means the formation of blood. A number of transcription factors (e.g. GATA-1, MLL) are critical both for stem cell formation and function and lineage-specific differentiation.

The first adult haematopoietic stem cells (HSCs) are generated in the aorto-gonad-mesonephros (AGM) region of the embryo. The classical hierarchy diagram (Fig 1.3) where all cells arise in orderly fashion from a HSC is helpful but simplified; in reality, HSCs are groups of cells with diverse potentials depending on transcription factors and the local microenvironment. HSCs are not detectable by microscopic techniques but their existence can be inferred from cell cultures. Culture of these early cells on agar generates groups of more mature and thus recognisable progenitor cells known as colony-forming units (CFUs). For myeloid development the earliest detectable precursor cell creates granulocytes, erythrocytes, monocytes and megakaryocytes and is thus called CFU_GEMM. HSCs may also be identified and separated from more committed progenitors by the use of flow cytometry as they have a characteristic immunophenotype.

HSCs have the capacity for self-renewal as well as differentiation and the system allows enormous amplification. A lifetime of human haematopoiesis with the generation of incalculable numbers of mature cells may rely on only a few thousand stem cells present at birth. These cells depend on their micoenvironment, the ‘niche’, for regulation of self-renewal and differentiation. Both haematopoietic and stromal stem cells have the capacity to produce cells associated with other tissues such as bone, liver, lung and muscle. This concept of ‘plasticity’ has therapeutic implications as stem cells are used to repair a variety of damaged tissues.

Regulators of haematopoiesis

Control of haematopoiesis is mediated via regulatory molecules (or ‘growth factors’ – Table 1.1). These are generally glycoproteins produced by stroma and differentiated blood cells. They may act on more than one cell lineage and frequently show additive and synergistic interactions with each other. Their actions are multiple, including the promotion of proliferation, differentiation and maturation, as well as changing functional activity. Proliferative regulators alter the behaviour of cells by interacting with specific receptors on the cell surface (Fig 1.4).

Receptors for haematopoietic regulators have been molecularly cloned and many are related in structure (haematopoietic receptor superfamily). The combination of regulator and membrane receptor leads to a structural change in the receptor and the triggering of a complex sequence of biochemical events (signal transduction). The end result is the generation of intracellular regulators in the cell cytoplasm which have the capacity to activate genes, which in turn encode proteins essential in cell activation.

Under normal circumstances regulators circulate in the plasma at virtually unidentifiable levels. The activities of many factors are likely to be localised and transient so that systemic levels are of limited significance. For instance, in the marrow, regulators acting at the earliest stages of haematopoiesis (e.g. c-kit ligand) are released from stromal cells in close proximity to haematopoietic precursor cells.

The colony-stimulating factors (CSFs) were originally defined by their ability to stimulate blood progenitor cells while the interleukins (ILs) were defined by their effects on mature lymphocytes. Subsequent discoveries have rendered this dual nomenclature unhelpful – thus IL-3 is a key stem cell growth factor. The term cytokine incorporates all growth factors.