Chapter 1
An introduction to haematopoiesis

Learning objectives

• To understand the process of formation of blood cells
• To understand the concept of a stem cell
• To appreciate the processes of lineage specification of blood cells
• To recognize the different types of mature blood cell
• To understand the normal role of each mature cell type in the blood

Where is blood formed?

As the developing embryo grows, it starts to require a means of delivering oxygen to tissues for respiration. The circulation and blood develop at the same time, from around 3 weeks' gestation, and there are close links between the cellular origins of the first blood cells and the vasculature. Haematopoietic stem cells originate in the para-aortic mesoderm of the embryo. Primitive red blood cells, platelet precursors and macrophages are initially formed in the vasculature of the extra-embryonic yolk sac, before the principal site of haematopoiesis shifts to the fetal liver at around 5-8 weeks' gestation. The liver remains the main source of blood in the fetus until shortly before birth, although the bone marrow starts to develop haematopoietic activity from as early as 10 weeks' gestation.

After birth, the marrow is the sole site of haematopoiesis in healthy individuals. During the first few years of life, nearly all the marrow cavities contain red haematopoietic marrow, but this recedes such that by adulthood haematopoiesis is limited to marrow in the vertebrae, pelvis, sternum and the proximal ends of the femora and humeri, with minor contributions from the skull bones, ribs and scapulae.

Although the sites of haematopoiesis in the adult are therefore relatively limited, other sites retain their capacity to produce blood cells if needed. In conditions in which there is an increased haematopoietic drive (such as chronic haemolytic anaemias and chronic myeloproliferative disorders), haematopoietic tissue will expand and may extend into marrow cavities that do not normally support haematopoiesis in the adult. Foci of haematopoietic tissue may also appear in the adult liver and spleen and other tissues (known as extramedullary haematopoiesis).

Haematopoietic stem cells

The process of haematopoiesis involves both the specification of individual blood cell lineages and cellular proliferation to maintain adequate circulating numbers of cells throughout life. This is accomplished using the unique properties of haematopoietic stem cells.

Long-term haematopoietic stem cells (HSCs) in the bone marrow are capable of both self-renewal and differentiation into the progenitors of individual blood cell lineages. The progenitor cells of individual lineages then undergo many rounds of division and further differentiation in order to yield populations of mature blood cells. This process can be represented as a hierarchy of cells, with HSCs giving rise to populations of precursor cells, which in turn give rise to cells increasingly committed to producing a single type of mature blood cell (Figure 1.1). Thus, the immediate progeny of HSCs are the multipotent progenitor cells, which have limited self-renewal capacity but retain the ability to differentiate into all blood cell lineages. Although there is still debate about exactly how lineage-restricted subsequent precursors are, the concept of sequential and irreversible differentiation is widely accepted. In Figure 1.1, the HSC is seen giving rise to two major lineages: the lymphoid lineage, in which a common lymphoid progenitor gives rise to B cells, T cells and natural killer (NK) cells; and a myeloid lineage, with a common myeloid progenitor giving rise to red cells, granulocytes and platelets. The division of haematopoiesis into myeloid and lymphoid compartments is fundamental to an understanding of haematological disease.

Figure 1.1 A schematic representation of the process of haematopoiesis. Multipotent stem cells give rise to lymphoid (pink) and myeloid (blue) lineages. The myeloid lineage further divides into granulocytic, erythroid and megakaryocytic (platelet) lineages. As cells progress through this process of differentiation, they accrue more functional specialization and lose their multipotency. GMP, granulocyte macrophage progenitor; HSC, haematopoietic stem cell; MEP, megakaryocyte/erythroid progenitor; NK, natural killer.

The process of haematopoiesis outlined above has several advantages. First, it permits the massive expansion of cell numbers needed to maintain an adequate population of mature blood cells. It also means that the production of each type of mature blood cell can be controlled individually, tailoring production to specific physiological requirements. Finally, it requires relatively little proliferative activity on the part of the long-term HSCs themselves, thereby minimizing the risk of developing mutations in these crucial cells during DNA replication and cell division.

HSCs were first detected and defined functionally through experiments in which a subset of cells from the bone marrow was shown to produce blood cells of all lineages when transplanted into lethally irradiated mice, which have no haematopoietic potential of their own. Subsequent work has used cell surface markers and flow cytometric techniques (see Chapter 5) to define this population: positivity for the cell surface marker CD34 combined with negativity for CD38 describes a population of multipotential cells that is capable of regenerating all cell lineages from the bone marrow. The cell surface marker CD34 is also used to isolate cells with multipotency and self-renewal capacity for stem cell transplantation.

Differentiating blood cells

Precisely how the ultimate lineage choice of differentiating progenitor cells is determined remains a subject of research. It has been argued that factors intrinsic to the HSC itself, such as stochastic fluctuations in transcription factor levels, may direct lineage specification. However, it is also known that proper regulation of HSCs and progenitor cells requires their interaction with extrinsic factors, such as non-haematopoietic cells in the bone marrow niche (e.g. endothelial cells and osteoblastic progenitors). HSCs and progenitor cells are not randomly distributed in the marrow, but exist in ordered proximity relative to mesenchymal cells, endothelial cells and the vasculature. Signalling from these non-haematopoietic cells, plus physicochemical cues such as hypoxia and blood flow, are therefore likely to influence the transcriptional activity and fate of HSCs.

Myelopoiesis

Signalling through myeloid growth factors such as granulocyte-macrophage colony stimulating factor (GM-CSF) is essential for the survival and proliferation of myeloid cells. The specification of the myeloid lineage is also known to require the interaction of a series of specific transcription factors, including C/EBPa, core binding factor and c-Myb. As well as being essential for the normal formation of myeloid cells, it is becoming clear that an appreciation of these factors and others like them is critical for an understanding of myeloid diseases such as acute myeloid leukaemia (see Chapter 11).

The separation of the erythroid and megakaryocytic components of myelopoiesis requires the action of transcription factors GATA1, NF-E2 and SCL, and signalling through the growth factors thrombopoietin and erythropoietin.

Granulocytes and their function

Morphologically, myeloblasts are the earliest recognizable granulocytic cells. They are large cells, with open nuclear chromatin (Figure 1.2a). The successive stages through which a myeloblast matures into circulating neutrophil granulocytes are termed promyelocytes (Figure 1.2b), myelocytes (Figure 1.2c), metamyelocytes and band forms. Cell division occurs in myeloblasts, promyelocytes and myelocytes, but not normally in metamyelocytes or band cells.

Figure 1.2 Neutrophil precursors from normal bone marrow. (a) Myeloblast (arrowed); the other nucleated cells near the myeloblast are an eosinophil granulocyte (centre) and two polychromatic erythroblasts. (b) Promyelocyte (arrowed); the other nucleated cells are two polychromatic erythroblasts and a neutrophil metamyelocyte. (c) Neutrophil myelocyte (arrowed); there are two neutrophil band cells adjacent to the myelocyte.

The maturation process of the neutrophil lineage is characterized by a reduction in the size of the cell, along with the development of granules containing agents essential for their microbicidal function. The nucleus also gradually begins to adopt its characteristic segmented shape (Figure 1.3).

Figure 1.3 Monocyte and two neutrophil granulocytes - the monocyte has a pale, greyish-blue vacuolated cytoplasm.

Mature neutrophils have the ability to migrate to areas of inflammation (chemotaxis), where they become marginated in the vessel lumen and pass into the tissues through interaction with selectins, integrins and other cell adhesion molecules. Once primed by cytokines such as tumour necrosis factor a (TNFa) and ?-interferon (IFN?), neutrophils are able to phagocytose opsonized microbes, and destroy them by deploying their toxic intracellular contents. This release of reactive oxygen species (the 'respiratory burst') provides a substrate for the enzyme myeloperoxidase (MPO), which then generates hypochlorous acid with direct cytotoxic effects. The granules of neutrophils also contain an array of antimicrobial agents, including...