CHAPTER 1
Haemopoiesis

Key topics

• Site of haemopoiesis
• Haemopoietic stem and progenitor cells
• Bone marrow stroma and niches
• The regulation of haemopoiesis
• Transcription factors
• Haemopoietic growth factors
• Growth factor receptors and signal transduction
• Adhesion molecules
• The cell cycle
• Epigenetics
• Apoptosis

This chapter deals with the general aspects of blood cell formation (haemopoiesis). The processes that regulate haemopoiesis and the early stages of formation of red cells (erythropoiesis), granulocytes and monocytes (myelopoiesis) and platelets (thrombopoiesis) are also discussed.

Site of haemopoiesis

In the first few weeks of gestation, the embryonic yolk sac is a transient site of primitive haemopoiesis. Definitive haemopoiesis derives from a population of stem cells first observed in the aorta–gonads–mesonephros (AGM) region of the developing embryo. These common precursors of endothelial and haemopoietic cells are called haemangioblasts and seed the liver, spleen and bone marrow.

From 6 weeks until 6–7 months of foetal life, the liver and spleen are the major haemopoietic organs and continue to produce blood cells until about 2 weeks after birth (Table 1.1; see Fig. 7.1b). The placenta also contributes to foetal haemopoiesis. The bone marrow takes over as the most important site from 6 to 7 months of foetal life. During normal childhood and adult life, the marrow is the only source of new red cells, granulocytes, monocytes and platelets. The developing cells are situated outside the bone marrow sinuses; mature cells are released into the sinus spaces, the marrow microcirculation and so into the general circulation.

In infancy all the bone marrow is haemopoietic, but during childhood and beyond there is progressive replacement of marrow throughout the long bones with fat cells, so that in adult life haemopoietic marrow is confined to the central skeleton and proximal ends of the femurs and humeri (Table 1.1). Even in these active haemopoietic areas, approximately 50% of the marrow consists of fat in the middle‐aged adult (Fig. 1.1). The remaining fatty marrow is capable of reversion to haemopoiesis, and in many diseases there is also expansion of haemopoiesis down the long bones. Moreover, in certain disease states, the liver and spleen can resume their foetal haemopoietic role (‘extramedullary haemopoiesis’).

Table 1.1 Dominant sites of haemopoiesis at different stages of development.

Figure 1.1 Normal bone marrow trephine biopsy (posterior iliac crest). Haematoxylin and eosin stain; approximately 50% of the intertrabecular tissue is haemopoietic tissue and 50% fat.

Haemopoietic stem and progenitor cells

Haemopoiesis starts with a pluripotent stem cell that can self‐renew by asymmetrical cell division but also gives rise to the precursor of the separate cell lineages. The stem cells are able to repopulate a bone marrow from which all stem cells have been eliminated by lethal irradiation or chemotherapy. Self‐renewal and repopulating ability define the haemopoietic stem cell (HSC). HSCs are rare perhaps 1 in every 20 million nucleated cells in bone marrow. Newer DNA sequencing techniques suggest that a typical adult has approximately 50 000 HSCs.

HSCs are heterogeneous, with some able to repopulate a bone marrow for more than 16 weeks, called long‐term HSCs, while others, although able to produce all haemopoietic cell types, engraft only transiently for a few weeks and are called short‐term HSCs. Although the exact cell surface marker phenotype of the HSC is still unknown, on immunological testing these cells are positive for the markers cluster of differentiation 34 (CD34), CD49f and CD90 and negative for CD38 and CD45RA and for cell lineage‐defining markers (Lin). Morphologically, HSCs have the appearance of small‐ or medium‐sized lymphocytes.

Cell differentiation occurs from the stem cells via committed haemopoietic progenitors, which are restricted in their developmental potential (Fig. 1.2). The existence of the separate progenitor cells can be demonstrated by in vitro culture techniques. Stem cells and very early progenitors are assayed by culture on bone marrow stroma as long‐term culture‐initiating cells, whereas later progenitors are generally assayed in semi‐solid media. As examples, in the erythroid series progenitors can be identified in special cultures as burst‐forming units (BFU‐E, describing the ‘burst’ with which they form in culture) and colony‐forming units (CFU‐E; Fig 1.2); the mixed granulocyte/monocyte progenitor is identified as a colony‐forming unit‐granulocyte/monocyte (CFU‐GM) in culture. Megakaryocytes derive from a megakaryocyte progenitor, itself derived from an earlier mixed erythroid–megakaryocyte progenitor.

Figure 1.2 Diagrammatic representation of the bone marrow pluripotent stem cells (haemopoietic stem cells, HSC) and the cell lines that arise from them. A megakaryocytic/erythroid progenitor (MkEP) and a mixed lymphoid/myeloid progenitor are formed from the pluripotent stem cells. Each gives rise to more differentiated progenitors. BFU‐E, burst‐forming unit erythroid; CFU‐E, colony‐forming unit erythroid.

In the haemopoietic hierarchy, the pluripotent stem cell gives rise to a mixed erythroid and megakaryocyte progenitor, which then divides into separate erythroid and megakaryocyte progenitors. The pluripotent stem cell also gives rise to a mixed lymphoid, granulocyte and monocyte progenitor, which divides into a progenitor of granulocytes and monocytes and a mixed lymphoid progenitor, from which B‐ and T‐cell lymphocytes and natural killer (NK) cells develop (Fig. 1.2). The spleen, lymph nodes and thymus are secondary sites of lymphocyte production (Chapter 9).

As the stem cell has the capability for self‐renewal (Fig. 1.3), the marrow cellularity remains constant in a normal, healthy steady state. There is considerable amplification in the system: one stem cell is capable of producing about 106 mature blood cells after 20 cell divisions (Fig. 1.3). In humans, HSCs are capable of about 50 cell divisions (the ‘Hayflick limit’), with progressive telomere shortening with each division affecting viability.

Under normal conditions most HSCs are dormant, with at most only a few percent active in cell cycle on any given day. Any given HSC enters the cell cycle approximately once every 3 months to 3 years in humans. By contrast, progenitor cells are much more numerous and highly proliferative. With ageing, the number of stem cells falls and the relative proportion giving rise to lymphoid rather than myeloid progenitors also falls. Stem cells also accumulate genetic mutations with age, an average of 8 exonic coding mutations by age 60 years (1.3 per decade). These, either passengers without oncogenic potential or drivers that cause clonal expansion, may be present in neoplasms arising from these stem cells (Chapters 11, 16).

The progenitor and precursor cells are capable of responding to haemopoietic growth factors with increased production of one or other cell line when the need arises. The development of the mature cells (red cells, granulocytes, monocytes, megakaryocytes and lymphocytes) is considered further in other sections of this book.

Figure 1.3 (a) Bone marrow cells are increasingly differentiated and lose the capacity for self‐renewal as they mature. (b) A single stem cell gives rise, after multiple cell divisions (shown by vertical lines), to >106 mature cells.

Bone marrow stroma and niches

The bone marrow forms a suitable environment for stem cell survival, self‐renewal and formation of differentiated progenitor cells. It is composed of various types of stromal cells and a microvascular network (Fig. 1.4). The stromal cells include adipocytes, fibroblasts, macrophages, megakaryocytes, osteoblasts, osteoclasts, endothelial cells and mesenchymal stem cells (which have the capacity to self‐renew and differentiate into osteocytes, adipocytes and chondrocytes). The stromal cells secrete extracellular molecules such as collagen, glycoproteins (fibronectin and thrombospondin) and glycosaminoglycans (hyaluronic acid and chondroitin derivatives) to form an extracellular matrix.

The HSCs reside in two types of niche. These provide some of the growth factors, adhesion molecules and cytokines which support stem...