Figure 1.1
Hierarchy of haematopoiesis– the multiple stages of blood cell development from haematopoietic stem cells to terminally differentiated cells through intermediate progenitors. The dashed lines represent an alternative differentiation pathway proposed by Adolfsson et al. 2005, based on the presence oflymphoid-primed multipotent progenitors.
Differentiation
HSCs in the bone marrow subsequently develop into other terminally differentiated cells such as erythrocytes, granulocytes and monocytes (Figure 1.1). HSCs give rise to both myeloid and lymphoid lineages of blood cells. The commitment of differentiated cells is irreversible: for example, monocytes are unable to form erythrocytes.
Self-renewal
The second fundamental attribute of HSCs is the ability to self-renew to provide a continuous source of blood cells throughout the human lifespan. The ability to self-renew is maintained through a number of tightly regulated mechanisms that are gradually being elucidated. The incidence of uncontrolled proliferation (as occurs in cancer) is rare compared with the number of times the haematopoietic system has to respond by controlled proliferation to injury or infection. One way in which this is regulated is through the loss of self-renewal properties in differentiated cells such as neutrophils and monocytes. For example, vast numbers of neutrophils are drawn to sites of infection, but they have a limited lifespan and have to be replaced by upstreamprogenitors. This requires the HSCs to exit dormancy and generate intermediate progenitors (see Figure 1.1), which are able to divide rapidly and replenish these peripheral cells.
Figure 1.2
Experimental haematopoietic models used to query the fate of different cell populations. Flow cytometryis the most widely applied method for characterising and, in combination with cell sorting, isolating stem cells. (a) Flow cytometry can isolate populations of lymphoid or myeloid stem cells in vivo. First, a population of cells with the same surface cell markers (for example, common lymphoid progenitors [CLP]) is isolated from a bone marrow sample by flow cytometry. This is transplanted into an irradiated mouse. The resulting cell line is then analysed by flow cytometry, in this case showing differentiationinto lymphocytes only. (b) The in vivo differentiation of individual haematopoietic progenitor cellscan be tracked by labelling each cell with a unique genetic barcode. Its progeny can then be tracked by high-throughput sequencing, permitting the contribution of clonal populations to the overall haematopoietic system to be identified. (c) Single-cell genome sequencing has helped to refine traditional views of cell differentiation. Single cells isolated from blood or bone marrow samples by flow cytometry can then be grouped according to their gene expression to establish the clonal relationship between individual cells.
Identification of upstream progenitor cells
The precursors of fully differentiated neutrophils and erythrocytes bear intermediate properties between them and the HSCs. They have an increasingly restricted developmental potential as they complete their development. Traditionally, these precursor cells have been identified by labelling cell surface markers with antibodies conjugated to fluorescent proteins, which can then be identified by flow cytometry. Cells sorted on the basis of these cell surface markers have been transplanted into irradiated mice, and only specific populations of cells have been found to develop from them. For example, when common lymphoid progenitors are transplanted into irradiated mice they give rise only to lymphocytes (Figure 1.2a). Similarly, upstream intermediate progenitors of myeloid and erythroid cells have been identified. However, the exact lineages and potential of different intermediaries have been revised over the years.
More recent work based on single-cell analyses has revealed novel insights into the process of haematopoiesis(Figures 1.2b,c). Normal blood cells can be sorted using flow cytometry into individual cells, and RNA can be extracted from them. From this, the expression levels of different genes can be identified using next-generation sequencingand, in combination with traditional transplantation studies, the ultimate fate of these cells can be determined. These studies have further revised the models of haematopoiesis, with some suggesting that haematopoietic development is a continuous process rather than one of sequential subpopulations that become increasingly restricted in terms of lineage potential.
