**4. Conclusions and future directions**

In this chapter, we have reviewed recent progress in our understanding of the development of the haematopoietic system. We have emphasized the critical role that the use of ES cells, in particular murine ES cells, has played in these recent advances. ES cells have been instrumental to identify and characterise the elusive haemangioblast. More recently, this model system allowed the merging of the two conflicting theories of the origin of blood cells (haemangioblast and haemogenic endothelium) in a single linear model of development. In addition the precise roles and requirements of many critical regulators of this process have been to a large extent elucidated using this approach.

Embryonic Stem Cell Differentiation – A Model System to Study Embryonic Haematopoiesis http://dx.doi.org/10.5772/57607 79

**Figure 7.** Examples of haematopoietic colonies obtained during ES cell differentiation A) Primitive erythroid colony – red in colour, compact and relatively small. B) Definitive myeloid colony – bigger in size, white and looser cells. C) Mixed haematopoietic colony. Scale bar 50µm.

Although only very succinctly discussed in this chapter, this system is also very amenable to examination of the cell signalling pathways that support the development of normal haema‐ topoiesis [33, 34, 82-84]. Interestingly these pathways are also implicated in leukaemogenesis [85, 86]. Finally with the advent of novel human ES cells or human iPS cells [87], that recapit‐ ulate better the ground state and are easier to work with, and development of new methods facilitating genome editing [88-92], this experimental system is very likely to be instrumental for delivering new advances in our understanding of human haematopoietic development, that is otherwise very difficult to study *in vivo*.
